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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 usunderstand how and why earthquakes, volcanoes, mountains, trenches, islands, oil deposits, andmuch more occur where they do. The basic idea ofplate 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 “flow” called the“mantle.” The “lithosphere” is broken into piecescalled “plates.” The semi-liquid rock down in themantle moves due to the heat of the Earth’s core andpushes 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-densitycontinental lithosphere with a granite “top” and higher-density oceanic lithosphere with a basalt “top.”

Granite. Lower-density rock Basalt. Higher-density rockmaking 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 ismade of solidified mantle rock. Thebedrock of the continents is made ofgranite while the ocean floor is made ofbasalt, so continental lithosphere has alower 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 thecontinents move and oceans grow and shrink as a consequence of plate tectonics. For example,earthquakes show that the normally-solid Earth can move. If you examine the coasts of SouthAmerica and Africa, you can see that they match rather nicely together; the match is even betterbetween their continental shelves. The east coast of North America and northern Africa alsomatch, 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 as well. The fossils, in particular, areinteresting. Currently, the plants and animals on different continents are quite different from oneanother. However, animal and plant fossils are the same on some continents (e.g., SouthAmerica and Africa) until a particular point in time, when they begin to start showing evolutionin different directions. In addition, we find remains of coral reefs in places that are now desertson land and in Antarctica, and well as signs of erosion by glaciers3 in the tropics. These mightbe explained by climate change (there were extreme ice ages and warm periods) except for thefact that both were true at the same time. Thus, all the evidence above suggests that thecontinents were once together (next to one another), and slowly over time – earthquake byearthquake – moved (“drifted”) apart, some moving from the tropics to the Poles and others fromthe Poles towards the tropics. Once the continents split apart, animals and plants began evolvingin their own unique directions depending on their new environmental conditions.

The “Fit” of the Continents.Alfred Wegner.

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

Observations from the ocean floor are crucial to understanding how and why the continentsmove. 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 (likeMt. St. Helens in Washington, Mt. Fuji in Japan, the Andes in South America, Popocatepetl nearMexico City, Krakatoa near the Indonesian islands of Java and Sumatra, etc.), and if a trenchoccurs next to an island chain, then the islands are giant volcanoes (e.g., Mariana Islands,

2 and more!3 Huge, slowly-moving sheets of ice.



Aleutian Islands in Alaska). The mid-ocean ridge is also a site of volcanic activity andearthquakes. In addition, several very different lines of evidence (sediment thickness,radiometric dating, seafloor magnetism) all show that the ocean floor is very young4 close to themid-ocean ridge and gets older and older the farther it is from the mid-ocean ridge.

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

The Mid-Ocean Ridge is thedark, red line in the picture on theleft. It indicates very youngocean floor (recently cooled lava).Yellow and green colors indicateold ocean floor (that cooled fromlava long ago). The oldest oceanfloor is shown in blue. NationalGeophysical Data Center,National Oceanic andAtmospheric Administration,Department of Commerce.

Most of the world’s trenches(blue lines) are found around theedge of the Pacific Ocean, andvolcanic mountains and islandsare next to the trenches, so we callthis region the “Ring of Fire.”USGS.

4 A “young” rock is made of material that “recently” became solid rock. In the case of the volcanic rocksof 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 beneaththe plates. Lava then comes up to fill in the resulting gap and cools into solid rock when itcomes into contact with cold ocean water, becoming new, young ocean floor. The ocean floorcools more as it ages (making it contract) and more and more sediments pile on top of it, so itbecomes more and more dense with time. As the ocean floor moves away from the mid-oceanridge (“sea-floor spreading”), it either pushes a continent (“continental drift”) or runs intoanother plate, leading to earthquakes. Both plates cannot occupy the same location, so the platewith 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 TrenchesThe 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 plateand mantle rock is an important factor) and the increased pressure. Water and sedimentsare also carried down by the plate. As the water and carbon dioxide (from the breakdownof calcium carbonate sediments) mix with mantle material, they lower its melting pointand 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 mixingwith the plate’s material, which alters the composition of the magma. Wherever a crackin the plate permits, the magma works its way toward the surface of the Earth until itcomes out as lava. The magma itself can force its way up: (1) it opens cracks byexpanding – remember: heating an object makes it larger – and (2) it melts thesurrounding material.



