Chapter 7: The Ocean Bottom

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Source: The text below is taken from Ck12 flexbooks, and open source, online textbook, and is not written by Mr Van Arsdale.

Introduction

An important piece of plate tectonic theory is the continental drift idea. This was developed in the early part of the 20th century, mostly by a single scientist, Alfred Wegener. His hypothesis states that continents move around on Earth’s surface and that they were once joined together as a single supercontinent. Wegener’s idea eventually helped to form the theory of plate tectonics, but while Wegener was alive, scientists did not believe that the continents could move.

The Continental Drift Idea

Find a map of the continents and cut each one out. Better yet, use a map where the edges of the continents show the continental shelf. In this case, your continent puzzle piece includes all of the continental crust for that continent and reflects the true size and shape of the continent. Can you fit the pieces together? The easiest link is between the eastern Americas and western Africa and Europe, but the rest can fit together too!

The continents fit together like pieces of a puzzle. This is how they looked 250 million years ago.

Alfred Wegener, an early 20th century German meteorologist believed that the continents could fit together. He proposed that the continents were not stationary but that they had moved during the planet’s history. He suggested that at one time, all of the continents had been united into a single supercontinent. He named the supercontinent Pangaea, meaning entire earthin ancient Greek. Wegener further suggested that Pangaea broke up long ago and that the continents then moved to their current positions. He called his hypothesis continental drift.

Evidence for Continental Drift

Besides the fit of the continents, Wegener and his supporters collected a great deal of evidence for the continental drift hypothesis. Wegener found that this evidence was best explained if the continents had at one time been joined together.

Wegener discovered that identical rocks could be found on both sides of the Atlantic Ocean. These rocks were the same type and the same age. Wegener understood that the rocks had formed side-by-side and that the land has since moved apart. Wegener also matched up mountain ranges that had the same rock types, structures, and ages, but that are now on opposite sides of the Atlantic Ocean. The Appalachians of the eastern United States and Canada, for example, are just like mountain ranges in eastern Greenland, Ireland, Great Britain, and Norway. Wegener concluded that they formed as a single mountain range that was separated as the continents drifted.

Wegener also found evidence from ancient fossils (Figure below). He found fossils of the same species of extinct plants and animals in rocks of the same age, but on continents that are now widely separated. Wegener suggested that the continents could not have been in their current positions because the organisms would not have been able to travel across the oceans. For example, fossils of the seed fern Glossopteris are found across all of the southern continents. But the plants’ seeds were too heavy to be carried across the ocean by wind. Mesosaurus fossils are found in South America and South Africa, but the reptile only could only swim in fresh water. Cynognathus andLystrosaurus were reptiles that lived on land. Both of these animals were unable to swim, let alone swim across wide seas! Their fossils have been found across South America, Africa, India and Antarctica. Wegener proposed that the organisms had lived side by side, but that the lands had moved apart after they were dead and fossilized.

Wegener used fossil evidence to support his continental drift hypothesis. The fossils of these organisms are found on lands that are now far apart. Wegener suggested that when the organisms were alive, the lands were joined and the organisms were living side-by-side.

Wegener also looked at evidence from ancient glaciers. Large glaciers are most commonly found in frigid climates, usually in the far northern and southern latitudes. Using the distribution of grooves and rock deposits left by ancient glaciers on many different continents, Wegener traced the glaciers back to where they must have started. He discovered that if the continents were in their current positions, the glaciers would have formed in the middle of the ocean very close to the equator. Wegener knew that this was impossible! However, if the continents had moved, the glaciers would have been centered over the southern land mass much closer to the South Pole.

Wegener also found evidence for his hypothesis from warm climate zones. Coral reefs and the swamps that lead to the formation of coal are now found only in tropical and subtropical environments. But Wegener discovered ancient coal seams and coral reefs in parts of the continents that were much too cold today. The coral reef fossils and coal had drifted to new locations since the coal and coral formed.

Although Wegener’s evidence was sound, most geologists at the time rejected his hypothesis of continental drift. These scientists argued that there was no way to explain how solid continents could plow through solid oceanic crust. At the time, scientists did not understand how solid material could move. Wegener’s idea was nearly forgotten until technological advances presented puzzling new information and gave scientists the tools to develop a mechanism for Wegener’s drifting continents.

