Evidence That Continents Move
Did you know that Earth's surface is always moving? Every year the North American continent moves about 2.5 cm toward the Asian continent and away from the European continent. This may not seem like a lot of movement, but over geologic time, the centimeters add up to great distances. 

Scientists did not always know that continents move. A scientist named Alfred Wegener (VAY guh nuhr) observed the similarities of continental coastlines now separated by oceans. Examine the maps and the outlines of the continents. Notice how the continents could fit together like pieces of a puzzle.

Wegener wanted to know if Earth’s continents were fixed in their positions. He proposed in 1912 that all the continents were once part of a supercontinent called Pangaea (pan JEE uh). Over time, Pangaea began breaking apart, and the continents slowly moved to their present positions. Wegener proposed the hypothesis of continental drift, which suggests that continents are in constant motion on the surface of Earth.

Continental Drift Hypothesis
Scientists used to be very skeptical of the continental drift hypothesis. The continents
appear to fit together like puzzle pieces, but this alone was not enough evidence.
Wegener needed more information to support his hypothesis. Wegener searched for
evidence in Earth’s geologic past. He studied mountain formations, climate clues from
glacial deposits, glacial grooves, and fossils, from all over the world.

Rock Evidence
Rocks and fossils are helpful for understanding Earth’s past. Different rocks and fossils
form in different environments. By observing the rocks and fossils in an area, scientists
can determine if that place used to be covered by seas or glaciers. Rocks and fossils are
evidence that Earth is always changing. 

Wegener observed that mountain ranges and rock formations on different continents,
separated by oceans today, join together like puzzle pieces. If you could superimpose
similar rock types onto the maps, these rocks would be in the area where Africa and
South America fit together. 

The Caledonian mountain range in northern Europe and the Appalachian mountains in
eastern North America are similar in age and structure. They are also composed of the
same rock types. If you placed North America and Europe next to each other, these
mountains would meet and form one long, continuous mountain beit. 

Today, geologists can determine when these rocks formed. For example, geologists now
believe that large-scale volcanic eruptions occurred on the western coast of Africa and
the eastern coast of South America at about the same time, about 180120 million years
ago, because the volcanic rocks from these eruptions are identical in both composition
and age.

Another important observation Wegener made was that scratches made by glaciers moving across rocks, called striations or glacial grooves, exist in regions now close to the equator, where it is too hot for glaciers to form.

Wegener explained this observation by matching the maps of where these rocks were found on different continents. If the continents are reassembled near the south pole, where it is cool enough for glaciers to form, the continents and striations fit together.

Fossil Evidence
Animals and plants that live on different continents can be unique to that continent alone. Bald eagles only live in North America. Wild emus live only in Australia. Because oceans separate continents, these animals cannot travel from one continent to another by natural means. However, fossils of similar organisms have been found on several continents separated by oceans. How did this happen? Wegener argued that these continents must have been connected in the past. 

Fossils are the preserved remnants or evidence of past living organisms. Fossils of a plant called Glossopteris (glahs AHP tur us) have been discovered in rocks from South America, Africa, India, Australia, and Antarctica. These continents are far apart today. The plant's seeds could not have traveled across the vast oceans that separate them.

When these continents were part of Pangaea 225 million years ago, Glossopteris lived in one region. The sediments preserved with these plants grew in a swampy environment. Therefore, the climate of this region, including Antarctica, was different than it is today. Antarctica had a warm and wet climate. The climate had changed drastically from what it was 55 million years earlier when glaciers existed.

Superposition
Geologists can tell rocks were deposited about the same time without modern technology because they used the principle of superposition. Superposition is the principle that in undisturbed rock layers, the oldest rocks are on the bottom.

By comparing a rock layer to the layers above and below it, the relative age can be determined. The relative age of a rock is the age in comparison to other rocks. Fossils can be used to
determine the relative age of a rock layer. Older fossils are small and relatively simple while younger fossils are more complex. This is because as organisms have changed over geologic time, they have become more complex.

Did you know Earth has not always had forests? Some of the oldest fossil trees scientists have found are 386 million years old! They were discovered in an abandoned quarry in New York In 2019. Scientists used the principle of superposition to understand the history of the forest. Scientists found fossilized fish in the layers of rock above the rock layers containing the fossilized forest. Using the law of superposition, scientists were able to support the hypothesis that the forest was flooded, killing trees and preserving roots.

