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GEOL Exam 1

Kennesaw State University
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Transcript
Transcript: What is Geology? - Part 1 Slide 1: What is Geology? Part 1 Welcome to Geology 11040 – How the Earth Works – an introductory Physical Geology course offered at Kent State University. Slide 2: What is Geology? So what is Geology? Well ‘logy’ means the ‘the study of’ and Geo is the Earth, so Geology is the study of the Earth. Perhaps you took a Geology course in high school – but if so, it was more likely called Earth Science – which is the same thing, and I will use both terms interchangeably throughout this course. Now Geology is a natural science, and a fairly young science compared to most other sciences including Biology, Chemistry, or Physics. So what do Geologists or Earth Scientists study? First, they are interested in what Earth is made of – its materials. Mostly solid materials like minerals, rocks, and fossils, but also liquid materials like magma, lava, surface water, subsurface fluids (oil and gas), and even vapors or gases such as those released during volcanic eruptions or released by the burning of fossil fuels. Second, Geologists are interested in Earth processes, such as the processes by which rocks, mountains, oceans, and even continents form and the processes by which rocks, mountains, oceans, and continents are destroyed. Thirdly, Earth Scientists are very interested in the history of Earth including the history of life on Earth. In fact many of the time periods and eras are defined in what dominated life at the time. We won’t be going into too much about the history of Earth in this course since our historical geology course Earth and Life through Time is devoted to that topic. In this physical geology course you’ll be learning primarily about earth materials and processes. Slide 3: Why is Geology Important? Never has Earth Sciences been as important or as practical as it is in today’s world. And that’s largely because of global population. Earth processes have a huge impact on our environment, but also with so many people our modern societies today are using Earth’s natural resources to a degree never seen before and in doing so humans are having a huge impact on the Earth’s environment as well. Essentially humans are now agents of geologic change. Let’s look at one dramatic impact humans have had on earth’s most important resource, water. The Aral Sea, in western Asia, was once the 4th largest lake in the world. In the 1960s, the Soviet Union undertook major water diversion projects on nearby rivers, capturing water that once fed into the Aral Sea for irrigation purposes. Now this worked great for crop production in the surrounding area but it was a disaster for the natural freshwater lake. These images show the shrinking of the Aral Sea from 1977 on the left to 1989 in the middle and to 2006 on the right. Today the Aral Sea is virtually gone. Wow…and it only took humans about 4 to 5 decades to do this. Slide 4: It’s Relevance Today…Soil Degradation Soil is another critical natural geologic resource that humans are using like never before. We’ll go over how soils form later in the course but it’s important to realize that a welldeveloped soil can take a few hundred years to form, and if humans degrade the existing soil faster than it can be replenished naturally, well then we have a problem. This map uses colors to visualize the state of soil on earth with the warm or hot color red showing the location of very degraded soil, orange showing somewhat degraded soils, and the cream colored regions showing stable soils. The United Nations has determined that 10s of millions of acres of cropland are lost annually to soil degradation especially in Africa, Latin America, and Asia. Even in the U.S., our soil is being eroded at a rate 10 times greater than it can be replenished! Wow. Slide 5: Course Themes As we proceed through this course, there will be a number of recurring themes that will crop up within the major topics we cover. One is that Earth is constantly changing and its components (its interior, its land surface, oceans, atmosphere, and life forms) interact with one another and evolve. We call this interconnected web of interlinking components the Earth System. Another theme is that internal and external processes drive geologic change – internal processes (represented by earthquakes, volcanoes, and mountains) are driven by the earth’s internal heat. External processes (represented by weathering, erosion, sediment transport by rivers and humans) are driven by heat from the sun and by gravity. A third recurring theme is that Plate Tectonics explains many Earth processes – which is why the first topic in this course is entirely devoted to Plate Tectonics. Another emphasis in this course is the Geologic processes and phenomena affect our environment and society. Natural hazards are dramatically increasing and natural resources are vitally important to society. People really need to know about these earth-based hazards and resources. Lastly, science stems from observations and our measurements of these observations. So we will use lots of visuals and talk a lot about societal impacts in this course. Slide 6: Earth System Science I want to emphasize the point of viewing the Earth as a System. When I was learning about Geology as a student some time ago, I learned primarily about the solid components of Earth –Geosphere – primarily Earth Materials, Earth’s crust and underlying mantle, Earth’s tectonic plates, etc. Now students of Geology understand our changing Earth as consisting of four realms or components all influencing and interacting with one another. In addition to the Geosphere there’s the atmosphere, the mixture of gases that surround Earth; the hydrosphere, all of the water on Earth including the oceans, rivers, lakes, but also including water in the subsurface and in the atmosphere; and lastly the biosphere, the realm of all living organisms, including mankind importantly. Within the Earth System certain materials cycle among rock, sea, air, and living organisms and this cycling and interaction of the different spheres or worlds is energized by the Sun, by Earth’s internal heat, and by the force of gravity. Slide 7: Plate Tectonics We will begin the course on the topic of Plate Tectonics, the theory that the outer layer of Earth consists of rigid moving plates. Plate Tectonics is one of the top 10 scientific discoveries of the 20th century and it serves as the foundation for understanding most geologic phenomena. This image outlines in black the boundaries of the large plates that constitute the rigid outer shell of earth. And it shows the location of active volcanoes on earth with red dots. We see that the majority (about 80%) of earth’s 1900 active volcanoes occur around the outer rim of the Pacific. This Pacific “Ring of Fire” is a direct result of active Plate Tectonics. Slide 8: Earth Materials and Riches in Rocks Another topic we will cover is the formation and characteristics of rocks and the mineral and rock resources that come from them. Most of the resources that humans use come from geologic materials and people living in developed societies use these resources extensively – on average every American born will use almost 3 million pounds of minerals, metals, and fuels in their lifetime. Earth’s resources are not infinite and some countries have already run out of some of these economically important resources. We will learn about the processes by which rocks are formed and the conditions which concentrate valuable resources, including minerals and natural ores. Slide 9: Natural Hazards: When Nature Strikes! Many geologic phenomena affect society, often in catastrophic and devastating ways. In this topic on Geologic Hazards, we will learn about the hazards of volcanoes, earthquakes, and landslides, why these hazards are increasing, and about