Magnetic Polarity Evidence

The puzzling new evidence came from studying Earth’s magnetic field and how it has changed. If you have ever been hiking or camping, you may have used a compass to help you find your way. A compass uses the Earth’s magnetic field to locate the magnetic North Pole. Earth’s magnetic field is like a bar magnet with the ends of the bar sticking out at each pole (Figure below). Currently, the field’s north and south magnetic poles are very near to the Earth’s north and south geographic poles.

Some iron-bearing minerals, like tiny magnetite crystals in igneous rocks, point to the north magnetic pole as they crystallize from magma. These little magnets record both the strength and direction of the Earth’s magnetic field. The direction is known as the field’s magnetic polarity. In the 1950’s, scientists began using magnetometers to look at the magnetic properties of rocks in many locations.

Earth’s magnetic field is like a magnet with its north pole near the geographic north pole and the south pole near the geographic south pole.

Geologists noted that magnetite crystals in fresh volcanic rocks pointed to the current magnetic north pole. This happened no matter where the rocks were located, whether they were on different continents or in different locations on the same continent. But for older volcanic rocks, this was not true. Rocks that were the same age and were located on the same continent pointed to the same point, but that point was not the current north magnetic pole. Moving back in time, rocks on the same continent that were the same age pointed at the same point. But these rocks did not point to the same point as the rocks of different ages or the current magnetic pole. In other words, although the magnetite crystals were pointing to the magnetic north pole, the location of the pole seemed to wander. For example, 400 millon year old lava flows in North America indicated that the north magnetic pole was located in the western Pacific Ocean, but 250 million year old lava flows indicated a pole in Asia, and 100 million year old lava flows had a pole in northern Asia. Scientists were amazed to find that the north magnetic pole changed location through time (Figure below)!

The magnetic north pole appears to move around with time. This diagram shows where the pole was 80 million years before present (mybp), then 60, 40, 20 and now. Since we know that the pole does not move, this path is called apparent polar wander.

There were three possible explanations for this puzzling phenomenon: (1) the continent remained fixed and the north magnetic pole moved (2) the north magnetic pole stood still and the continent moved (3) both the continent and the north pole moved.

The situation got stranger when scientists looked at where magnetite crystals pointed for rocks of the same age but on different continents. They found these rocks pointed to different magnetic north poles! For example, 400 million years ago the European north pole was different from the North American north pole at that same time. At 250 million years, the north poles were also different for the two continents. The scientists again looked at the three possible explanations. If the correct explanation was that the continents had remained fixed while the north magnetic pole moved, then there had to be two separate north poles. Since there is only one north pole today, they decided that the best explanation had to involve only one north magnetic pole. This meant that the second explanation must be correct, that the north magnetic pole had remained fixed but that the continents had moved.

To test this, geologists fitted the continents together as Wegener had done. They discovered that there had indeed been only one magnetic north pole but that the continents had drifted. They renamed the phenomenon of the magnetic pole that seemed to move but actually did not apparent polar wander. This evidence for continental drift gave geologists renewed interest in understanding how continents could move about on the planet’s surface. And we know that the magnetic pole wanders, too, so the correct explanation was that both the continents and the magnetic poles move (Figure below).

The left side image shows the apparent north pole locations for two different continents, Europe and North America, if the continents were always in their current locations. When continental drift is taken into account, the two paths merge into one since there is only one magnetic north pole.

Sea Floor Spreading

Perhaps surprisingly, it was World War II that gave scientists the tools to find the mechanism for continental drift that had eluded Wegener and his colleagues. Scientists used maps and other data gathered during the war to develop the seafloor spreading hypothesis. This hypothesis traces oceanic crust from its origin at a mid-ocean ridge to its destruction at a deep sea trench. Scientists realized that seafloor spreading could be the mechanism for continental drift that they had been looking for.

Seafloor Bathymetry

During the war, battleships and submarines carried echo sounders to locate enemy submarines (Figure below). Echo sounders produce sound waves that travel outward in all directions, bounce off the nearest object, and then return to the ship. The round-trip time of the sound wave is then recorded. By knowing the speed of sound in seawater, scientists can calculate the distance to the object that the sound wave hit. During the war, the sound wave rarely encountered an enemy submarine, and so most of the sound waves ricocheted off the ocean bottom.

