# 5.2: Plate Tectonics

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## Plate Tectonics

 LEARNING OUTCOMES ESSENTIAL QUESTIONSWhat is the driving force of plate tectonics and how does this impact earthquakes and volcanoes around the world?How does the theory of plate tectonics help explain the different types of earthquakes and volcanoes around the planet?

## Theory of Continental Drift

The continental drift hypothesis was developed in the early part of the 20th century, mostly by Alfred Wegener. Wegener said that continents move around on Earth’s surface and that they were once joined together as a single supercontinent. While Wegener was alive, scientists did not believe that the continents could move.

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. That’s the true size and shape of a continent and many can be pieced together like a puzzle. The easiest link is between the eastern Americas and western Africa and Europe, but the rest can fit together too.

Alfred Wegener proposed that the continents were once united into a single supercontinent named Pangaea, meaning all earth in ancient Greek. He 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 way the continents fit together, Wegener and his supporters collected a great deal of evidence for the continental drift hypothesis. For one, identical rocks of the same type and age are found on both sides of the Atlantic Ocean. Wegener said the rocks had formed side-by-side and that the land had since moved apart.Mountain ranges with the same rock types, structures, and ages 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.Ancient fossils of the same species of extinct plants and animals are found in rocks of the same age but are on continents that are now widely separated. Wegener proposed that the organisms had lived side by side, but that the lands had moved apart after they were dead and fossilized. He suggested that the organisms would not have been able to travel across the oceans. For example, the fossils of the seed fern Glossopteris were too heavy to be carried so far by wind. The reptile Mesosaurus could only swim in fresh water. was a swimming reptile but could only swim in fresh water.Cynognathus and Lystrosaurus were land reptiles and were unable to swim.Grooves and rock deposits left by ancient glaciers are found today on different continents very close to the equator. This would indicate that the glaciers either formed in the middle of the ocean and/or covered most of the Earth. Today glaciers only form on land and nearer the poles. Wegener thought that the glaciers were centered over the southern land mass close to the South Pole and the continents moved to their present positions later on.Coral reefs and coal-forming swamps are found in tropical and subtropical environments, but ancient coal seams and coral reefs are found in locations where it is much too cold today. Wegener suggested that these creatures were alive in warm climate zones and that the fossils and coal later had drifted to new locations on the continents.Although Wegener’s evidence was sound, most geologists at the time rejected his hypothesis of continental drift. Scientists argued that there was no way to explain how solid continents could move through solid oceanic crust. Wegener’s idea was nearly forgotten until technological advances presented even more evidence that the continents moved and gave scientists the tools to develop a mechanism for Wegener’s drifting continents.

SEAFLOOR BATHYMETRY
World War II gave scientists the tools to find the mechanism for continental drift that had eluded Wegener. Maps and other data gathered during the war allowed scientists 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 and is the mechanism for continental drift.During World War II, battleships and submarines carried echo sounders to locate enemy submarines. Echo sounders produce sound waves that travel outward in all directions, bounce off the nearest object, and then return to the ship. By knowing the speed of sound in seawater, scientists calculate the distance to the object based on the time it takes for the wave to make a round-trip. During the war, most of the sound waves ricocheted off the ocean bottom. This animation shows how sound waves are used to create pictures of the seafloor and ocean crust.After the war, scientists pieced together the ocean depths to produce bathymetric maps, which reveal the features of the ocean floor as if the water were taken away. Even scientist were amazed that the seafloor was not completely flat. What was discovered was a large chain of mountains along the deep seafloor, called mid-ocean ridges. Scientists also discovered deep sea trenches along the edges of continents or in the sea near chains of active volcanoes. Finally, large, flat areas called abyssal plains we found. When they first observed these bathymetric maps, scientists wondered what had formed these features.

SEAFLOOR MAGNETISM
Sometimes, for reasons unknown, the magnetic poles switch positions. North becomes south and south becomes north. During normal polarity, the north and south poles are aligned as they are now. With reversed polarity, the north and south poles are in the opposite position. During WWII, magnetometers attached to ships to search for submarines located an astonishing feature; the normal and reversed magnetic polarity of seafloor basalts creates a pattern. Stripes of normal polarity and reversed polarity alternate across the ocean bottom. These stripes also forms a mirror image of itself on either side of the mid-ocean ridges. But the stripes end abruptly at the edges of continents, sometimes at a deep sea trench. The characteristics of the rocks and sediments change with distance from the ridge axis as seen in the Table below.
 At ridge axis Distance from axis Rock Ages Youngest Becomes older Sediment Thickness None Becomes thicker Crust Thickness Thinnest Becomes thicker Heat Flow Hottest Becomes cooler
A map of sediment thickness is found here. The oldest seafloor is near the edges of continents or deep sea trenches and is less than 180 million years old. Since the oldest ocean crust is so much younger than the oldest continental crust, scientists realized that seafloor was being destroyed in a relatively short time.