Evidence That Plates Move
For the scientific community to take the hypothesis of continental drift seriously, Wegener needed evidence that the continents had moved and a scientifically sound explanation for how they moved. How could continents move through the solid rocky ocean floor? 

‘Wegener did not explain how continents could move through a solid Earth before his death in 1930. Later scientists built upon his work by mapping the bottom of the sea, finding evidence that the seafloor is moving. They used these findings to revolutionize geology by proposing a more complete theory called plate tectonics. The theory of plate tectonics states that Earth’s surface is divided into large plates of rigid rocks that move with respect to each other. Each plate moves over Earth’s hot and semi-plastic mantle.

Studying the Seafloor
During the late 1940s after World War II, scientists began exploring the seafloor in greater detail. They were able to determine the depth of the ocean using a device called an echo sounder. Once ocean depths were determined, scientists used these data to create a topographic map of the seafloor. 

These new topographic maps of the seafloor revealed that deep continuous valleys and vast mountain ranges stretched for many miles deep below the ocean’s surface. The mountain ranges in the middle of the oceans are called mid-ocean ridges. Mid-ocean ridges are much longer than any mountain range on land. 

Marie Tharp played a major role in mapping the seafloor. She processed the data while her supervisor, Bruce Heezen, and other colleagues collected data at sea. She noticed the continuous low points in the ocean called rift valleys. The rift valleys implied that the seafloor could be spreading apart.

After Tharp created detailed maps of the seafloor, which led to the groundbreaking discovery of these rift valleys and mid ocean ridges, people began to change their minds about the hypothesis of continental drift, and the way scientists understood Earth changed drastically.

Seafloor Spreading
By the 1960s, scientists discovered a new process that helped explain continental drift. This process is called seafloor spreading. Seafloor spreading is the process by which
new oceanic crust forms along @ mid-ocean ridge and older oceanic crust moves away from the ridge. 

When the seafloor spreads, the mantle below melts and forms magma. Because magma is less dense than solid mantle material, it rises through cracks in the crust along the mid- ocean ridge. While magma is in Earth's interior, pressure keeps gases, such as carbon dioxide, dissolved in the magma. As the molten rock moves to the surface, where it is under less pressure, the dissolved gases are released, usually during eruptions or through hydrothermal vents. Pressure from water above usually prevents smaller bubbles from escaping the seafloor. Bubbles like those shown at the beginning of the chapter in shallow waters near volcanoes can be caused by magma that has come close to the surface without erupting, releasing gas through a crack in the crust. Bacteria are also commonly found near volcanic activity. Some bacteria produce gases like methane and release bubbles.

When lava cools and crystallizes on the seafloor, it forms a type of rock called basalt. Because the lava erupts into water, it cools rapidly, and forms rounded structures called pillow lavas. 

As the seafloor continues to spread apart, the older oceanic crust moves away from the mid-ocean ridge. The oldest oceanic crust is only 340 million years old. New oceanic crust is created at mid-ocean ridges, so the youngest rock is found here. Think of the ocean crust as moving like a conveyor belt; the first thing you place on it will end up moving farther and farther away. Scientists argue that if the seafloor spreads, the continents must also be moving.

Magnetic Reversals
Recall that the iron-rich, liquid outer core is like a giant magnet that creates Earth's magnetic field. The direction of the magnetic field is not constant. Today's magnetic field is described as having normal polarity—a state in which magnetized objects, such as compass needles, will orient themselves to point north. A magnetic reversal is the process by which Earth's magnetic field reverses direction. The opposite of normal polarity s called reversed polarity—a state in which magnetized objects would reverse direction and orient themselves to point south. Magnetic reversals occur every few hundred thousand to few million years

Basalt on the seafloor contains iron-rich minerals that can get magnetized. Each mineral acts like a small magnet. Magnetic minerals align themselves with Earth’s magnetic field.When lava erupts along a mid-ocean ridge, it cools and crystallizes. At the time the lava cools, the direction and orientation of Earth's magnetic field is recorded.

Scientists studying basalt on the seafloor used a magnetometer (mag nuh TAH muh tur)to measure and record the magnetic signature of the rocks. These measurements revealed a pattern. Scientists have discovered parallel magnetic stripes that recorded the reversals in the magnetic field's polarity on either side of the mid-ocean ridge. Each pair of stripes has the same age and magnetic orientation. The pairs of magnetic stripes confirm that the ocean crust formed at mid-ocean ridges was carried away from the center of the ridges in opposite directions. This evidence confirmed the hypothesis that the rift valleys identified by Marie Tharp were i fact spreading centers where oceanic crust moved apart.