A ship sends out sound waves to create a picture of the seafloor below it. The echo sounder pictured has many beams and as a result it creates a three dimensional map of the seafloor beneath the ship. Early echo sounders had only a single beam and created a line of depth measurements.

After the war, scientists pieced together the bottom depths to produce a a map of the seafloor. This is known as a bathymetric map and is similar to a topographic map of the land surface. While a bathymetric map measures the distance of the seafloor below sea level, a topographic map gives the elevation of the land surface above sea level. Bathymetric maps reveal the features of the ocean floor as if the water were taken away.

The bathymetric maps that were produced at this time were astonishing! Most people had thought that the ocean floor was completely flat but the maps showed something completely different. As we know now, majestic mountain ranges extend in a line through the deep oceans. Amazingly, the mountain ranges are connected as if they were the seams on a baseball. These mountain ranges are named mid-ocean ridges. The mid-ocean ridges and the areas around them rise up high above the deep seafloor (Figure below).

A modern map of the eastern Pacific and Atlantic Oceans. Darker blue indicates deeper seas. A mid-ocean ridge can be seen running through the center of the Atlantic Ocean. Deep sea trenches are found along the west coast of Central and South America and in the mid-Atlantic east of the southern tip of South America. Isolated mountains and flat featureless regions can also be spotted.

Another astonishing feature is the deep sea trenches that are found at the edges of continental margins or in the sea near chains of active volcanoes. Trenches are the deepest places on Earth. The deepest trench is the Marianas Trench in the southwestern Pacific Ocean, which plunges about 11 kilometers 35,840 feet (35,840 feet) beneath sea level. Near the trenches, the seafloor is also especially deep.

Besides these dramatic features, there are lots of flat areas, called abyssal plains, just as the scientists had predicted. But many of these plains are dotted with volcanic mountains. These mountains are both large and small, pointy and flat-topped, by themselves as well as in a line. When they first observed the maps, the amazing differences made scientists wonder what had formed these features.

Seafloor Magnetism

In the previous lesson, you learned that magnetometers used on land were important in recognizing apparent polar wander. Magnetometers were also important in understanding the magnetic polarity of rocks in the deep sea. During WWII, magnetometers that were attached to ships to search for submarines discovered a lot about the magnetic properties of the seafloor.

In fact, using magnetometers, scientists discovered an astonishing feature of Earth’s magnetic field. Sometimes, no one really knows why, the magnetic poles switch positions. North becomes south and south becomes north! When the north and south poles are aligned as they are now, geologists say the polarity is normal. When they are in the opposite position, they say that the polarity is reversed.

Scientists were surprised to discover that the normal and reversed magnetic polarity of seafloor basalts creates a pattern of magnetic stripes! There is one long stripe with normal polarity, next to one long stripe with reversed polarity and so on across the ocean bottom. Another amazing feature is that the stripes are form mirror images on either side of the mid-ocean ridges. The ridge crest is of normal polarity and there are two stripes of reversed polarity of roughly equal width on each side of the ridge. Further distant are roughly equal stripes of normal polarity, beyond that, roughly equal stripes of reversed polarity, and so on. The magnetic polarity maps also show that the magnetic stripes end abruptly at the edges of continents, which are sometimes lined by a deep sea trench (Figure below).

Scientists found that magnetic polarity in the seafloor was normal at mid-ocean ridges but reversed in symmetrical patterns away from the ridge center. This normal and reversed pattern continues across the seafloor.

The scientists used geologic dating techniques to find the ages of the rocks that were found with the different magnetic polarities. It turns out that the rocks of normal polarity are located along the axis of the mid-ocean ridges and these are the youngest rocks on the seafloor. The ages of the rocks increases equally and symmetrically on both sides of the ridge.

Scientists also discovered that there are virtually no sediments on the seafloor at the axis, but the sediment layer increases in thickness in both directions away from the ridge axis. This was additional evidence that the youngest rocks are on the ridge axis and that the rocks are older with distance away from the ridge (Figure below). The scientists were surprised to find that oldest seafloor is less than 180 million years old while the oldest continental crust is around 4 billion years old. They realized that some process was causing seafloor to be created and destroyed in a relatively short time.