Countless studies over the last century have verified the motion and the boundaries of the platessuggested by plate tectonics. Scientists currently use the GPS (global positioning system) tomonitor plate motion, but it is simple enough to do without advanced technology: measure thedistance between two objects (houses, trees, fences, etc…) on two different plates, and then doso again after each earthquake. Most plates move at least few centimeters per year (an inch or abit 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 andsolidifies 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 theEarth is the heat of the Earth’s core5.The core heats the bottom of themantle6, lowering its density andcausing it to rise. Cooler mantlematerial slides in to replace it, and theprocess repeats in giant, slow-motionconvection cells. In addition, the end ofa subducting (“diving”) plate can bemore dense than the mantle, because ithas cooled down and has thick, heavylayers of sediments piled on top of it. Itdives 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 dragsthe 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.

EarthquakesEarthquakes occur when the Earth moves suddenly. The magma of the mantle flows inconvection cells which steadily rub against the bottom of the plates, pushing them slowly awayfrom mid-ocean ridges and towards trenches. However, the plates run into one another, so theyresist being moved. Instead of moving, the plates are “squeezed” and “twisted” where theycollide. Eventually, they cannot deform any more, and the pressure to move builds up until itovercomes the resistance7: 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 cellsover 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 isexerted at the edges, so this is where most of the deformation (bending, squeezing) takes placeand the tension builds up8. Plates can resist moving for several reasons. A plate can rub againstthe side or bottom of another plate, and the friction between them keeps them from moving. Inaddition, the rock in front of a moving plate (another plate or the semi-solid asthenosphere) canblock it, possibly forcing it to bend.

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



Rock Magnetism

Evidence from rock magnetism providedsome of the crucial evidence that made themajority of scientists switch from earliertheories about the Earth to the moderntheory of the plate tectonics in the 1960s.When lava cools into solid rock, magnetiteparticles (little iron-rich minerals) in thelava orient themselves with the Earth’smagnetic field. (They behave like littlecompass needles.) Thus, the rock recordsinformation about the direction andstrength of the Earth’s magnetic field at thetime when the rock “formed.” Studies of volcanic lava flows on land showed scientists that theEarth’s magnetic “north pole” changes location with time9. In addition, sometimes themagnetic north pole “flips,” going to South Pole10! By comparing results from volcanoes ondifferent continents, scientists found that the continents must have shifted location11 inagreement with the fossil (and other kinds) of evidence. The “straw that broke the camel’sback” came from observations of the sea floor. The rocks of the ocean floor are magnetizedpointing north and south in long “stripes” that run parallel to the mid-ocean ridge and are amirror images of one another across the ridge (their magnetism “points” in the same directionand they have the same thickness). This suggests that rocks on either side of the ridge formedat the same time (because they were influenced by the same magnetic field) and then movedoutward from the ridge together12. It is hard to think of another logical explanation.

9 Scientists monitor it today. It shifts a couple hundred feet each year.10 This happens every million years or so. In fact, the Earth’s magnetic field has been weakening for the last fewcenturies and we are due for a “flip:” We may actually experience a “flip” over the next few centuries. The Earth’smagnetic field helps protect us from the charged particles (“radiation”) of the “solar wind” from the Sun which notonly affect background radiation, but disrupt electromagnetic networks (e.g., radio, cell phones, even electric powergrids), so this could cause some surmountable – but expensive – problems for future generations.11 This was the only way to get their data from different continents to make sense: either there were multiplemagnetic north poles or the continents were moving.12 In addition, the thickness of the “stripes” on the ocean floor is proportional to the time between the “flips” of theEarth’s magnetic field determined from volcanoes on land. In other words, the more time that has passes, thethicker the stripe is, just what you would expect if rock is slowly moving away from the mid-ocean ridge and moremore rock is being added at the mid-ocean ridge.

USGS



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

Like everyone else13, scientists believe that the interiorof the Earth is hot, because lava comes out ofvolcanoes. We cannot drill all the way through thelithosphere to reach the mantle (the lithosphere is about60 miles thick!), so we must use other methods to learnabout the inside of the Earth. Every earthquake thatoccurs ripples outward, giving us clues about theEarth’s interior. By measuring how long it takes theearthquake to reach each monitoring station of ourworldwide network14, scientists have built up a “picture”of the interior of the Earth using the same principles asthose of “ultrasound” (e.g., used to visualize a baby in thewomb15). Earthquake waves travel at different speedsdepending upon the kind of rock and the rock’stemperature and pressure, something that can be measuredin laboratory experiments. By combining travel timesfrom many different earthquakes and earthquake speedsfor different rocks, scientists can work out the thickness ofthe layers of the Earth, their composition, temperature,pressure, and more. In addition to providing informationabout the Earth’s crust, lithosphere, and mantle, these results suggest that the Earth’s iron corehas two parts, a solid inner core and a fluid outer core. The motion of the fluid part of the core ispresumably responsible for the Earth’s magnetic field, since iron is a magnetic substance. Itmakes sense that the core is iron, since iron is the densest substance made by stars in largequantities before they “die” in a supernova. If the early Earth formed from more and moreasteroids being pulled together by gravity, then the collisions would have produced vast amountsof heat, making the early Earth a molten ball of rock. The higher-density elements like ironwould have tended to sink towards the center, while the lower-density substances at the surfacewould have cooled and become solid by radiating their heat into space. The cool, outer later thenserved as an insulator, trapping the heat inside except where it leaks out, mainly at places likevolcanoes. High density radioactive materials would have sunk towards the center, and theirdecay continues to warm the Earth’s core to this day.