what can be done to mitigate their impacts. This is an aerial photo of the Oso landslide (seen in the upper center) in Washington state. In March 2014 the landslide catastrophically engulfed 49 homes, killing 43 people, and dammed the river, causing extensive flooding upstream. Excluding landslides caused by volcanic eruptions, earthquakes or dam collapses, the Oso slide is the deadliest single landslide event in the United States history. Slide 10: Geology of Water Freshwater is earth’s most important natural resource. This topic covers where freshwater occurs, how freshwater sculpts and shapes landscapes and transports earth materials from the continental interiors and mountains to the margins of the oceans. Especially important is how humans extract and use freshwater and how people have created a global water crisis through the overuse and misuse of water resources. For example, Lake Erie has been experiencing harmful algal blooms that are toxic and contaminating an important freshwater resource. The increase in harmful algal blooms is being studied by scientists who have determined that the heavy use of pesticides and fertilizers in Ohio and other places is at least part of the reason why there are more blooms. Slide 11: Geology of Energy and Environmental Impacts There has been a dramatic increase in use of and demand for energy resources with industrialization and global population growth. This topic covers the nature and origin of the most widely used energy sources (oil, gas and coal, the fossil fuels), the discovery and search for fossil fuels, and the methods and environmental impacts of extracting and burning these energy sources. Slide 12: Climate Change No doubt you have heard and read a lot about climate change in the present. But what was climate like in the geologic past and how do Earth Scientists recognize what climate was like just one-two million years ago during the ice ages but hundreds of millions of years ago when Earth may have been completely immersed in ice? And what earth processes might have been responsible for past climates and how and why is climate changing today? Transcript: What is Geology? - Part 2 Slide 1: What is Geology? - Part 2 Welcome back to the Introduction to Geology 11040 How the Earth Works. Questions are one really good way to increase our understanding of a subject and to promote interest. So for fun think about a few no-stakes (non-graded) questions on your own. Slide 2: Answer these Questions Here are the questions. The age of the Earth is a) 4600 years, b) 46000 years, c) 46 million years, d) 4.6 billion years, and e) 46 billion years. Rocks can be folded: True or False? Continents are older than oceans: True or False? The Rocky Mountains are older than the Appalachian Mountains: True or False? Ice is a mineral: True or False? And lastly, the largest historical earthquake in the lower 48 states occurred in a) California, b) Oregon, c) Missouri, d) Ohio. Now the answers: The age of the Earth is d) 4.6 billion years. The reason I start with this question in this course is to emphasize that the Earth is really old, so old in fact that it’s hard for humans to fathom the vast amount of geologic time that encompasses Earth history. In fact scientists give a name for the past 4.6 billion years –Deep Time, a topic we will come back to soon. Another reason I start with this question is that it allows me to follow up with another question – namely how do we know the Earth is 4.6 billion years old? Interestingly, there are no known rocks or minerals on earth that are 4.6 billion years old; the oldest radiometrically dated rocks and minerals are about 4 billion years old and there are very few of those. Most rocks are much, much younger. We don’t have the time or the background to go into how we know the Earth’s age right now, but my point is this: “it’s not so much what you know, but how you know it.” Slide 3: Yes, Rocks can be Folded Here’s a beautiful photo that proves that yes, rocks can be folded. Rock folds are some of the most spectacular features of Earth structures. They are common features in many belts and they occur at all scales – I have hand size samples of rocks in my office that have beautiful folds. There are larger size folds that are about the size of road cuts, and there are very large folds that are the size of mountains. I ask this question to show you how answers to questions in Geology often come from simple observations of the rock record. I also ask it to get you thinking about how folds form and the tremendous forces that must be involved to cause rocks to fold. Slide 4: Yes, Continents are older than the Oceans Ok, which do you think is older; continents or oceans? Continents are indeed older than ocean basins – in fact they are much older. For instance, the North American continent started forming about 2 billion years ago and was largely complete 1 billion years ago. In comparison, the Atlantic Ocean that borders North America to the east started forming less than 200 million years ago. I ask this question to get you thinking about why there are oceans and continents, and to stress that they are fundamentally different geologic features. Oceans are born and die all the time, but when continents form, they tend to stick around. This map of the U.S. is also helpful in answering which mountain chain is older – the Appalachians on the eastern side of the U.S. or the Rocky Mountains to the west on the other side of the Great Plains? The answer lies in how tall the mountains are – note how the Rocky Mountains stand tall compared to the Appalachian Mountains to the east. Older mountains have had lots of time to be eroded, so they tend to be worn down and have a more subdued topography. In contrast, younger mountains tend to be higher and can often have dramatic topography. Slide 5: Grand Tetons, Wyoming Let’s take a look at some mountains out west. Here’s a favorite photo of mine of Geology majors attending our summer field course in the Rocky Mountains – great looking bunch of people I might add – and behind them are the spectacular Grand Tetons, which are majestic mountains in Wyoming that are only about 2 million years old. If you get a chance to go west, I encourage you to visit the Grand Teton National Park – you won’t be disappointed. It’s one of my favorite places! Slide 6: Ice is by Definition a Mineral Many students are surprised to learn that ice is indeed a mineral– so these beautiful glaciers could be considered flowing rivers of rock, all made of a single mineral – ice. Now everyone knows that ice melts and becomes water. So is water a mineral? Actually no, because by definition minerals are solid. So what about mineral water? Heehee. Mineral water is water that is obtained from mineral springs and that contains dissolved solids and gases. OK, so what about ice cubes? Are they minerals? Actually, no! Because ice cubes are not naturally occurring, and a mineral by definition must also be naturally occurring. Anyway, we’ll talk more about ice and water in this course as both are important components of the Earth’s System. Slide 7: U.S. Seismic Hazard Map Here’s a map of seismic hazard in the United States. The hotter red and orange colors represent regions where there is very high potential of earthquakes and hence high seismic hazard risk, and the cooler colors, the blues and greys, are areas of few or no earthquakes historically and therefore low seismic hazard. Very likely you are well aware of the high seismic hazard along the entire west coast (shown in the red), but you may be surprised to see that there is also high seismic hazard potential in the great plains area along what is known as the New Madrid seismic zone (the bulls-eye pattern in the interior). It turns out the historically largest earthquake (2 earthquakes actually) in the lower 48 states actually occurred in Missouri in 1811 and 1812, in the interior of the North American plate. So although most earthquakes