Seafloor is youngest near the mid-ocean ridges and gets progressively older with distance from the ridge. Orange areas show the youngest seafloor. The oldest seafloor is near the edges of continents or deep sea trenches.

The scientists also discovered that the seafloor was thinner at the ridge axis and grew thicker as the crust became older. This is because over time, additional magma cools to form rock. The added sediments also increase the thickness of the older crust.

The Seafloor Spreading Hypothesis

Scientists brought all of these observations together in the early 1960s to create the seafloor spreading hypothesis. They suggested that hot mantle material rises up toward the surface at mid-ocean ridges. This hot material is buoyant and causes the ridge to rise, which is one reason that mid-ocean ridges are higher than the rest of the seafloor.

The hot magma at the ridge erupts as lava that forms new seafloor. When the lava cools, its magnetite crystals take on the current magnetic polarity. The polarity is locked in when the lava solidifies and the magnetite crystals are trapped in position. Reversals show up as magnetic stripes on opposite sides of the ridge axis. As more lava erupts, it pushes the seafloor that is at the ridge horizontally away from ridge axis. This continues as the formation of new seafloor forces older seafloor to move horizontally away from the ridge axis.

The magnetic stripes continue across the seafloor. If the oceanic crust butts up against a continent, it pushes that continent away from the ridge axis as well. If the oceanic crust reaches a deep sea trench, it will sink into it and be lost into the mantle. In either case, the oldest crust is coldest and lies deepest in the ocean.

It is the creation and destruction of oceanic crust, then, that is the mechanism for Wegener’s drifting continents. Rather than drifting across the oceans, the continents ride on a conveyor belt of oceanic crust that takes them around the planet’s surface.

One of the fundamental lines of evidence for continental drift is the way the coastlines of continents on both sides of the Atlantic Ocean fit together. So let’s look at how seafloor spreading moves continents in the Atlantic by looking more closely at figure 3 above. New oceanic crust is forming at the mid-ocean ridge that runs through the center of the Atlantic Ocean basins, which is called the Mid-Atlantic Ridge. Stripes of different magnetic polarity are found on opposite sides of the Mid-Atlantic Ridge. These stripes go all the way to the continents, which lie on opposite sides of the Atlantic. So new seafloor forming at the Mid-Atlantic Ridge is causing the Americas and Eurasia to move in opposite directions!

Putting the Peices Together – Plate Tectonics

Wegener’s continental drift hypothesis had a great deal of evidence in its favor but it was largely abandoned because there was no plausible explanation for how the continents could drift. In the meantime, scientists developed explanations to explain the locations of fossils on widely different continents (land bridges) and the similarity of rock sequences across oceans (geosynclines), which were becoming more and more cumbersome. When seafloor spreading came along, scientists recognized that the mechanism to explain drifting continents had been found. Like the scientists did before us, we are now ready to merge the ideas of continental drift and seafloor spreading into a new all-encompassing idea: the theory of plate tectonics.

Earth’s Tectonic Plates

Now you know that seafloor and continents move around on Earth’s surface. But what is it that is actually moving? In other words, what is the “plate” in plate tectonics? This question was also answered due to war, in this case the Cold War.

Although seismographs had been around for decades, during the 1950s and especially in the early 1960s, scientists set up seismograph networks to see if enemy nations were testing atomic bombs. Seismographs record seismic waves. Modern seismographs are sensitive enough to detect nuclear explosions. While watching for enemy atom bomb tests, the seismographs were also recording all of the earthquakes that were taking place around the planet. These seismic records could be used to locate an earthquake’sepicenter, the point on Earth’s surface directly above the place where the earthquake occurs. Earthquakes are associated with large cracks in the ground, known as faults. Rocks on opposite sides of a fault move in opposite directions.

Earthquakes are not spread evenly around the planet, but are found mostly in certain regions. In the oceans, earthquakes are found along mid-ocean ridges and in and around deep sea trenches. Earthquakes are extremely common all around the Pacific Ocean basin and often occur near volcanoes. The intensity of earthquakes and volcanic eruptions around the Pacific led scientists to name this region the Pacific Ring of Fire (Figure below). Earthquakes are also common in the world’s highest mountains, the Himalaya Mountains of Asia, and across the Mediterranean region.

