Plate Tectonics - PC\|MACimages.pcmac.org/.../11A__R-Plate_Tectonics_Reading.pdfPlate Tectonics “Plate tectonics” is the name given to our modern theory of how the Earth works

$Page 1: Plate Tectonics - PC\|MACimages.pcmac.org/.../11A__R-Plate_Tectonics_Reading.pdfPlate Tectonics “Plate tectonics” is the name given to our modern theory of how the Earth works$
Oceanography 10, T. James Noyes, El Camino College 11A-1


Plate Tectonics “Plate tectonics” is the name given to our modern theory of how the Earth works. It helps us understand how and why earthquakes, volcanoes, mountains, trenches, islands, oil deposits, and much more occur where they do. The basic idea of plate tectonics is that the Earth is covered by a cool, thin outer layer called the lithosphere which floats on a warmer, denser layer which can slowly flow called the mantle. The lithosphere is broken into pieces called plates. The semi-solid rock down in the mantle moves due to the heat of the Earth’s core and pushes the plates1 away from and into one another, resulting in the earthquakes, volcanoes, mountains, trenches, and so on.

There are two kinds of lithosphere, lower-density continental lithosphere with a granite “top” and higher-density oceanic lithosphere with a basalt “top.”

Granite. Lower-density rock Basalt. Higher-density rock making up the continents. making up the ocean floor.

1 The density of one side of a plate may be larger than the density of the other side of the plate. This may cause a plate to move.

The lithosphere is made up of two parts. The upper part is made of lower-density, lighter-colored granite or higher-density, darker-colored basalt. The lower part is made of solidified mantle rock. The bedrock of the continents is made of granite while the ocean floor is made of basalt, so continental lithosphere has a lower density than oceanic lithosphere.



The Observations and the Basic Ideas of Plate Tectonics A vast amount of evidence from both the continents and the ocean floor supports the idea that the continents move and oceans grow and shrink. If you examine the coasts of South America and Africa, you can see that they match rather nicely together; the match is even better between their continental shelves. The east coast of North America and northern Africa also match, as do other parts of the world. Not only do the shapes match, but fossils, mineral deposits (e.g., coal, diamonds), and mountain ranges2 match along the edges of the continents as well. The matching of these features suggests that the continents were once connected to one another and therefore they must have moved to reach their present positions. If the continents were not connected in the past, there would be no reason for the features to match: it would simply be a coincidence. This is possible – unlikely things do happen (e.g., someone wins the lottery) – but when determining what we believe, we typically favor ideas that are more probable, not less likely. In other words, arguing that these features of the continents are likely to match by chance would be something like saying that if you choose 7 pieces out of jigsaw puzzle box, they are likely to fit together AND produce a nice picture. This is possible, but certainly not something that most of us would expect to happen.

The “Fit” of the Continents. Alfred Wegner.

Just a few examples of fossils that “match” on different continents. Alfred Wegner and the USGS.

The fossils, in particular, are interesting. Currently, the plants and animals on different continents are different from one another. They have evolved to adapt to the present environment (e.g., climate, ecosystem). However, land animal and plant fossils are the same on some continents (e.g., South America and Africa) until a particular point in time, when they begin to start showing evolution in different directions. This suggests that their environment began to change and that they were no long able to interbreed, presumably due the separation of the continents. We also find remains of tropical plants and animal like coral reefs near the Poles, and signs of erosion by glaciers3 in the tropics. These observations suggest that continents that are now close to the Equator were once at the Poles and vice versa4.

2 and more! 3 Huge, slowly-moving sheets of ice. 4 These observations might be explained by climate change since there were extreme ice ages and warm periods except for the fact that both were true at the same time.



Observations from the ocean floor are crucial to understanding how and why the continents move. As we discussed way back in topic 1A, most ocean trenches lie close to land or islands. In addition, earthquakes (often starting deep inside the Earth) and volcanoes occur near trenches. If the trench occurs next to a continent, the continent has volcanic mountains near the coast (like Mt. St. Helens in Washington, Mt. Fuji in Japan, the Andes in South America, Popocatepetl near Mexico City, Krakatoa near the Indonesian islands of Java and Sumatra, etc.), and if a trench occurs next to an island chain, then the islands are giant volcanoes (e.g., Mariana Islands, Aleutian Islands in Alaska). The mid-ocean ridge is also a site of volcanic activity and earthquakes. In addition, several very different lines of evidence (sediment thickness, radiometric dating, seafloor magnetism) all show that the ocean floor is very young5 close to the mid-ocean ridge and gets older and older the farther it is from the mid-ocean ridge.