Tectonic Plates
Recall that the theory of plate tectonics states that Earth’s surface is divided into rigid plates that move relative to one another. These plates are “floating” on top of a hot and semi-plastic mantle. 

Earth’s outermost layers are cold and rigid compared to the layers within Earth’s interior. The cold and rigid outermost rock layer is called the lithosphere. It is made up of the crust and the uppermost mantle. The lithosphere is thin below mid-ocean ridges and thick below continents. Earth’s tectonic plates are large pieces of lithosphere. These lithospheric plates fit together like the pieces of a giant jigsaw puzzle. 

The layer of Earth below the lithosphere is called the asthenosphere (as THEN uh sfihr). This layer is so hot that although it is solid, it behaves like a plastic material. This enables Earth'’s plates to move because the hotter, plastic mantle material beneath them can flow. The interactions between lithosphere and asthenosphere help to explain plate tectonics. 

Plate tectonics has become the unifying theory of geology. It explains the connection between continental drift and the formation and destruction of crust along plate boundaries. It also helps to explain the occurrence of earthquakes, volcanoes, and Mountains.

Plate Motion
The main objection to Wegener's continental drift hypothesis was that he could not explain why or how continents move. 

Evidence of matching fossils, coastlines, mountain ranges, glacial features, and seafloor spreading support his hypothesis. The theory of plate tectonics explains how continents can move and how oceans can be created and destroyed. The theory of plate tectonics explains how Earth has changed over time. As scientists research and learn more about the driving forces behind plate tectonics we learn more about how the Earth has changed over time.

 
Before scientists had enough evidence to accept plate tectonics as a scientific theory, they thought that the continents could not move and that the oceans were unchanging.
Now scientists know that the Earth has changed over time and continues to change and cycle through plate motion. Plate motion leads to the formation of supercontinents and
the breakup of continents.

Convection
Scientists now understand that continents move because the plastic layer within the mantle, called the asthenosphere, moves underneath, moving the rigid rocks in the lithosphere. The lithosphere is the rigid outermost layer of Earth that includes the uppermost mantle and crust. 

You are probably already familiar with the process of convection—the circulation of material caused by differences in density. When the temperature of a material changes, so does the density. For example, hot air balloons have burners to heat the air in the: balloon. When the air inside the balloon is heated, it becomes less dense than its surroundings and rises up. When the air inside the balloon gets far from the burner, it cools down. This air is denser than its surroundings and will begin to sink. The rising and falling of air because of density creates a convection current. 

Plate tectonic activity is related to convection in the mantle. Radioactive elements, such as uranium, thorium, and potassium, heat Earth’s interior. When materials such as solid rock are heated, they expand and become less dense. Hot mantle material rises upward and cools when it approaches the surface of Earth. Thermal energy is transferred from hot mantle material to the colder surface above. As the mantle cools, it becomes denser and then sinks, forming a convection current. These currents in the asthenosphere act like a conveyor belt moving the lithosphere above. 

Did you know that Earth is the only known object in our solar system that currently has plate tectonic activity? When plates move, they collide, separate, or slide past each other along their boundaries. This forms volcanoes, earthquakes, mountains, and mid-ocean ridges. The presence of these features on Earth's surface is evidence that Earth is changing over time.

Basal Drag
Convection currents in the mantle produce a force that causes motion called basal drag. Convection currents in the asthenosphere circulate and drag the lithosphere similar to the way a conveyor belt moves items along at a supermarket checkout. 

Ridge Push
Recall that mid-ocean ridges have greater elevation than the surrounding seafloor. Because mid-ocean ridges are higher, gravity pulls the surrounding rocks down and away from the ridge. Rising mantle material at mid-ocean ridges creates the potential for plates to move away from the ridge with a force called ridge push. Ridge push moves lithosphere in opposite directions away from the mid-ocean ridge. 

Slab Pull
As you read earlier in this lesson, when tectonic plates collide, the denser plate will sink into the mantle along a subduction zone. This plate is called a slab. Because the slab is old and cold, it is denser than the surrounding mantle and will sink. As a slab sinks, it pulls on the rest of the plate with a force called slab pull. Slab pull is thought to be a more significant force than ridge push in moving tectonic plates.