The bold pink swatch outlines the volcanoes and active earthquake areas found around the Pacific Ocean basin, which is called the Pacific Ring of Fire.

Scientists noticed that the earthquake epicenters were located along the mid-ocean ridges, trenches and large faults that mark the edges of large slabs of Earth’s lithosphere (Figure below). They named these large slabs of lithosphere plates. The movements of the plates were then termed plate tectonics. A single plate can be made of all oceanic lithosphere or all continental lithosphere, but nearly all plates are made of a combination of both.

A map of earthquake epicenters shows that earthquakes are found primarily in lines that run up the edges of some continents, through the centers of some oceans, and in patches in some land areas.

The lithosphere is divided into a dozen major and several minor plates. The plates’ edges can be drawn by the connecting the dots that are earthquakes epicenters. Scientists have named each of the plates and have determined the direction that each is moving (Figure below). Plates move around the Earth’s surface at a rate of a few centimeters a year, about the same rate fingernails grow.

The lithospheric plates and their names. The arrows show whether the plates are moving apart, moving together, or sliding past each other.

How Plates Move

We know that seafloor spreading moves the lithospheric plates around on Earth’s surface but what drives seafloor spreading? The answer is in lesson one of this chapter: mantle convection. At this point it would help to think of a convection cell as a rectangle or oval (Figure below). Each side of the rectangle is a limb of the cell. The convection cell is located in the mantle. The base is deep in the mantle and the top is near the crust. There is a limb of mantle material moving on one side of the rectangle, one limb moving horizontally across the top of the rectangle, one limb moving downward on the other side of the rectangle, and the final limb moving horizontally to where the material begins to move upward again.

Now picture two convection cells side-by-side in the mantle. The rising limbs of material from the two adjacent cells reach the base of the crust at the mid-ocean ridge. Some of the hot magma melts and creates new ocean crust. This seafloor moves off the axis of the mid-ocean ridge in both directions when still newer seafloor erupts. The oceanic plate moves outward due to the eruption of new oceanic crust at the mid-ocean ridge.

Convection in the mantle is the driving force of plate tectonics. Hot material rises at mid-ocean ridges and sinks at deep sea trenches, which keeps the plates moving along the Earth’s surface.

Beneath the moving crust is the laterally moving top limb of the mantle convection cells. Each convection cell is moving seafloor away from the ridge in opposite directions. This horizontal mantle flow moves with the crust across the ocean basin and away from the ridge. As the material moves horizontally, the seafloor thickens and both the new crust and the mantle beneath it cool. Where the limbs of the convection cells plunge down into the deeper mantle, oceanic crust is dragged into the mantle as well. This takes place at the deep sea trenches. As the crust dives into the mantle its weight drags along the rest of the plate and pulls it downward. The last limbs of the convection cells flow along the core. The material is heated and so is ready to rise again when it reaches the rising limb of the convection cell. As you can see, each convection cell is found beneath a different lithospheric plate and is responsible for the movement of that plate.

Plate Boundaries

Back at the planet’s surface, the edges where two plates meet are known asplate boundaries. Most geologic activity, including volcanoes, earthquakes, and mountain building, takes place at plate boundaries where two enormous pieces of solid lithosphere interact.

Think about two cars moving around a parking lot. In what three ways can those cars move relative to each other? They can move away from each other, they can move toward each other, or they can slide past each other. These three types of relative motion also define the three types of plate boundaries:

  • Divergent plate boundaries: the two plates move away from each other.
  • Convergent plate boundaries: the two plates move towards each other.
  • Transform plate boundaries: the two plates slip past each other.

What happens at plate boundaries depends on which direction the two plates are moving relative to each other. It also depends on whether the lithosphere on the two sides of the plate boundary is oceanic crust, continental crust, or one piece of each type. The type of plate boundary and the type of crust found on each side of the boundary determines what sort of geologic activity will be found there: earthquakes, volcanoes, or mountain building.