do occur along the edges of geologic plates, some significant earthquakes have occurred in the middle of plates and we will talk about why that is when we cover geologic hazards. Another interesting point about earthquakes is that humans are now causing earthquakes by the process of removing gas from the subsurface via fracking. Slide 8: Geology is the study of Earth Materials, Processes and History Geology, the study of Earth, has changed a lot since I was a student learning about Earth Science. The older view of Geology focused on identifying Earth materials (rocks, minerals, fossils), boots on the ground mapping of the earth’s surface, and development of the Plate Tectonic theory as the over-riding paradigm for understanding our planet. Modern views of Earth Sciences focuses much more on human environmental impacts; recognizing that people are a major force on earth, causing significant environmental changes and dramatically changing Earth’s landscape. Energy and mineral resource extraction and usage are exceedingly important to societies. And Earth is studied today by remote sensing techniques (satellites, radar, infrared cameras, etc.) and is viewed as an interconnected system (the Earth System). Slide 9: Mapping the Invisible Here’s a great example of using remote sensing to document an environmental problem that occurred near Los Angeles. In 2015 a leak was discovered in a natural gas storage facility near Los Angeles. The gas leak was invisible and no one new how extensive it was, although residents in a nearby neighborhood were experiencing significant health problems and many residents had to be evacuated. Slide 10: Methane The center photo of these 3 photos shows the Aliso Canyon area during the gas leak. Obviously it looks very normal. However, the images on the left and right were taken by infrared cameras which reveal a huge cloud of methane gas. The purple stuff on the left and the orange cloud and vent on the right. The image on the right clearly shows the gas pouring out of the ground like an erupting volcano – you could view this essentially as a human made methane volcano. These types of remote sensing images enabled scientists to measure the amount of methane being released and to understand the magnitude of the problem. The Governor of California declared a state of emergency for this area and sometime later the leak was capped, but not before a tremendous amount of methane was released into the atmosphere. Methane is a very potent Greenhouse Gas, nearly 100 times, or two orders of magnitude, more potent than carbon dioxide. The methane released from this one storage facility was thought to be about equivalent to greenhouse gases released by 7 million cars driven for an entire year. Slide 11: Geologic Processes take place across Vast Scales of Space and Time Geologic materials exist at many different scales of space from the size of sand grains (and smaller) to the size of mountains and tectonic plates. Likewise geologic processes occur across vast scales of time – on the order of seconds and minutes for earthquakes to millions of years for the construction of mountains. One way to think about different scales of space or size is to think about size variations by multiples or factors of 10, which scientists also refer to as an order of magnitude. Now humans are about 1-2 meters in length (a meter is 3 feet). Something that’s about 10 times our length might be a 3 story apartment building – that would be a one order of magnitude different or 101 . A very tall structure, say the Eiffel tower, would be closer to 100 times our length or two orders of magnitude or 102 . What about Mount Rushmore, the giant sculpture of Presidents carved into granite in the Black Hills of South Dakota? We would say that’s roughly 1000 times larger than us, or 3 orders of magnitude larger, or 103 . This image nicely depicts scales of space using multiples of 10 from a carbon atom (10-10 meters across) to the size of the universe (1026 meters across). In this physical geology course we will refer to scales about as small as coffee beans (10-2 meters or 100 times smaller than us) up to the size of earth which is on the order of 107 meters across. Slide 12: Grasping 4.6 billion years Earth is not only large, but it also extends almost unimaginably far back in time. So how can we fathom or comprehend 4. 6 billion years of time? Again, let’s go backwards in time using factors of 10 years. Most of us have no problem thinking in timescales of 1 year or 101 years ago, but what was happening say about 100 years ago, or 102 years ago? The first cars were being mass produced. 1000 years ago? That was the Middle Ages and people thought that the sun and planets orbited earth. About 10,000 or 104 years ago was the dawn of civilizations. The rise of Homo Sapiens occurred on the order of 100,000 or 105 years ago, and early hominids first appeared about one million or 106 years before that. 10 million years ago was the Age of Mammals, 100 million years ago was the Age of Dinosaurs, and 1,000 million years ago (1 billion years) only single celled organisms existed. And Earth’s history started 4.6 billion years ago which is Earth’s birthday – woohoo! Slide 13: Another view of Earth’s History Another way to view Deep Time (the last 4.6 billion years) is to compress it into a single calendar year – which is a manageable amount of time for us. So Earth’s birthday would be January 1, the oldest dated rocks would be about mid-February, single cell organisms would come on the scene in early March, but the first fossils of animals with hard parts wouldn’t arrive until middle October. And the first dinosaurs, which lived on the order of 1-200 millions of years ago, would not arrive until December 11 and they would disappear December 26. The first modern humans would come on the scene only 23 minutes before midnight, the last day of the year. Slide 14: Geologic History Geologic history is often depicted with a vertical geologic timescale with the top of the scale being the present and time going further and further backwards going down the scale. This is a simple geologic timescale that highlights the most recent 542 million years. These 542 million years are divided into three parts, the Paleozoic Era where ‘Paleo’ means ancient and ‘zoic’ means life. This 200 million year era was when hard-shelled organisms evolved, and also fishes and plants. The Mesozoic or middle life period was another approximately 200 million years from 250 to 65 million years ago when dinosaurs wandered earth. And this is the Age of Dinosaurs. The extinction of the dinosaurs 65 million years ago was the start of the Cenozoic Era, the Age of Mammals. I know that’s a lot of information to take in about geologic time, but hopefully it didn’t just go into one Era and out the other Era. Note how the lowest segment is the Precambrian – which is the first 4 billion or 7/8 of geologic time, lumped into one word called the “Precambrian.” That doesn’t seem quite right but geologists tend to know much more about the more recent 500 million years of Earth history than we do about the Precambrian, so maybe that’s OK. Slide 15: Oceans and Continents Geology is an outside science – it’s the study of our physical world and therefore it’s helpful to know very broadly about the primary geographical features of Earth – namely the oceans and the continents and where some of the major geologic features are located – like mountains. Now I know that everyone is familiar with the names and locations of the four oceans, the Pacific, Atlantic, Indian and Arctic Ocean at the top of the world. And the continents Australia, Asia, Africa, Europe, North and South America and Antarctica at the bottom of the world. Now can you locate these geologic features, the Rocky Mountains, Denali, the Alps, the Himalaya Mountains, the Great Rift Valley, and the Andes Mountains? We’ve already talked about the Rocky Mountains, but while you