Earthquake Locations. Notice how earthquakes tend to occur near the locations of the mid-ocean ridge and trenches (shown in the pictures below). NASA.

The Mid-Ocean Ridge is the dark, red line in the picture on the left. It indicates very young ocean floor (recently cooled lava). Yellow and green colors indicate old ocean floor (that cooled from lava long ago). The oldest ocean floor is shown in blue. National Geophysical Data Center, National Oceanic and Atmospheric Administration, Department of Commerce.

Most of the world’s trenches (blue lines) are found around the edge of the Pacific Ocean, and volcanic mountains and islands are next to the trenches, so we call this region the “Ring of Fire.” USGS.

5 A “young” rock is made of material that “recently” became solid rock. In the case of the volcanic rocks of the sea floor, the sea floor is “young” if it cooled from lava into solid rock “recently.” (When geologists use the word “recently,” they could be discussing that last few million of years.)



The theory of plate tectonics provides the following explanation for these features. At the mid-ocean ridges, the plates are separating, pulled apart by the motion of convections cells of semi-solid mantle rock beneath the plates. Lava then comes up to fill in the resulting gap and cools into solid rock when it comes into contact with cold ocean water, becoming new, young ocean floor. The ocean floor cools more as it ages (making it contract) and more and more sediments pile on top of it, so it becomes more and more dense with time. As the ocean floor moves away from the mid-ocean ridge (“sea-floor spreading”), it either pushes a continent (“continental drift”) or runs into another plate, leading to earthquakes. Both plates cannot occupy the same location, so the plate with the higher density dives into the interior of the Earth (We say that it “subducts” = “dives.”), making the sea-floor deeper (i.e., a trench). The diving plate begins to melt, creating hot, low-density magma which works its way back towards the surface, creating volcanoes.

Volcanoes Near Trenches The mantle rock above the diving ocean crust undergoes what is called “partial melting.” The diving ocean crust is heated by the surrounding mantle (friction between the plate and mantle rock is an important factor) and the increased pressure. Water and sediments are also carried down by the plate. As the water and carbon dioxide (from the breakdown of calcium carbonate sediments) mix with mantle material, they lower its melting point and its density, allowing the resulting magma to rise upwards until it hits the non-subducting plate above. The magma begins melting the bottom of the plate and mixing with the plate’s material, which alters the composition of the magma. Wherever a crack in the plate permits, the magma works its way toward the surface of the Earth until it comes out as lava. The magma itself can force its way up: (1) it opens cracks by expanding – remember: heating an object makes it larger – and (2) it melts the surrounding material.



Countless studies over the last century have verified the motion and the boundaries of the plates suggested by plate tectonics. Scientists currently use the GPS (global positioning system) to monitor plate motion, but it is simple enough to do without advanced technology: measure the distance between two objects (houses, trees, fences, etc.) on two different plates, and then do so again after each earthquake. Most plates move at least few centimeters per year (an inch or a bit more per year) and no faster than about 15 centimeters per year (about 6 inches per year).

The Plates of the Earth’s Lithosphere. USGS.

Notice that a plate can contain both continental and oceanic lithosphere. For example, the North American plate includes both North America (continental lithosphere) and the bottom of the western Atlantic Ocean (oceanic lithosphere). Lava coming out of the mid-ocean ridge cools and solidifies onto whatever plate is nearby and becomes part of that plate, whether it is continental or oceanic lithosphere.



Why do the plates move? What drives plate tectonics? The ultimate cause of motion inside the Earth is the heat of the Earth’s core6. The core heats the bottom of the mantle7, lowering its density and causing it to rise. Cooler mantle material slides in to replace it, and the process repeats in giant, slow-motion convection cells. In addition, the end of a subducting (“diving”) plate can be more dense than the mantle, because it has cooled down and has thick, heavy layers of sediments piled on top of it. It dives down owing to its higher density, and can pull the rest of the plate with it. (This is called “slab pull.”) The diving plate also drags the nearby mantle rock with it, helping to push the convection cell in the mantle. (This is called “slab suction.”)

Motion (“Convection Cells”) within the Earth. USGS.