Divergent Plate Boundaries

Plates move apart, or diverge, at mid-ocean ridges where seafloor spreading forms new oceanic lithosphere. At these mid-ocean ridges, lava rises, erupts, and cools. Magma cools more slowly beneath the lava mostly forming the igneous intrusive rock gabbro. The entire ridge system, then, is igneous. Earthquakes are also common at mid-ocean ridges since the movement of magma and oceanic crust result in crustal shaking. Although the vast majority of mid-ocean ridges are located deep below the sea, we can see where the Mid-Atlantic Ridge surfaces at the volcanic island of Iceland (Figure below).

The Leif the Lucky Bridge straddles the Mid-Atlantic ridge separating the North American and Eurasian plates on Iceland.

Although it is uncommon, a divergent plate boundary can also occur within a continent. This is called continental rifting (Figure below). Magma rises beneath the continent, causing it to thin, break, and ultimately split up. As the continental crust breaks apart, oceanic crust erupts in the void. This is how the Atlantic Ocean formed when Pangaea broke up. The East African Rift is currently splitting eastern Africa away from the African continent.

The Arabian, Indian, and African plates are rifting apart, forming the Great Rift Valley in Africa. The Dead Sea fills the rift with seawater.

Convergent Plate Boundaries

What happens when two plates converge depends on the types of crust that are colliding. Convergence can take place between two slabs of continental lithosphere, two slabs of oceanic lithosphere, or between one continental and one oceanic slab. Most often, when two plates collide, one or both are destroyed.

When oceanic crust converges with continental crust, the denser oceanic plate plunges beneath the continental plate. This process occurs at the oceanic trenches and is called subduction (Figure below). The entire region is known as a subduction zone. Subduction zones have a lot of intense earthquakes and volcanic eruptions. The subducting plate causes melting in the mantle. The magma rises and erupts, creating volcanoes. These volcanoes are found in a line above the subducting plate. The volcanoes are known as acontinental arc. The movement of crust and magma causes earthquakes. The Andes Mountains, which line the western edge of South America, are a continental arc. The volcanoes are the result of the Nazca plate subducting beneath the South American plate (Figure below).

Subduction of an oceanic plate beneath a continental plate forms a line of volcanoes known as a continental arc and causes earthquakes.

This satellite image shows the trench lining the western margin of South America where the Nazca plate is subducting beneath the South American plate. The resulting Andes Mountains line western South America and are seen as brown and red uplands in this image.

The volcanoes of northeastern California—Lassen Peak, Mount Shasta, and Medicine Lake volcano—along with the rest of the Cascade Mountains of the Pacific Northwest, are the result of subduction of the Juan de Fuca plate beneath the North American plate (Figure below). Mount St. Helens, which erupted explosively on May 18, 1980, is the most famous and currently the most active of the Cascades volcanoes.

The Cascade Mountains of the Pacific Northwest are formed by the subduction of the Juan de Fuca plate beneath the North American plate. The Juan de Fuca plate forms near the shoreline at the Juan de Fuca ridge.

Sometimes the magma does not rise all the way through the continental crust beneath a volcanic arc. This usually happens if the magma is rich in silica. These viscous magmas form large areas of intrusive igneous rock, called batholiths, which may someday be uplifted to form a mountain range. The Sierra Nevada batholith cooled beneath a volcanic arc roughly 200 million years ago (Figure below). Similar batholiths are likely forming beneath the Andes and Cascades today.

The granite batholith of the Sierra Nevada Mountain range is well exposed here at Mount Whitney, the highest mountain in the range at 14,505 feet (4,421 meters) and the second highest mountain in North America.

When two oceanic plates converge, the older, denser plate will sink beneath the other plate and plunge into the mantle. As the plate is pushed deeper into the mantle, it melts, which forms magma. As the magma rises it forms volcanoes in a line known as an island arc, which is a line of volcanic islands (Figure below).

A convergent plate boundary subduction zone between two plates of oceanic lithosphere. Melting of the subducting plate causes volcanic activity and earthquakes.

The Japanese, Indonesian, and Philippine islands are examples of island arc volcanoes. The volcanic islands are set off from the mainland in an arc shape as seen in this satellite image of Japan (Figure below).