are thinking about the others, you should know that Denali is the new name for what was formerly called Mt. McKinley, originally named after President McKinley from Ohio. Slide 16: Mountain Locations Okay, here are the locations of those geologic features. Denali is in Alaska and is the highest point in North America. The Rockies (RM) are in the western US (we knew that already). The Andes Mountains (AM) are the long mountain chain on the western side of South America. The Alps are young spectacular mountains in southern Europe. The Himalaya Mountains (HM) are the huge mountains in Asia formed by the collision of India into Asia about 45 million years ago (and still growing today). And lastly, the Great Rift Valley (GRV) is in the northeastern part of Africa and it represents a place where the African continent is breaking apart. Okay, very good. Slide 17: Why does the Ocean Roar? I’d like to end this Introduction to How the Earth Works with one of my favorite Geology jokes. Why does the Ocean roar? Well, you would roar too if you had crabs all over your bottom! Hope you enjoy the course. Transcript: Intro to Plate Tectonics – Part 2 Slide 1: Continental Drift Hypothesis In the early 1900’s what evidence did Wegener present to the community of scientists and in his 1915 book for the existence of Pangea and its breakup by continental drift, the slow movement of continents over time? Slide 2: Wegener’s used Fit of Continents too Wegener noted the jigsaw puzzle fit of the continents as others had before him, but with a twist. Instead of using the coastlines of the continents, Wegener realized that the true edge of continents actually extend out a ways from the coastline, to where the underwater continental shelves start to steepen or slope down into the ocean. By including the continental shelves as the drowned true edge of continents, and as shown in the cyan color bordering the continents of this map, Wegener was able to show that the continents fit together even better, with fewer gaps between the puzzle pieces. Slide 3: Glacial Till As an Arctic climate scientist who worked extensively on ice sheets in Greenland, Wegener knew that glaciers are rivers of ice that flow across the land surface. He recognized that glaciers carry rock debris that gets laid down or deposited on the Earth’s surface when the glaciers melt. The rock debris left behind is typically very chaotic with a mixture of different rock fragments and of different sizes (a jumbled mess essentially) called glacial till. The jumbled, mixed nature of the different fragments, such as you can see in this photo, are evidence that this rock from South Africa was deposited by a glacier. Slide 4: Wegener used Glacial Evidence Wegener recognized and mapped 260 million year old Paleozoic glacial till deposits in parts of South America, Africa, southern Australia, and even in tropical India, places today that are much too warm to harbor glaciers! He also recognized glacial grooves and scratches on rocks (called striations) that were carved by rocks embedded in the base of the glacier as it was moving. And he noted that on a map these striations were oriented so that they appeared to be radiating outward from a location in southern Africa. Slide 5: Glacial Deposits and Striations fit together near the South Pole in Wegener’s Pangea Wegener hypothesized that these ancient glacial features, which are now widely separated from one another, and even extend to north of the equator today, could have been a single ice sheet located near the South Pole at the end of the Paleozoic (around 250 to 260 million years ago). Slide 6: Wegener Used Climatic Evidence As a meteorologist, Wegener recognized that Earth has climate zones in which different land and marine deposits and life forms exist. Coal deposits and marine limestone reefs form in warm tropical deposits, for instance, whereas salt deposits and sand dunes form in hot and dry climates. Since his Pangea supercontinent extended from the South Pole to the equatorial regions, Wegener expected to find different rock types within different climate belts across Pangea – and that’s what he found. For instance, he found coal and limestone deposits that would be predicted to form in the tropics of Pangea shown here in the green. Slide 7: Wegener used Fossil Evidence Wegener also used fossil evidence to connect continents that are now widely separated. Since land animals and plants cannot cross oceans, they evolve independently on different continents. But when continents are together, land animals and plants can migrate across them. By examining the late Paleozoic and early Mesozoic fossils, Wegener was able to show that certain species had indeed lived on several continents, including species of ferns and reptiles as shown in this reconstruction. Slide 8: Wegener matched Geology Across Oceans Finally, Wegener examined geologic maps and recognized that rock types, structures, and ages matched up across continents and fit together very well in the absence of the Atlantic Ocean. The left image shows how the geologic core of South America and Africa match up when they are adjacent. And the right image shows how a single mountain belt exists when Africa and North America are in contact with one another. Slide 9: Pangea (~250 my ago) = “All Land” On the left is a simple cartoon image of Wegener’s proposed Pangea Supercontinent reconstruction annotated with the names of the continents. And on the right is an amazing more realistic image of what Wegener’s supercontinent-superocean world may have looked like around 250 million years ago. So I have question – do you think Wegener makes a strong argument for moving continents and the existence Pangea? Slide 10: Criticism of Continental Drift Hypothesis After publishing his book and when presenting his idea that continents move slowly or drift to the scientific community, Wegener’s proposal was ridiculed and rejected by nearly everyone. He convinced essentially no one at the time and although he kept looking for further evidence, he ultimately died in 1930 at the age of 40 on an ice sheet in Greenland when he was on a supply expedition. And since he was the sole advocate for continental drift, the drift hypothesis ‘drifted away’ and died with him. Now why couldn’t Wegener convince anyone of continental drift? Some people point out that being a German at the time probably didn’t help him (during and after World War I); others argue that being a meteorologist also probably didn’t help him either since he was not trained and educated as a Geologist. But certainly the most important reason however, was that Wegener couldn’t explain how continental crust could drift or plow through oceanic crust. He lacked a mechanism for continental drift. Slide 11: How do continents plow through oceans? In his writings and at conferences Wegener wasn’t able to provide a mechanism or process by which the continents could move and he couldn’t explain the great forces that would be needed to move huge masses of continents. Slide 12: New Evidence after WWII The argument for continental drift disappeared and was largely forgotten about until after World War II when new evidence came to light largely driven by renewed political and scientific interest in the oceans. Remember, Wegener used only land based evidence. The oceans and the Earth’s magnetic field were two other key elements in the Plate Tectonic story. Transcript: Paleomagnetism – Part 1 Slide 1: Paleomagnetism The idea of continental drift, the notion that large masses of continents are somehow drifting through oceans of basalt, died with Wegener in 1930, but was inadvertently rediscovered after World War II in the 1950s with renewed interest in studying rock magnetism and especially the record of ancient magnetism preserved in rocks. In order to understand Paleo-magnetism, we first have to understand the Earth’s magnetic field. Slide 2: Aurora Borealis from Space How do we know the Earth