Use continental shapes and seafloor features as evidence of plate motion

Have you ever looked at a world map and noticed something strange? The east coast of South America looks like it could fit into the west coast of Africa, almost like puzzle pieces. You are not the first person to notice this pattern. Scientists have wondered about this for hundreds of years.

In the early 1900s, a German scientist named Alfred Wegener proposed a bold idea. He said all the continents were once joined together in one giant landmass. He called this supercontinent Pangaea (meaning "all lands" in Greek). Most other scientists laughed at him. They could not figure out what force could move something as big as a continent.

The big question Wegener could not answer was how continents moved. The answer came from the bottom of the ocean. In this lesson, you will use the shapes of continents and features of the seafloor as evidence that Earth's surface is broken into moving pieces called tectonic plates.

Scientists use many types of evidence to support the idea that continents move. The best evidence comes from two main sources: the shapes of continents and the features of the seafloor. Let's explore the key ideas.

The diagram below shows how the continents looked when they were joined as Pangaea about 250 million years ago. It also shows where matching fossils have been found across continents that are now far apart. Notice how the shapes fit together and the fossil zones line up perfectly.

Look at how the eastern edge of South America (green) tucks into the western edge of Africa (yellow). Scientists have tested this fit with computers, and it is very close — especially when you include the edges of the continental shelf (the shallow underwater part of a continent). The matching fossil zones shown by the dashed ovals are a cause-and-effect clue. These animals lived on land and could not swim across an ocean. The simplest explanation is that the land was once connected.

The key mechanism behind plate motion is called seafloor spreading. Deep beneath the ocean, Earth's interior is extremely hot. Hot rock from the mantle (the thick layer below Earth's crust) rises up at mid-ocean ridges. When it reaches the surface, it cools and hardens into new ocean floor.

As new crust forms at the ridge, it pushes older crust outward in both directions. This is like adding new paper to the middle of a conveyor belt. Over millions of years, the seafloor moves away from the ridge and eventually reaches a deep ocean trench. There, the old ocean floor sinks back into the mantle in a process called subduction.

Three Key Seafloor Features

• Mid-ocean ridges — Long underwater mountain chains where new crust is born. The Mid-Atlantic Ridge runs down the middle of the Atlantic Ocean for over 16,000 km.
• Deep ocean trenches — Narrow, deep valleys in the ocean floor where old crust sinks back into the mantle. The Mariana Trench is the deepest point on Earth at about 11,000 m.
• Magnetic stripes — Bands of rock on the seafloor with alternating magnetic directions. They form a mirror-image pattern on each side of a ridge, proving the floor spreads outward.

In the diagram above, you can see the full system. Hot material from the mantle rises at the ridge center. New crust pushes outward in both directions. The magnetic stripes are the "smoking gun" evidence. Each purple band formed when Earth's magnetic field pointed north (normal). Each light band formed when the field flipped and pointed south (reversed). Because the stripes are a perfect mirror image on both sides of the ridge, we know the floor is spreading from the center.

One of the strongest pieces of evidence for plate motion is the pattern of seafloor ages. Scientists collected rock samples from the ocean floor using special drilling ships. They measured the age of the rocks using the process of radiometric dating (a method that uses the natural decay of atoms in rocks to figure out how old they are).

The results showed a very clear pattern. Look at the table below. It shows how the age of ocean floor rock changes as you move away from the Mid-Atlantic Ridge.

Do you see the pattern? The rock is youngest right at the ridge and gets older as you move farther away. This is exactly what you would expect if new rock is being created at the ridge and pushed outward. The crosscutting concept of Patterns helps us here. When scientists see a pattern repeating in data, it is a clue that a process is at work.

Also notice the magnetic polarity column. It flips back and forth between "Normal" and "Reversed." Earth's magnetic field has flipped hundreds of times over millions of years. When hot lava cools at the ridge, tiny magnetic minerals inside it lock in the direction of Earth's field at that moment. That creates the alternating magnetic stripes we see on the seafloor.

Scientists can actually calculate how fast tectonic plates move. They use the age of the seafloor and the distance from the ridge. Let's try a real example using the data from the Mid-Atlantic Ridge.

Scientists use many different lines of evidence to support plate tectonics. Each type has strengths and limitations. The table below compares the main types of evidence you have learned about.

The evidence you've studied in this lesson is the foundation of plate tectonics — one of the biggest ideas in all of science. In high school and beyond, you will learn even more about this topic. Here's a preview of where these ideas lead.

The crosscutting concept of Systems and System Models is very important here. Earth's surface is a system. The plates, the mantle, the ridges, and the trenches all interact. Scientists build models of this system to predict future plate positions. In about 250 million years, the continents may come together again to form a new supercontinent!