13 For example, what is hell suppose to be like? e.g., “A snowball’s chance in hell.”14 The network was built to monitor nuclear test-ban treaties. Nuclear explosions set off vibrations that can bedetected by the network, so the network is used to make sure that nations live up to their agreements. (In otherwords, 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 nodifficulty detecting and measuring even small earthquakes.15 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 twokinds of divergent boundaries: where oceanic lithosphere is separating and where continentallithosphere is separating.

Oceanic lithosphere is moving apart at the mid-ocean ridge. The motion of magma beneath theplates rubs against the bottom of the plates, pulling them apart (this is called “rifting”), and thenmagma rises up in between the plates to fill the gap. This magma (lava) meets the cool oceanwater and cools into new, solid lithosphere on each side of theridge. Initially, the crust at the ridge has a low density, so itfloats higher than the neighboring lithosphere. The crust agesas 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 newocean floor (oceanic lithosphere), not continental lithosphere. As the continents get farther andfarther apart, a new ocean grows in between them. This is happening right now at places wherethe mid-ocean ridge “runs into” land like Gulf of California (between Baja California andmainland Mexico). There are some places in the world where a continent is just beginning tosplit (e.g., the East African Rift Valley in Africa).

At the top of the mid-ocean ridgethere is a “rift valley” where theplates are pulling apart. It is notvery deep though, not nearly asdeep as a true ocean trench.



The motion of the semi-solid mantle can pull continental lithosphere apart. Magma rises up at the mid-oceanridge, 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 bothsides (e.g., the African plate). How can the plate bemoving 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 ismoving or the location of the mid-ocean ridges relativeto the plate, or both. The red arrows really show thedirection from newer rock to older rock. They cannot show the “absolute” motion of the platerelative to some fixed, permanent reference point because the entire surface of the Earth is inmotion, including the mid-ocean ridges and trenches.

Similarly, plates always “move towards” a trench. At the trench, one plate is subducting (divingdown 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 shrinkuntil they are completely destroyed.

USGS



Convergent Boundaries

Convergent boundaries are places where plates are coming together(“colliding”) or “converging.” There are three kinds of convergentboundaries: where oceanic lithosphere meets continentallithosphere, where oceanic lithosphere meets oceanic lithosphere,and where continental lithosphere meets continental lithosphere.

The upper part (“crust”) of continental lithosphere is made ofgranite, whereas the upper part of oceanic lithosphere is made ofbasalt, so continental lithosphere has a lower density than oceaniclithosphere. When they meet, the higher-density oceaniclithosphere is forced to dive down into the Earth (“subduct”).To dive, the oceanic plate must “bend”downwards, so this makes the ocean floor deeperwhere the two plates meet: a trench. As theoceanic plate dives down into the Earth, it melts,creating low-density magma which rises upbeneath the other plate. It melts its way throughthe continental lithosphere. More and more lavacomes up, cools, and piles up, forming volcanicmountains at the edge of the continent. This ishappening along the coast of South America,where it created the Andes Mountains.It also is happening along the coast ofmuch of Central America and thePacific Northwest of the UnitedStates, where it produced the Cascadesmountain range and the famousvolcano Mt. St. Helens.

Both shallow and deep earthquakesare common along this kind of plateboundary, because the plates grindagainst one another near the surfaceand down deep, stopping their motion.Eventually the pressure to moveovercomes the friction between theplates and the diving plate slipsforward. In other words, anearthquake occurs. The continentallithosphere is compressed (squeezed)by the oceanic lithosphere pushinginto it; it “snaps back” into shape likea bent pencil will become straightonce you release it.

Mt. St. Helens, 1980. USGS

Above: Cascades Mountains, inWashington, Oregon, andCalifornia. USGS.