has a magnetic field? One way we know is the existence of the amazing northern and southern lights – the Aurora Borealis and Aurora Australis. This is a beautiful photo of the Aurora Borealis taken from space. These are the lights seen above the magnetic poles of the northern hemisphere. Slide 3: Aurora from Earth The lights are even more amazing to see from the ground, they are these amazing green lights that dance around the sky. They exist because a magnetic field permeates the space around Earth. The dancing lights are actually collisions between electrically charged particles from the sun that enter the Earth's atmosphere. These charged particles flow toward Earth’s magnetic poles and cause gases in the atmosphere to glow. Of course the other way we know Earth has a magnetic field is because we can use compasses, which point toward the magnetic North Pole. Slide 4: Iron Filings surround a Bar Magnet So what does the magnetic field look like? Now you may have done this interesting experiment in a secondary school science class. If you take a bar magnet and place it on a paper thinly coated with randomly oriented iron filings, the filings will move and orient themselves into the direction shown here, and what they are doing is aligning themselves parallel to the magnetic force field that surrounds the bar magnet. So in essence this allows us to see the shape of the normally invisible magnetic field. Now many rocks have tiny magnetic minerals in them, magnetite being perhaps the best known magnetic mineral. And these magnetic minerals in rocks align to Earth’s magnetic field just like those iron filings aligned to the magnetic field of the bar magnet. Slide 5: The Earth’s Magnetic Field Earth is essentially a huge dipole magnet – meaning it has force field lines that go around the Earth and enter and exit Earth at the magnetic poles. You can see how the shape of the Earth’s magnetic field is thought to be pretty much the same as the magnetic field surrounding a bar magnet – which is also dipolar, consisting of two poles. Slide 6: Why Does Earth have a magnetic field? So why does Earth have a magnetic field? It turns out that internally Earth is layered, kinda like a layer cake, and that it has a very dense iron rich core, part of which is liquid iron, that’s the outer part of the core. The motion of earth spinning through space causes circulation of the outer liquid iron core, which generates a magnet field that surrounds and shields Earth from dangerous solar winds. Just as a sidebar, this graphic nicely shows how Earth is layered, with its outermost layering being atmosphere, the oceans and ice caps forming the hydrosphere on the surface beneath the atmosphere, the thin outer shell of granitic continental and basaltic oceanic crust below that, which is underlain by a very thick and dense layer of mantle, which in turn surrounds the densest iron core. Each layer I just described is more dense or heavier than the previous layers, so we know that Earth is layered by density. Slide 7: Geographic and Magnetic Poles do not Coincide Now the geographic pole, the location where the Earth’s rotational axis intersects Earth’s surface, does not coincide exactly with the magnetic pole. Today, the magnetic pole is several hundred miles away from the geographic pole and measurements over time show that the magnetic pole moves around a bit, but it’s never too far from the geographic pole. Since compasses point toward magnetic north, people need to correct their compasses to point toward true north. The angle between the magnetic pole and the geographic pole is known as the magnetic declination, which varies depending on where you are on Earth. If you go to magnetic-declination.com you will see that its 8 and 1/2 degrees west in Kent, whereas in Duluth Minnesota, where I grew up, its only about 1 degree west. So when Minnesotans go hiking in the northwoods, they don’t really need to worry about magnetic declination since the angular difference is so small, but here in northeast Ohio the magnetic declination is quite significant! Slide 8: Curved Magnetic Field Lines If we look again at Earth’s magnetic field we see that the field lines are curved – that is they are about parallel to the Earth’s surface near the equator and then start to tilt downward toward the surface moving away from the equator. Since your compass needle wants to align itself with the magnetic field lines, this curving of the magnetic field causes your compass needle to tilt – and this tilt is the magnetic inclination. Now practically compass needles, like the Brunton Compass, are weighted so that they can’t tilt, because the needle needs to be kept horizontal so that it can spin freely. Slide 9: Magnetic Inclination This image shows how the tilt of a compass needle would change moving from the equator to the North Pole if it weren’t artificially weighted, that is held horizontal. The inset circle in the lower right shows that inclination is the angle of tilt down from the horizontal. Looking at the Earth’s surface near the equator, the tilt or inclination of the needle is zero as the field lines there are parallel to Earth’s surface near the equator. In contrast, at the North Pole, the inclination is 90 degrees or vertical, as the needle would be perpendicular to the Earth’s surface, just as the magnetic field lines are oriented. And in-between the equator and the pole, the inclination steepens gradually going from the equator to the pole. The take-home message about magnetic inclination is that it varies from zero (or no tilt) at the equator and increases with increasing latitude toward the polar regions, until it’s very steep or even vertical (90 degrees). When certain rocks form, they freeze in the magnetic inclination at the location where the rock formed – so rocks with low magnetic inclination must have formed near the equator, and rocks with high or steep inclination must have formed at high latitudes near the poles. Transcript: Paleomagnetism – Part 2 Slide 1: Paleomagnetism: Ancient Magnetism Preserved in Rocks Welcome back! There are two aspects about Paleomagnetism, the study of ancient magnetism preserved in certain rock types. The first is that rocks have a magnetic inclination, or tilt, that is related to the latitude where the rock formed. And the second, which we will cover today, is that rocks record regular reversals of the magnetic field, that is the flipping of the N and S magnetic poles in the past. So what type of rocks preserves a record of the Earth’s magnetic field when it forms? Slide 2: Recent Lava Flows at Hawaii Volcanoes National Park Lava is liquid or molten melt, from the subsurface, that flows onto the land surface or the ocean floor, cools quickly, and crystallizes into volcanic rock. As lava cools like this one in Hawaii, magnetic crystals that form within the melt align themselves with the magnetic field much like iron filings do when placed next to a bar magnet. Active lava flows today are in a sense ‘freezing-in’ magnetic inclinations that will be permanently recorded in the volcanic rock. So here’s a question, what do you expect would be the ballpark magnetic inclination angle for Hawaiian basalt flow like these? Would you expect a low, medium, or steep magnetic inclination to be preserved? The right answer would be a low inclination since Hawaii is located in the tropics at low latitudes. Slide 3: The Magnetic Memory of Rocks This photo shows a stack or sequence of multiple basalt flows that erupted millions of years ago in Washington State. In sequences like this, where each layer flowed onto the surface sequentially, we know that the oldest visible flow would be at the base of the sequence, at about lake level, and that younger flows formed on top of older flows in sequence with the youngest visible flow seen at the top of the cliff and along the flat plateau extending away from the cliff in the background. Now Washington State is at an intermediate