Earthquakes Earthquakes occur when the Earth moves suddenly. The magma of the mantle flows in convection cells which steadily rub against the bottom of the plates, pushing them slowly away from mid-ocean ridges and towards trenches. However, the plates run into one another, so they resist being moved. Instead of moving, the plates are “squeezed” and “twisted” where they collide. Eventually, they cannot deform any more, and the pressure to move builds up until it overcomes the resistance8: then the plates move (“slip”) all at once, and snap back into shape. This is an earthquake. Thus, instead of moving slowly and gently with the convection cells over time, the plates “give” (deform) until they cannot “give” any more, and move all at once.

Most earthquakes occur near the edges of plates, because most of the resistance to motion is exerted at the edges, so this is where most of the deformation (bending, squeezing) takes place and the tension builds up9. Plates can resist moving for several reasons. A plate can rub against the side or bottom of another plate, and the friction between them keeps them from moving. In addition, the rock in front of a moving plate (another plate or the semi-solid asthenosphere) can block it, possibly forcing it to bend.

6 The Earth’s core is warm, because of heat produced during the initial collisions between the rocky asteroids that produced the earth and the decay of dense, radioactive materials. 7 There is a debate within the geophysics community about whether convection cells reach all the way down to the core. Some scientists think that there 2 layers within the mantle, each with its own convections cells. There is some data which support this view, but if this is true, then it is not clear why large deep earthquakes occur: stay tuned! 8 Have you ever tried moving a couch on carpeting by yourself? It does not move at all until you provide enough pressure, but then it moves easily. When you let up the pressure, the friction with the carpet brings it to a stop. 9 There are other mechanisms that can cause earthquakes. For example, as a plate dives into the Earth, some parts get warmer than others, so they get larger and the plate cracks where warm rock meets cold rock. This is why glass trays can break when you take them out of the oven or glass pitchers break when you pour hot water into them: one part cools more quickly than another part.



Rock Magnetism

Evidence from rock magnetism provided some of the crucial evidence that made the majority of scientists switch from earlier theories about the Earth to the modern theory of the plate tectonics in the 1960s. When lava cools into solid rock, magnetite particles (little iron-rich minerals) in the lava orient themselves with the Earth’s magnetic field. (They behave like little compass needles.) Thus, the rock records information about the direction and strength of the Earth’s magnetic field at the time when the rock “formed.” Studies of volcanic lava flows on land showed scientists that the Earth’s magnetic “north pole” changes location with time10. In addition, sometimes the magnetic north pole “flips,” going to South Pole11! By comparing results from volcanoes on different continents, scientists found that the continents must have shifted location12 in agreement with the fossil evidence and other kinds of evidence. The “straw that broke the camel’s back” came from observations of the sea floor. The rocks of the ocean floor are magnetized pointing north and south in long “stripes” that run parallel to the mid-ocean ridge and are a mirror images of one another across the ridge (their magnetism “points” in the same direction and they have the same thickness). This suggests that rocks on either side of the ridge formed at the same time (because they were influenced by the same magnetic field) and then moved outward from the ridge together13. It is hard to think of another logical explanation.

10 Scientists monitor it today. It shifts a couple hundred feet each year. 11 This happens every million years or so. In fact, the Earth’s magnetic field has been weakening for the last few centuries and we are due for a “flip:” We may actually experience a “flip” over the next few centuries. The Earth’s magnetic field helps protect us from the charged particles (“radiation”) of the “solar wind” from the Sun which not only affect background radiation, but disrupt electromagnetic networks (e.g., radio, cell phones, even electric power grids), so this could cause some surmountable – but expensive – problems for future generations. 12 This was the only way to get their data from different continents to make sense: either there were multiple magnetic north poles or the continents were moving. 13 In addition, the thickness of the “stripes” on the ocean floor is proportional to the time between the “flips” of the Earth’s magnetic field determined from volcanoes on land. In other words, the more time that has passes, the thicker the stripe is, just what you would expect if rock is slowly moving away from the mid-ocean ridge and more more rock is being added at the mid-ocean ridge.

USGS



How do we know what the interior of the Earth is like?