Scientists brought these observations together in the early 1960s to create the seafloor spreading hypothesis. In this hypothesis, hot buoyant mantle rises up a mid-ocean ridge, causing the ridge to rise upward. The hot magma at the ridge erupts as lava that forms new seafloor. When the lava cools, the magnetite crystals take on the current magnetic polarity and as more lava erupts, it pushes the seafloor horizontally away from ridge axis.The magnetic stripes continue across the seafloor. As oceanic crust forms and spreads, moving away from the ridge crest, it pushes the continent away from the ridge axis. If the oceanic crust reaches a deep sea trench, it sinks into the trench and is lost into the mantle. Scientists now know that the oldest crust is coldest and lies deepest in the ocean because it is less buoyant than the hot new crust.Seafloor spreading is the mechanism for Wegener’s drifting continents. Convection currents within the mantle take the continents on a conveyor-belt ride of oceanic crust that over millions of years takes them around the planet’s surface.

## Earth’s Tectonic Plates

When the concept of seafloor spreading came along, scientists recognized that it was the mechanism to explain how continents could move around Earth’s surface. Scientific data and observation now allows us to merge the ideas of continental drift and seafloor spreading into the theory of plate tectonics.

Seafloor and continents move around on Earth’s surface, but what is actually moving? What portion of the Earth makes up the “plates” in plate tectonics? This question was also answered because of technology developed during the Cold War. The plates are made up of the lithosphere. During the 1950s and early 1960s, scientists set up seismograph networks to see if enemy nations were testing atomic bombs. These seismographs also recorded all of the earthquakes around the planet. The seismic records could be used to locate an earthquake’s epicenter, the point on Earth’s surface directly above the place where the earthquake occurs. Earthquake epicenters outline these tectonic plates. Mid-ocean ridges, trenches, and large faults mark the edges of these plates along with where earthquakes occur.

The lithosphere is divided into a dozen major and several minor plates. The plates’ edges can be drawn by connecting the dots that mark earthquakes’ epicenters. 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. Movement of the plates over Earth’s surface is termed plate tectonics. Plates move at a rate of a few centimeters a year, about the same rate fingernails grow.

 HOW PLATES MOVE If seafloor spreading drives the plates, what drives seafloor spreading? Picture two convection cells side-by-side in the mantle. Hot mantle from the two adjacent cells rises at the ridge axis, creating new ocean crust. The top limb of the convection cell moves horizontally away from the ridge crest, as does the new seafloor. The outer limbs of the convection cells plunge down into the deeper mantle, dragging oceanic crust as well. This takes place at the deep sea trenches. The material sinks to the core and moves horizontally. The material heats up and reaches the zone where it rises again.

## Tectonic Plate Boundaries

Plate boundaries are the edges where two plates meet. Most geologic activities, including volcanoes, earthquakes, and mountain building, take place at plate boundaries. How can two plates move relative to each other?

• 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.

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.

DIVERGENT PLATE BOUNDARIES
Plates move apart at mid-ocean ridges where new seafloor forms. Between the two plates is a rift valley. Lava flows at the surface cool rapidly to become basalt, but deeper in the crust, magma cools more slowly to form gabbro. So the entire ridge system is made up of igneous rock that is either extrusive or intrusive. Earthquakes are common at mid-ocean ridges since the movement of magma and oceanic crust results in crustal shaking. The vast majority of mid-ocean ridges are located deep below the sea. Here is an animation from the United States Geologic Survey (USGS) of divergent plate boundary at mid-ocean ridge and another animation by IRIS.As divergence occurs, shallow earthquakes can occur along with volcanoes along the rift areas. When the process begins, a valley will develop such as the Great Rift Valley in Africa. Over time that valley can fill up with water creating linear lakes (see Figure A below). If divergence continues, a sea can form like the Red Sea (Figure B). and finally an ocean like the Atlantic Ocean. Check out the eastern half of Africa and notice the lakes that look linear. Eastern Africa is tearing apart from these linear lakes, to the Great Rift Valley, and up to the Red Sea. The ultimate divergent boundary is the Mid-Atlantic Ridge which began forming when Pangea broke apart. The spreading along this ridge formed the Atlantic Ocean.
 Figure A: Linear lakes within East African Rift Zone Figure B: Continued spreading will form a sea, here the Red Sea.
CONVERGENT PLATE BOUNDARIES
When two plates converge, the result depends on the type of lithosphere the plates are made of. There are three types of convergent boundaries. I. Ocean-to-continent convergence occurs when oceanic crust converges with continental crust, forcing the denser oceanic plate to plunge beneath the continental plate. This process called subduction, occurs along oceanic trenches called subduction zones where lots of intense earthquakes and volcanic eruptions can occur. The denser, subducting plate begins to heat up under extreme pressure near the mantle and melts to create causes melting in the volcanoes. These coastal volcanic mountains are found in a line above the subducting plate. The volcanoes are known as a continental arc. The movement of crust and magma causes earthquakes. Click here for a map of earthquake epicenters at subduction zones.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. The Juan de Fuca plate is created by seafloor spreading just offshore at the Juan de Fuca ridge. Figure C below is of the west coast of South America. The Andes Mountains are created here where the Nazca Plate, composed of oceanic lithosphere, is subducted under the continental lithosphere of the South American plate.