latitude today, but the magnetic minerals in these ancient basalt flows do not align with the Earth’s current magnetic field – they aligned to the field that existed when the rock originally cooled from lava millions of years ago. In a sense, rocks have a magnetic memory from when they initially formed, from their birthdays. Slide 4: Basalt Flow, Grand Coulee, WA Let’s take a look at another sequence of basalt flows in a cliff near Grand Coulee, Washington. And let’s say, just for funsies, that paleomagnetic scientists, I sometimes call them paleo-magicians, measured the magnetic inclination in three flows, the highest one, one in the middle of the sequence and one near the bottom of the cliff. And let’s say the highest flow has a very gentle magnetic inclination – a tilt of only 5 degrees. The middle flow, however, has somewhat steeper inclination of about 30 degrees and the lowest flow has the steepest inclination, say 60 degrees. Since these flows are all from the same geographic location today, why are their magnetic inclinations different? The answer is that the latitude of these rocks changed as they were erupted over time. The lowest oldest flow formed when North America was at higher latitudes, and over time, the continent moved southward toward the equator, as suggested by the decreasing magnetic inclination. And since, eruption of the highest, youngest flow, this part of the continent has migrated northward to its present location at an intermediate latitude. Slide 5: Which moves – the Continents or the Magnetic Pole? Studies of magnetic inclinations in ancient volcanic rocks in the 1950’s revealed that many flows preserved inclinations that were very different from the expected inclination based on their current distance from the equator (or the latitude). Geologists realized that these unexpected inclinations could be explained if the continents had moved since the volcanic flow initially formed millions of years ago – and hence these studies inadvertently resurrected Wegener’s hypothesis of Continental Drift. Now it had been 25 years since Alfred Wegener died and about 40 years since his book about Pangea and Continental Drift was published. Many of these young scientists had never even heard of Wegener, so you can imagine their surprise when they rediscovered his idea of Continental Drift. As a short aside, the paleomagnetic results generated a debate about what actually moves – the continents or the magnetic poles? These diagrams show how the new results can be explained by a) assuming a fixed pole and a drifting continent or by b) assuming a fixed continent and a wandering pole. Suffice it to say that with further study, the scientists learned that the magnetic poles move only slightly and that it’s the continents that have somehow drifted substantially. Slide 6: Studies of Magnetic Inclination So it was studies of magnetic inclination in volcanic rocks in the 1950’s that resurrected Wegener’s hypothesis of Continental Drift, although scientists still did not know how they could drift. More information or data was needed and that would come, in part, from learning about the second key aspect of Earth’s magnetic field, that it changes polarity or reverses itself fairly regularly. Slide 7: Normal and Reversed Polarity Normal polarity of Earth today is represented by a dipole magnet with an arrow pointing toward the South Pole with force field lines exiting the South Pole and entering at the North Pole (as shown on the left). During reverse polarity the arrows point in the opposite directions (exiting the North Pole and entering the South Pole shown on the right). All volcanic rocks formed in the last 700,000 years or so record normal polarity like we have today. However, volcanic rocks formed between 700,000 years and about 1.2 million years or so show reverse polarity. Slide 8: Volcanic Rock Layers Permanently Record Magnetic Reversals This sketch of a volcano shows the different volcanic layers that erupted onto the surface as the volcano grew. The purple lines represent the magnetic field lines at the volcanoes’ location on Earth. The angle of tilt of these lines represents the magnetic inclination angle (shown here as moderately steep) and the arrowheads on these lines reflect periods of normal polarity and reverse polarity (with arrows pointing in the opposite direction). We see that the youngest layer on top, which is 0.4 million years old or m.y. (or 400,000 years old) has normal polarity, but the 800,000 year old volcanic rock below it has a reverse polarity. We therefore know that there must have been a polarity flip or reversal between the eruption of these two rock layers. And we know from the third and oldest sample that there must have been another reversal again between 1.2 million years and 800,000 years. Slide 9: Magnetic Reversal Timescale By dating easily accessible piles of layered volcanic rocks on continents like shown in this drawing, and measuring their polarity, the paleomagicians were able to reconstruct a timescale of all of the magnetic reversals that have occurred in the past several hundred million years of Earth history. This drawing shows the intervals of normal (in grey) and reverse (in white) polarity extending back to about 2.3 million years ago. Slide 10: Normal, Reverse Looking again at the pile of volcanic flows from Grand Coulee, Washington, we can now add the additional information of normal and reverse polarity in this volcanic sequence, with N being the abbreviation for normal polarity and R being the abbreviation for Reverse polarity. The discovery that the magnetic poles have flipped positions fairly regularly in the geologic past was a key element in recognizing that it was not just the continents that moved but that oceanic crust also moved as I’ll explain later. Slide 11: Marine Geology Prior to World War II, we knew very little about the ocean floor and about the oceanic crust other than it was made of primarily basaltic volcanic rocks. During World War II, navy battles in the South Pacific and the presence of German U-boats off the east coast of the United States, resulted in renewed political and scientific interest in the oceans. Use of submarines in the future would require knowledge of the variation of ocean floor depths (or ocean topography) and this and other information from the oceans was key for discovering plate tectonics. Transcript: The Ocean Floor – Part 1 Slide 1: Observations of the Ocean Floor What would the ocean floor look like without water? Prior to World War II people knew very little about the ocean floor. Military needs during World War II gave a boost to sea floor exploration. With submarines and warfare, suddenly countries became very interested in mapping the ocean floor. Prior to World War II, researchers used plumb lines (that is a weight at the end of cable) to determine the ocean depth at one location (a pinpoint), an incredibly tedious task, but multibeam sonar technology greatly enhanced ocean floor observations. Slide 2: Sonar Mapping (Echo Sounding) of the Ocean Floor Sound Navigation and Ranging or Sonar mapping is also known as echo sounding, which is the same method that bats use to navigate. Sound waves from a ship bounce off the ocean floor and return as an echo. The time it takes for a sound wave to echo back is related to depth, the distance to the ocean floor. By cruising back and forth with multibeam sonar as shown here, physical features of the ocean floor can be mapped in detail. Today, these types of maps are made even more quickly using satellite data. Slide 3: Harry Hess Dr. Harry Hess is another rock star that I want to introduce to you. He was a Princeton University Geology Professor and during World War II he served as Captain of the Cape Johnson in the south Pacific. He was keen on using the new Sonar technology to study depth variation of the ocean floor so he would do sonar mapping as he moved from battle to battle. Amazing. He