Like everyone else14, scientists believe that the interior of the Earth is hot, because lava comes out of volcanoes. We cannot drill all the way through the lithosphere to reach the mantle (the lithosphere is about 60 miles thick!), so we must use other methods to learn about the inside of the Earth. Every earthquake that occurs ripples outward, giving us clues about the Earth’s interior. By measuring how long it takes the earthquake to reach each monitoring station of our worldwide network15, scientists have built up a “picture” of the interior of the Earth using the same principles as those of “ultrasound” (e.g., used to visualize a baby in the womb16). Earthquake waves travel at different speeds depending upon the kind of rock and the rock’s temperature and pressure, something that can be measured in laboratory experiments. By combining travel times from many different earthquakes and earthquake speeds for different rocks, scientists can work out the thickness of the layers of the Earth, their composition, temperature, pressure, and more. In addition to providing information about the Earth’s crust, lithosphere, and mantle, these results suggest that the Earth’s iron core has two parts, a solid inner core and a fluid outer core. The motion of the fluid part of the core is presumably responsible for the Earth’s magnetic field, since iron is a magnetic substance. It makes sense that the core is iron, since iron is the densest substance made by stars in large quantities before they “die” in a supernova. If the early Earth formed from more and more asteroids being pulled together by gravity, then the collisions would have produced vast amounts of heat, making the early Earth a molten ball of rock. The higher-density elements like iron would have tended to sink towards the center, while the lower-density substances at the surface would have cooled and become solid by radiating their heat into space. The cool, outer later then served as an insulator, trapping the heat inside except where it leaks out, mainly at places like volcanoes. High density radioactive materials would have sunk towards the center, and their decay continues to warm the Earth’s core to this day.

14 For example, what is hell suppose to be like? e.g., “A snowball’s chance in hell.” 15 The network was built to monitor nuclear test-ban treaties. Nuclear explosions set off vibrations that can be detected by the network, so the network is used to make sure that nations live up to their agreements. (In other words, the network is used to detect nuclear “cheaters.”). Nuclear explosions are different from natural earthquakes, so the network can easily tell them apart. Nuclear explosions create very small earthquakes, so the network has no difficulty detecting and measuring even small earthquakes. 16 I believe that the technique was developed by earth scientists and borrowed by the medical community.

USGS

Ultrasound. Courtesy of Sam Pullara, (Creative Commons 2.0 Generic)



CASE STUDIES IN PLATE TECTONICS We will now discuss the different possible situations that can occur along the edges of plates, and look at specific places in the world where these events are happening.

USGS

Divergent Boundaries Divergent boundaries are places where plates are moving apart or “diverging.” There are two kinds of divergent boundaries: where oceanic lithosphere is separating and where continental lithosphere is separating. Oceanic lithosphere is moving apart at the mid-ocean ridge. The motion of magma beneath the plates rubs against the bottom of the plates, pulling them apart (this is called “rifting”), and then magma rises up in between the plates to fill the gap. This magma (lava) meets the cool ocean water and cools into new, solid lithosphere on each side of the ridge. Initially, the crust at the ridge has a low density, so it floats higher than the neighboring lithosphere. The crust ages as it moves away from the ridge, and its density increases (because it cools), so it sinks down (a bit) into the mantle. The motion of magma deep in the earth can also begin to pull continental lithosphere apart. In this case, a continent can be split apart. The magma which rises up to fill in the resulting gap, though, is just like magma which rises up at the mid-ocean ridge, so when it cools, it forms new ocean floor (oceanic lithosphere), not continental lithosphere. As the continents get farther and farther apart, a new ocean grows in between them. This is happening right now at places where the mid-ocean ridge “runs into” land like Gulf of California (between Baja California and mainland Mexico). There are some places in the world where a continent is just beginning to split (e.g., the East African Rift Valley in Africa).

At the top of the mid-ocean ridge there is a “rift valley” where the plates are pulling apart. It is not very deep though, not nearly as deep as a true ocean trench.



The motion of the semi-solid mantle can pull continental lithosphere apart. Magma rises up at the mid-ocean ridge, cools, and becomes new ocean lithosphere. This is happening now in the Gulf of California.

Growing and Shrinking Plates Plates always “move away” from the mid-ocean ridge. However, some plates have the mid-ocean ridge on both sides (e.g., the African plate). How can the plate be moving in opposite directions on opposite sides? How can the plate be moving both east and west? The answer is that the plate is growing (getting bigger); new material is being added along both boundaries. It is not entirely clear whether it is the plate that is moving or the location of the mid-ocean ridges relative to the plate, or both. The red arrows really show the direction from newer rock to older rock. They cannot show the “absolute” motion of the plate relative to some fixed, permanent reference point because the entire surface of the Earth is in motion, including the mid-ocean ridges and trenches. Similarly, plates always “move towards” a trench. At the trench, one plate is subducting (diving down into the Earth) and melting. Some plates are completely surrounded by trenches (e.g., Filipino plate), so they are slowly being pushed down into the earth. They will shrink and shrink until they are completely destroyed.