Japan is an island arc composed of volcanoes off the Asian mainland, as seen in this satellite image.

When two continental plates collide, they are too thick to subduct. Just like if you put your hands on two sides of a sheet of paper and bring your hands together, the material has nowhere to go but up (Figure below)! Some of the world’s largest mountains ranges are created at continent-continent convergent plate boundaries. In these locations, the crust is too thick for magma to penetrate so there are no volcanoes, but there may be magma. Metamorphic rocks are common due to the stress the continental crust experiences. As you might think, with enormous slabs of crust smashing together, continent-continent collisions bring on numerous earthquakes.

When two plates of continental crust collide, the material pushes upward forming a high mountain range. The remnants of subducted oceanic crust remain beneath the continental convergence zone.

The world’s highest mountains, the Himalayas, are being created by a collision between the Indian and Eurasian plates (Figure below). The Appalachian Mountains are the remnants of a large mountain range that was created when North America rammed into Eurasia about 250 million years ago.

The Himalaya Mountains are the result of the collision of the Indian Plate with the Eurasian Plate, seen in this photo from the International Space Station. The high peak in the center is world’s tallest mountain, Mount Everest (8,848 meters; 29,035 feet).

Transform Plate Boundaries

Transform plate boundaries are seen as transform faults. At these earthquake faults, two plates move past each other in opposite directions. Where transform faults bisect continents, there are massive earthquakes. The world’s most notorious transform fault is the 1,300 kilometer (800 mile) long San Andreas Fault in California (Figure below). This is where the Pacific and North American plates grind past each other, sometimes with disastrous consequences.

At the San Andreas Fault in California, the Pacific Plate is sliding northeast relative to the North American plate, which is moving southwest. At the northern end of the picture, the transform boundary turns into a subduction zone.

California is very geologically active. A transform plate boundary creates the San Andreas Fault. A convergent plate boundary between an oceanic plate and a continental plate creates the Cascades volcanoes. Just offshore, the Juan de Fuca ridge is subducting beneath the North American plate at a divergent plate boundary.

Earth’s Changing Surface

Geologists now know that Wegener was right when he said that the continents had once been joined into the supercontinent Pangaea and are now moving apart. Most of the geologic activity that we see on the planet today is due to the interactions of the moving plates. Where plates come apart at a divergent boundary, there is volcanic activity and small earthquakes. If the plates meet at a convergent boundary, and at least one is oceanic, there is a chain of volcanoes and many earthquakes. If both plates at a convergent boundary are continental, mountain ranges grow. If the plates meet at a transform boundary, there is a transform fault. These faults do not have volcanic activity but they have massive earthquakes.

If you look at a map showing the locations of volcanoes and earthquakes in North America, you will see that the plate boundaries are now along the western edge. This geologically active area makes up part of the Pacific Ring of Fire. California, with its volcanoes and earthquakes, is an important part of this region. The eastern edge of North America is currently mostly quiet, although mountain ranges line the area. If there is no plate boundary there today, where did those mountains come from?

Remember that Wegener used the similarity of the mountains in eastern North America, on the west side of the Atlantic, and the mountains in Great Britain, on the eastern side of the Atlantic, as evidence for his continental drift hypothesis. These mountains were formed at a convergent plate boundary as the continents that made up Pangaea came together. So about 200 million years ago these mountains were similar to the Himalaya today (Figure below)!

The Appalachian Mountains of eastern North America were probably once as high as the Himalaya, but they have aged since the breakup of Pangaea.

Before the continents collided they were separated by an ocean, just as the continents rimming the Pacific are now. That ocean crust had to subduct beneath the continents just as the oceanic crust around the Pacific is being subducted today. Subduction along the eastern margin of North America produced continental arc volcanoes. Ancient lava from those volcanoes can be found in the region.

Currently, Earth’s most geologically active area is around the Pacific. The Pacific is shrinking at the same time the Atlantic is growing. But hundreds of millions of years ago, that was reversed: the Atlantic was shrinking as the Pacific was growing. What we’ve just identified is a cycle, known as thesupercontinent cycle, which is responsible for most of the geologic features that we see and many more that are long gone. Scientists think that the creation and breakup of a supercontinent takes place about every 500 million years.