also made some key observations of the ocean floor sediment and of volcanic island chains that I will come back to later in this presentation. Slide 4: Key Discoveries from the Oceans This is one of the early sonar images of a segment of a ridge in the Pacific Ocean. Warm red colors represent shallow water depth or ocean ridges and cooler blues depict deeper depths or lower ocean floor elevations. These types of images were the first to show that the ocean floor was not merely flat like a shallow bowl or saucer – sonar revealed a rich underwater world of dramatic physical features. In addition to underwater mountains like the Pacific ridge, other key discoveries included great fracture zones (which are some of the largest faults on Earth), deep sea trenches (which I already mentioned earlier with regard to the Pacific Ring of Fire), and seamounts. Slide 5: The Mid-Atlantic Ridge Sonar studies in the Atlantic Ocean revealed a 60,000 kilometer (or 40,000 mile) long mountain range which splits nearly the entire Atlantic Ocean from north to south as shown in this map illustration. These ocean floor highs are not tremendously tall mountains; they rise about 2 kilometers above the deeper flat plains of the ocean floor, which is why we call them ridges, and they are roughly symmetrical. Clearly these ridges are not straight features but curve, maintaining its presence in the middle of the Atlantic Ocean. Slide 6: Mid-Atlantic Ridge in Cross-Section Another way to visualize the ocean floor topography is with a cross-section, or a vertical slice through the ocean. The bottom image is a cross-section slice that’s oriented about east-west and extends from the U.S. east coast (near Florida) on the left (or the west) through the Atlantic ocean and over to Africa on the right. This cross-section shows the peak of the ridge in the middle of the Atlantic Ocean with both sides of the ridge gradually dropping off in elevation to the deepest and flat parts of the ocean. These vast flat regions of oceans are called abyssal plains, or I like to say abysmal plains, and represent some of the flattest regions found anywhere on Earth. Near the continental margins of the Atlantic, the edges of the oceans start to rise up and form a slope that extends to the continental shelf, or the shallow drowned margin of continents. Slide 7: Ocean Ridges form the Longest Single Mountain Chain on Earth The Pacific Ocean also has a ridge, although it’s not in the middle of the Pacific but is located closer to the Americas in the east Pacific. This spectacular world ocean floor map shows how the ridges wind their way between the continents much like the seam on a baseball forming the longest single mountain chain on Earth. Note also how the ridges appear segmented or chopped up by closely spaced fracture zones. Slide 8: Deep Sea Trenches Bordering Volcanic Islands The other physical feature about the Pacific Ocean that’s different from the Atlantic Ocean is that it’s bordered by deep ocean trenches. These Google images show a deep trench adjacent to the Aleutian volcanic islands in the north Pacific and the Japanese volcanic islands in the west Pacific. These narrow trenches at the edge of the Pacific are 8 to 12 kilometers deep troughs that occur adjacent to arcs of volcanic islands or directly adjacent to continents like South America. Slide 9: Oceanic Fracture Zones Let’s look a little more closely at those fracture zones that segment the ocean ridges. The map on the left is an image of the south Pacific ridge that is cut by two very long and closely spaced fractures that are about perpendicular to the ridge segments. Note how the ridge seems to be offset or displaced by each fracture. The right ridge shows an oblique view created by multibeam sonar showing the offset segments of ocean ridges (in orange) between two fracture zones, which consist of broken up fractured oceanic crust. The blue regions on both those images represent the flatter deeper parts of the ocean floor, located away from the ridges. Slide 10: Ocean Ridge, Great Fracture Zones, and Seamounts In addition to ocean ridges and great fracture zones (labeled here as transform faults), Sonar mapping revealed isolated lines or chains of underwater extinct volcanoes such as these shown in the upper right of this beautiful seafloor image. Lines of active volcanic islands in the middle of oceans have been known for about for a long time – such as the Hawaiian Islands in the middle of the Pacific. But sonar revealed lines of extinct underwater volcanoes called seamounts. Slide 11: Atolls of the Maldives Atolls are circular reefs like the one shown here in the upper left that sometimes occur next to each other in a straight line. In the 1800’s Charles Darwin, on his famous Voyage of the Beagle, explained that atolls start out as reefs that surround or fringe an active volcano. When the volcano becomes extinct, subsides, and is eventually submerged, the marine organisms continue to grow and build upward, first forming a barrier reef around the sinking volcano and then forming an atoll when the volcano is completely submerged. Slide 12: Harry Hess Recognized that Volcanic Islands Moved As Captain Harry Hess was moving around the south Pacific during World War II, he recognized that volcanic islands moved horizontally as they progressively sank and evolved into atolls. He noted that submerged extinct volcanoes often had flat tops from being eroded when they were islands. Flat topped submerged volcanos are now known as guyots (g-u-y-o-t-s). Perhaps most importantly, Captain Hess noticed that seamount chains always moved away from ocean ridges. As a prestigious Geology professor, Hess was well aware of the new magnetic inclination data that resurrected Wegener’s continental drift. But now he correctly deduced, in an Oh-Ah moment, that oceanic crust was also slowly moving along the earth’s surface and that everything was moving away from the oceanic ridges. Slide 13: Thin Ocean Sediments and High Heat Flow I want to return to this close-up image of mid-ocean ridge to mention two other observations of the ocean floor: 1) that it’s covered by a thin layer of ocean sediments and 2) that lots of heat is rising from below the ridges. Ocean sediment is formed by clay and shell-like material from organisms constantly raining down and settling on the ocean floor. Since the ocean sediment was quite thin, Hess realized that the oceans must be fairly young. If the oceans were very old, then there would be more time for sediment to settle and accumulate on the ocean floor and the sediment should be very thick. He also observed that the thin sedimentary layer on the ocean floor was a little thicker moving away from the ridges, which suggested to him that the ridges were younger than the abyssal plain regions which had been around long enough to collect more sediment. In fact the abyssal plain regions are flat (as seen in the right side of this image) because the sediment there blankets the oceanic floor there and smooths out the rough topography that makes up the ridges. The high heat flowing out of the ridge area suggested to Hess that the ridge area was underlain by liquid magma which was erupting volcanic basalt onto the ocean floor at the ridges. Slide 14: New Maps of Earthquakes Finally, new maps of earthquake locations produced in the 1950s revealed that oceanic earthquakes were localized in distinct narrow belts along the ocean ridges and at deep sea trenches. This 1954 map nicely shows how the Atlantic Ocean earthquakes occur north-south, right along the curved Mid-Atlantic Ridge on the left side of this map. Slide 15: Sea-Floor Spreading and Subduction In 1960, Professor Harry Hess wrote an elegant explanation for the many new observations about the ocean floor that he dubbed an “Essay in Geopoetry.” He proposed that molten magma formed beneath ocean ridges erupted and solidified to form new oceanic crust. Earthquakes along the ridges