USGS



Convergent Boundaries Convergent boundaries are places where plates are coming together (“colliding”) or “converging.” There are three kinds of convergent boundaries: where oceanic lithosphere meets continental lithosphere, where oceanic lithosphere meets oceanic lithosphere, and where continental lithosphere meets continental lithosphere. The upper part (“crust”) of continental lithosphere is made of granite, whereas the upper part of oceanic lithosphere is made of basalt, so continental lithosphere has a lower density than oceanic lithosphere. When they meet, the higher-density oceanic lithosphere is forced to dive down into the Earth (“subduct”). To dive, the oceanic plate must “bend” downwards, so this makes the ocean floor deeper where the two plates meet: a trench. As the oceanic plate dives down into the Earth, it melts, creating low-density magma which rises up beneath the other plate. It melts its way through the continental lithosphere. More and more lava comes up, cools, and piles up, forming volcanic mountains at the edge of the continent. This is happening along the coast of South America, where it created the Andes Mountains. It also is happening along the coast of much of Central America and the Pacific Northwest of the United States, where it produced the Cascades mountain range and the famous volcano Mt. St. Helens. Both shallow and deep earthquakes are common along this kind of plate boundary, because the plates grind against one another near the surface and down deep, stopping their motion. Eventually the pressure to move overcomes the friction between the plates and the diving plate slips forward. In other words, an earthquake occurs. The continental lithosphere is compressed (squeezed) by the oceanic lithosphere pushing into it; it “snaps back” into shape like a bent pencil will become straight once you release it.

Mt. St. Helens, 1980. USGS

Above: Cascades Mountains, in Washington, Oregon, and California. USGS.

Left: Andes Mountains (brown & red) and nearby trench (dark blue) National Geophysical Data Center, National Oceanic and Atmospheric Administration, Department of Commerce.

USGS



When two pieces of oceanic lithosphere meet, the plate with the higher density subducts (dives down into the Earth). However, in this case, both plates are made of basalt, so neither is more dense than the other simply by virtue of their composition. Instead, it is the older plate that subducts. The older plate has cooled down more, because it has been a longer time since it formed along the mid-ocean ridge. In addition, there has been more time for sediments to pile on top of it. Both the cooling of the plate (reducing its size) and the weight of the sediments make the older plate more dense than the younger plate. It dives down into the Earth, making the ocean floor deeper (a trench), and partially melts, producing magma that rises, melts its way through the other plate, and bursts out at the surface. The molten material cools into solid rock on the surface and piles up higher and higher as more and more magma comes up. If these volcanoes grow high enough, they produce a chain of islands along the trench on the other plate. This is happening at the Mariana Trench and Mariana Islands, and along the south side of Alaska at the Aleutian Islands.

Above: Mt. Cleveland in the Aleutians, NASA.

Left: Many of the Aleutian Islands of Alaska are active volcanoes, USGS.

Once oceanic lithosphere gets old enough, it must start diving into the Earth: it is simply too dense to float at the surface. If the oceanic lithosphere is pushing continental lithosphere, the plate can fracture along the boundary. A trench forms between the continental lithosphere and oceanic lithosphere, splitting the plate.

USGS

Trench



The upper layer (“crust”) of continental lithosphere is made of low-density granite, so continental lithosphere is never dense enough to sink down into the mantle. Thus, when two pieces of continental lithosphere collide, the continental crust17 does not dive down deep into the Earth. Instead, the rock is squeezed upwards (and downwards), building tall mountains. Each time the plates slip forward towards one another, the resulting earthquake lifts the mountains higher and higher, and pushes down some continental crust a bit lower18. For example, the subcontinent of India is being pushed northeast into Asia, producing the Himalayas, the mountain range with the tallest mountain in the world, Mt. Everest.