Left: Andes Mountains (brown &red) and nearby trench (dark blue)National Geophysical DataCenter, National Oceanic andAtmospheric 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 thiscase, both plates are made of basalt, so neitheris more dense than the other simply by virtueof their composition. Instead, it is the olderplate that subducts. The older plate has cooleddown more, because it has been a longer timesince it formed along the mid-ocean ridge. Inaddition, there has been more time forsediments to pile on top of it. Both the cooling of the plate (reducing its size) and the weight ofthe sediments make the older plate more dense than the younger plate. It dives down into theEarth, 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 coolsinto 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 theother plate. This is happening at the Mariana Trench and Mariana Islands, and along the southside of Alaska at the Aleutian Islands.

Above: Mt. Cleveland in the Aleutians, NASA.

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

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

USGS

Trench



The upper layer (“crust”) of continental lithosphere is made of low-density granite, socontinental lithosphere is never dense enough to sink down into the mantle. Thus, when twopieces of continental lithosphere collide, the continental crust16 does not dive down deep into theEarth. Instead, the rock is squeezed upwards (and downwards), building tall mountains. Eachtime the plates slip forward towards one another, the resulting earthquake lifts the mountainshigher and higher, and pushes down some continental crust a bit lower17. For example, thesubcontinent of India is being pushed northeast into Asia, producing the Himalayas, themountain range with the tallest mountain in the world, Mt. Everest.

USGS Himalayas, NASA

USGS Himalayas, NASA

16 The part of the lithosphere beneath the continental crust can still dive down into the Earth. This can actuallycause neighboring continental and oceanic lithosphere to break into separate plates. The oceanic lithosphere willthen begin subducting beneath the continental lithosphere, creating a trench along the coast.17 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 isbroken into segments or “pieces.”There are “jumps” (gaps) betweenthe segments, and these jumps orgaps are called “transform faults.”A transform fault is a place wheretwo plates slide next to one anotherin opposite directions horizontally.The lithosphere on either side of thefault moves away from segment ofthe mid-ocean ridge where it was“born.” The plates on either side ofthe fault rub against one another, so friction between them keeps them from moving until thepressure becomes too much and they “slip” (an earthquake). We live near one large transformfault called the “San Andreas Fault” that runs between a segment of the mid-ocean ridge in theGulf 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 justat the mid-ocean ridge18. The magma heats the bottom of the lithosphere;the lithosphere expands more in some places than others, which opens upcracks through which magma flows to the surface of the lithosphere. Lavabreaks through again and again, slowly building an underwater volcano19.The volcano is a “seamount,” an underwater mountain, until in reaches thesurface and becomes an “island.”

The volcano grows on the lithosphere of a plate. Since plates move, theisland is slowly carried off the hotspot by the motion of the plate20,causing the volcano to become extinct (it loses its source of lava). Astime passes, the island (and the lithosphere it rides on) cool down moreand more, causing them to contract (shrink). This makes the islandsmaller, so it sinks, and this can carry it beneath the waves, making it intoa seamount. In addition, as the lithosphere and volcano cool, they becomemore dense, so they sink down a bit into the mantle. Oceanographersoften find “flat-topped” seamounts called tablemounts or “guyots” (gee-ohs). They were once islands at the surface and had their top eroded awayby waves before sinking deeper into the ocean. Corals are able to survivewhen their island sinks by growing straight upwards towards the surface.They form coral atolls, islands whose tops are made of coral reef, notvolcanic 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.

18 Hotspots can exist near the mid-ocean ridge (e.g., Iceland), and cause even more volcanism than normal.19 Hotspots can also be located under a continent. For example, the geysers of Yellowstone National Park arecaused by the heat of a “hotspot.” These hotspots can also cause volcanoes.20 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 theisland sinks. USGS.



After one volcano leaves the hotspot, another volcanogrows on the hotspot. This volcano also leaves, andanother volcano grows. This happens again and again,creating an “island-seamount chain.” The classicexample is the Hawaiian Islands which are one end ofthe much larger Hawaii Islands-Emperor SeamountChain. Currently the southeast corner of the “big”island21 is above hotspot; this is where you can seeactive volcanoes at “Hawaiian Volcanoes NationalPark.” The New England Seamounts along the “GreatMeteor hot-spot track” near the east coast of the UnitedStates also formed in this manner.

Hawaiian Islands. The red dot shows thelocation 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. Lookout Maui, if this is the case, becausethis would make it harder to predict where volcanoes will occur! Stay tuned: there are newthings to be discovered!

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

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