Figure C: South America’s western coast and Andes Mountains

Figure D: Oceanic plate convergence along Pacific Ocean (Ring of Fire)

II. Oceanic-to-oceanic plate boundary occurs when two oceanic plates converge, causing the older, denser plate will subduct into the mantle. An ocean trench marks the location where the plate is pushed down into the mantle. The line of volcanoes that grows on the upper oceanic plate is an island arc.

The Ring of Fire, as seen in Figure D above, is a ring around the Pacific Ocean of subduction zones, which most are oceanic-to-oceanic convergence. Here is an animation of an ocean continent plate boundary. Along these subduction zones, volcanic islands (also called volcanic arcs) form. Examples of these regions include Japan, Indonesia, and the Aleutian Islands.

Figure E: India’s collision with Asia forming the Himalayan Mountains.

III. Continental-to-continental convergence occurs when two continental plates collide, instead of subduction, the two similar tectonic plates will buckle up to create large mountain ranges like a massive car pile-up. This continental-to-continental convergence creates intense folding and faulting rather than volcanic activity.

Examples of mountain ranges created by this process are the Himalayan mountains (see Figure E above) as India is colliding with Asia, the Alps in Europe, and the Appalachian mountains in the United States as the North American plate collided with the African plate when Pangea was forming. The Kashmir India earthquake of 2005 that killed over 80,000 people occurred because of this process. And most recently, the 2008 earthquake in China which killed nearly 85,000 people before the Summer Olympics was because of this tectonic force. 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.

Transform plate boundaries occur when two tectonic plates slide (or grind) past parallel to each other. The most famous transform boundary is the San Andreas Fault where the Pacific plate that Los Angeles and Hawaii are on is grinding past the North American plate that San Francisco and the rest of the United States is on at the rate of 3 inches a year. Recently, geologists have stated that San Francisco should expect another disastrous earthquake in the next 30 years. Another important transform boundary is the North Anatolian Fault in Turkey. This powerful fault last ruptured in 1999 in Izmit, Turkey which killed 17,000 people in 48 seconds. INTRAPLATE BOUNDARIES
A small amount of geologic activity, known as intraplate activity, does not take place at plate boundaries but within a plate instead. Mantle plumes are pipes of hot rock that rise through the mantle. The release of pressure causes melting near the surface to form a hotspot. Eruptions at the hotspot create a volcano. Hotspot volcanoes are found in a line. Can you figure out why? Hint: The youngest volcano sits above the hotspot and volcanoes become older with distance from the hotspot.An animation of the creation of a hotspot chain is seen here:http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_hawaii.html.Geologists use some hotspot chains to tell the direction and the speed a plate is moving.Hotspot magmas rarely penetrate through thick continental crust. One exception is the Yellowstone hotspot.

## Earth’s Changing Surface

Geologists know that Wegener was right because the movements of continents explain so much about the geology we see. Most of the geologic activity that we see on the planet today is because of the interactions of the moving plates.

In the map of North America, where are the mountain ranges located? Using what you have learned about plate tectonics, try to answer the following questions:

1. What is the geologic origin of the Cascades Range? The Cascades are a chain of volcanoes in the Pacific Northwest. They are not labelled on the diagram but they lie between the Sierra Nevada and the Coastal Range.
2. What is the geologic origin of the Sierra Nevada? (Hint: These mountains are made of granitic intrusions.)
3. What is the geologic origin of the Appalachian Mountains along the Eastern US?

Remember that Wegener used the similarity of the mountains on the west and east sides of the Atlantic as evidence for his continental drift hypothesis. The Appalachian mountains formed at a convergent plate boundary as Pangaea came together.

Before Pangaea came together, the continents were separated by an ocean where the Atlantic is now. The proto-Atlantic ocean shrank as the Pacific ocean grew. Currently, the Pacific is shrinking as the Atlantic is growing. This supercontinent cycle is responsible for most of the geologic features that we see and many more that are long gone (Figure below).

This animation shows the movement of continents over the past 600 million years beginning with the breakup of Rodinia:http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_plate_reconstruction_blakey.html