USGS Himalayas, NASA

USGS Himalayas, NASA

17 The part of the lithosphere beneath the continental crust can still dive down into the Earth. This can actually cause neighboring continental and oceanic lithosphere to break into separate plates. The oceanic lithosphere will then begin subducting beneath the continental lithosphere, creating a trench along the coast. 18 Some continental crust goes down (it has to go somewhere), but it does not sink down into the mantle. In other words, it does not sink down very far.

Himalayas



Transform Boundaries The mid-ocean ridge is not one, continuous ridge, but instead is broken into segments or “pieces.” There are “jumps” (gaps) between the segments, and these jumps or gaps are called “transform faults.” A transform fault is a place where two plates slide next to one another in opposite directions horizontally. The lithosphere on either side of the fault moves away from segment of the mid-ocean ridge where it was “born.” The plates on either side of the fault rub against one another, so friction between them keeps them from moving until the pressure becomes too much and they “slip” (an earthquake). We live near one large transform fault called the “San Andreas Fault” that runs between a segment of the mid-ocean ridge in the Gulf of California and a segment off the coast of Northern California.

San Andreas Fault

San Andreas Fault, USGS

Mid-OceanRidge

Mid-OceanRidge

Transform Fault

USGS



Hotspots Molten rock rises at certain, special spots beneath the lithosphere, not just at the mid-ocean ridge19. The magma heats the bottom of the lithosphere; the lithosphere expands more in some places than others, which opens up cracks through which magma flows to the surface of the lithosphere. Lava breaks through again and again, slowly building an underwater volcano20. The volcano is a “seamount,” an underwater mountain, until it reaches the surface and becomes an “island.” The volcano grows on the lithosphere of a plate. Since plates move, the island is slowly carried off the hotspot by the motion of the plate21, causing the volcano to become extinct (it loses its source of lava). As time passes, the island (and the lithosphere it rides on) cool down more and more, causing them to contract (shrink). This makes the island smaller, so it sinks, and this can carry it beneath the waves, making it into a seamount. In addition, as the lithosphere and volcano cool, they become more dense, so they sink down a bit into the mantle. Oceanographers often find “flat-topped” seamounts called tablemounts or “guyots” (gee-ohs). They were once islands at the surface and had their top eroded away by waves before sinking deeper into the ocean. Corals are able to survive when their island sinks by growing straight upwards towards the surface. They form coral atolls, islands whose tops are made of coral reef, not volcanic lava.

A volcano begins to grow on the “hotspot,” but is carried off the hotspot by the plate upon which the volcano grows. Without lava, the volcano cools and shrinks, but a new volcano grows on the hotspot.

19 Hotspots can exist near the mid-ocean ridge (e.g., Iceland), and cause even more volcanism than normal. 20 Hotspots can also be located under a continent. For example, the geysers of Yellowstone National Park are caused by the heat of a “hotspot.” These hotspots can also cause volcanoes. 21 Hotspots can and do move as well, but observations suggest they move slower than the plates move above them, so it is primarily the motion of the plates that produce hotspot island chains.

Coral Reef Development. Coral grows upwards as the island sinks. USGS.



After one volcano leaves the hotspot, another volcano grows on the hotspot. This volcano also leaves, and another volcano grows. This happens again and again, creating an “island-seamount chain.” The classic example is the Hawaiian Islands which are one end of the much larger Hawaii Islands-Emperor Seamount Chain. Currently the southeast corner of the “big” island22 is above hotspot; this is where you can see active volcanoes at “Hawaiian Volcanoes National Park.” The New England Seamounts along the “Great Meteor hot-spot track” near the east coast of the United States also formed in this manner.

Hawaiian Islands. The red dot shows the location of active volcanoes. NASA.

Part of the Hawaiian Island-Emperor Seamount Chain, USGS.

Steam Vent, HVNP. T. James Noyes Steam emerging from Iki Crater, HVNP. T. James Noyes

Recent research has shown that there is not one big magma chamber directly under Hawaii. Its head may have cooled and been dragged away by the plate, or there may be lots of little “fingers” of magma instead of one big chamber. If this is the case, lookout Maui, because this would make it harder to predict where volcanoes will occur! Stay tuned: there are new things to be discovered!

22 The big island of Hawaii is actually 2 smaller islands linked together by the lava that flowed from the volcanoes.

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Plate Tectonics - PC\|MACimages.pcmac.org/.../11A__R-Plate_Tectonics_Reading.pdfPlate Tectonics “Plate tectonics” is the name given to our modern theory of how the Earth works