Intraplate Activity

While it is true that most geological activity takes place along plate boundaries, some is found away from the edges of plates. This is known asintraplate activity. The most common intraplate volcanoes are above hotspots that lie beneath oceanic plates. Hotspot volcanoes arise because plumes of hot material that come from deep in the mantle rise through the overlying mantle and crust. When the magma reaches the plate above, it erupts, forming a volcano. Since the hotspot is stable, when the oceanic plate moves over it, and it erupts again, another volcano is created in line with the first. With time, there is a line of volcanoes; the youngest is directly above the hot spot and the oldest is furthest away. Recent research suggests that hotspots are not as stable as scientists once thought, but some larger ones still appear to be.

The Hawaiian Islands are a beautiful example of a chain of hotspot volcanoes. Kilauea volcano on the south side of the Big Island of Hawaii lies above the Hawaiian hot spot. The Big Island is on the southeastern end of the Hawaiian chain. Mauna Loa volcano, to the northwest, is older than Kilauea and is still erupting, but at a lower rate. Hawaii is the youngest island in the chain. As you follow the chain to the west, the islands get progressively older because they are further from the hotspot (Figure below).

This view of the Hawaiian islands shows the youngest islands in the southeast and the oldest in the northwest. Kilauea volcano, which makes up the southeastern side of the Big Island of Hawaii, is located above the Hawaiian hotspot.

The chain continues into the Emperor Seamounts, which are so old they no longer reach above sea level. The oldest of the Emperor seamounts is about to subduct into the Aleutian trench off of Alaska; no one knows how many older volcanoes have already subducted. It’s obvious from looking at the Emperor seamounts that the Pacific plate took a large turn. Radiometric dating has shown that turn to have taken place about 43 million years ago (Figurebelow). The Hawaii hotspot may also have been moving southward during this time. Still, geologists can use some hotspot chains to tell not only the direction but the speed a plate is moving.

The Hawaii-Emperor chain creates a large angular gash across the Pacific basin in this satellite image. The bend in the chain is due to a change in the direction of motion of the Pacific plate 43 million years ago.

Hot spots are also found under the continental crust, although it is more difficult for the magma to make it through the thick crust and there are few eruptions. One exception is Yellowstone, which creates the activity at the Yellowstone hotspot. In the past, the hotspot produced enormous volcanic eruptions, but now its activity is best seen in the region’s famous geysers.

Questions to Research:

  1. Briefly describe Alfred Wegener’s theory and the evidence he used to support it.  Also identify what question Wegener failed to resolve.  If you need more information, try the USGS Historical Perspectives page.
  2. Explain the contributions of Harry Hess, that allowed for the development of a theory in plate tectonics known as “Sea Floor Spreading”. If you need more information, try the USGS Developing A Theory page.
  3. Draw pictures of the different types of plate boundaries (Divergent, Convergent, and Transform) with arrows to indicate movement.  Under each, identify the oceanic features ( undersea mountains, trenches, islands, etc.) they tend to form.   Finally, include an example of each that could be found on a map (i.e. The Great Rift Valley of Africa is a divergent plate boundary.) If the text above is not enough, try the USGS Understanding Plate Movements page.
  4. Examine the image below (image from: wikipedia commons, click for a larger version).  Based on what you have learned about sea floor spreading, explain what the different colors on this map mean.
    age of the ocean bottom
  5. Hot Spots: Describe why there are volcanic islands (i.e. Hawaii) in the very middle of the Pacific Plate?
  6. World seismicity map: Explain how map’s like these added some of the most significant evidence to the Theory of Plate Tectonics.
  7. Examine a map and graph of the Pacific Plate subducting under the South American Plateand explain how the movement of oceanic plates is related to the depths and locations of earthquakes under South America.
  8. Subduction Zones: Of these four “Subduction Zones” you are asked to examine (Japan, Alaska, Tonga, South America) which one is the oceanic plate being pushed down at the steepest angle.
  9. Ocean Bottom Features: Of all these ocean features, what does the majority of the ocean bottom look like?
  10. Guyotes and Sea Mounts: These are areas of incredible biological diversity.  They often see large gatherings of fish, sharks, and squid.  How are they formed?