suggested that the new crust was cracking and splitting at the ridges and was moving away from the ridge on both sides. The process Hess described is now known as Sea-floor Spreading, the gradual widening of an ocean basin as new oceanic crust forms at and moves away from an ocean ridge. Since new ocean floor was being created at the ridges, Hess realized that it had to be consumed somewhere else, and he proposed that it was being consumed or sinking back into the earth at the trenches, a process now known as subduction. Slide 16: Hess’s Hypothesis: Continents “Drift” Hess’s elegant Essay in Geopoetry is famous for instantly providing the long-sought explanation for how continental drift occurs. The idea is that continents are attached to the oceans and passively pushed across the Earth’s surface by the formation and destruction of oceanic crust. They don’t plow through the ocean floor but instead ride along with the ocean floor as it does all the work of spreading. Transcript: The Ocean Floor – Part 2 Slide 1: Observations of the Ocean Floor Welcome back. Ocean studies were critical to the discovery of plate tectonics. Recognition of major physical features of the ocean floor, including ocean ridges, great fracture zones, deep sea trenches, and seamounts led to Dr. Hess’s idea of sea-floor spreading as the mechanism for continental drift. Another key discovery from the oceans related to magnetic reversals provided a key test for the concept of sea-floor spreading and cemented the realization that the continents and the ocean floor together make up the mobile plates. Slide 2: Passive Continental Margin This block diagram nicely summarizes the physical features making up the ocean floor. The ridges, great fracture zones, abysmal plains, and seamounts constitute the ocean floor interior. Note however that there are two different types of submerged continental margins where the oceans and continents meet. The continental margin on the left side consists of a broad shallow shelf that steepens to form a slope that extends down to where the ocean floor starts to rise up from the abysmal plain region. Continental shelf-slope-and-rise regions are sometimes down cut by narrow and deep valleys called submarine canyons. Broad continental shelves form along passive continental margins that are not plate boundaries and thus lack earthquakes and volcanoes. An example is the east coast of North America. In contrast, active continental margins have a narrow drowned shelf that slopes steeply into a trench which typically borders active volcanic islands or continents with active volcanoes. Active continental margins do constitute a plate boundary with lots of earthquakes and examples include the continental margins which surround the Pacific Ocean and make up the Pacific Ring of Fire. Slide 3: 2 nd Key Discovery from Oceans The second key discovery from the ocean studies was the documentation of magnetic stripping along the ocean floors like that shown here off the Pacific Northwest coast. The bold dashed lines represent oceanic ridges and the colored zones or stripes represent patterns or stripes of alternating magnetic strength preserved on the ocean floor. These stripes are always oriented parallel to ocean ridge segments and the pattern of stripes are the same on both sides of the ridges. This symmetrical magnetic stripping pattern exists in all oceanic crust and was documented when magnetometers towed behind research vessels recorded changes in magnetic strength across the ocean floor. Slide 4: Strength of Magnetic Field Varies away from Mid-Ocean Ridges The top diagram shows how magnetic strength swings up and down from high to low away from ocean ridges. Note how the broad high or positive magnetism in the middle of the diagram coincides with mid-ocean ridge and how the positive and negative (or up and down) swings are the same on either side going away from the ridge. If we color the positive high strength stripes dark and the negative (or low) strengths light, the magnetic pattern appears like stripes on a zebra oriented parallel to ridges. These magnetic zebra stripe patterns, which are recorded on all ocean floor, were interesting but very mysterious patterns until the paleomagicians recognized that the magnetic field reverses itself or flips. Slide 5: Volcanic Sequence Remember that volcanic sequences preserved on continents record normal and reverse magnetic polarity and indicate reversals or flips of the Earth’s magnetic field in the past. The volcanic sequence on the left represents a vertical stack of volcanic rocks with the oldest 2.3 million year flow on the bottom and the youngest 300,000 year old flow on the top. N stands for normal polarity like we have today and R stands for reversed polarity. On continents, volcanic rocks get younger vertically from bottom to top (as younger flows erupt atop older flows on the land surface). But according to the sea-floor spreading hypothesis, oceanic flows get younger horizontally towards the ridges where new oceanic flows are erupting today. To show this on the right I have tipped the volcanic continental sequence sideways with the oldest flow on the left side and youngest on the right side. Looking down on this sideways sequence hopefully you can now see how the ocean floor would appear with zebra stripes recording past magnetic reversals. Slide 6: Formation of Magnetic Stripping on Ocean Floor This diagram shows the creation of oceanic crust by sea floor spreading over time. New oceanic basalt forming continuously at the ridges records or freezes in the polarity of the Earth’s field at the time of crystallization of the melt. At is moves away from the ridge on both sides, the oceanic crust and sea floor becomes increasingly older and the magnetic stripes preserve the time of prior periods of normal and reverse polarity in the geologic past. This explanation for the magnetic stripping recorded in the ocean regions was an extraordinary test of and perfectly consistent with Hess’s sea-floor spreading idea. Slide 7: World Ocean Floor Topography Map Let’s return to this amazing image of the world’s ocean floor topography, which represents one of the most important datasets collected about Earth. We now know that the underwater mountains or ridges are where new oceanic real estate is forming and you can perhaps imagine the pattern of magnetic zebra stripes which would be parallel to these ridges. The presence of past magnetic reversals permanently recorded in the sea floor meant that they could be used to date the entire sea floor without the need to sample any oceanic rocks. The magnetic reversal timescale obtained from the easily accessible continental volcanic rocks could be correlated with the record of ocean floor magnetic reversals. Slide 8: Age of the Sea Floor This beautiful color image of sea floor age was created with using magnetic ocean floor reversals. It reveals how the youngest sea floor in red occurs at the ridges with oceanic crust getting progressively older away from the ridges in both directions. The coolest color blue represents the oldest oceanic crust preserved on Earth today which is only 180-200 million years old, the middle of the Mesozoic Era when dinosaurs roamed the Earth. You can see that the oldest (deep blue colored) oceanic crust occurs in the western Pacific Ocean region and on the continental margins of the Atlantic Ocean. Note how this image of the age of the sea floor mimics the world ocean floor topography map. Their remarkable similarity tells us that the ocean mountains stand high because they are the warmest parts of the ocean floor (recall that lots of heat emanates from the ridge regions) – and therefore they are less dense than the off-ridge portions of the ocean floor. Essentially they are thermal mountains elevated by their lower density compared to the rest of the older, cooler parts of oceanic crust. Density is

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