Earth's Structure

As the tectonic plates move they are creating mountains, trenches, islands, volcanoes, and rearranging continents.

Usually rock formations have horizontal layers, because the sediment is deposited over long periods of time.

Vasquez Rocks, [below], in Southern California, shows a different kind of rock formation, one created by plate tectonics.

Over thousands of years, tectonic plates, which are continually and slowly moving, can move great rocks out of the ground, or even rearrange positions of continents. 

Let's examine how this works:

The sim model below, has layers of sand, to represent ancient layers of rock.


Normally, sediment can be moved by wind or water over thousands of years. 

And sediment forms sedimentary rock when it is compacted and hardened by the layers above it.

Watch what happens if we move Earth's surface in our sim model: It forms a bulge that looks like mountains.

 making The movement of the Earth's surface push the sedimentary rock layers towards the surface. Then the layers close to the surface could be exposed, making formations such as Vasquez Rocks [below].

In nature, what actually causes Earth's surface to move? To understand that, we have to learn about the layers of the Earth.

The layer we walk on every day is called the crust. It is made of solid rock. Everything we do, is on this layer. The crust is between 5 to 70 kilometers thick [ 3 to 40 miles], depending on the spot.

Under the crust is the mantle, which is about 100 times thicker than the crust. It is super hot - between 2,000 - 4,000 degrees celsius, depending on the depth. 

The mantle is so hot, because when Earth was formed billions of years ago, many small meteors crashed together... 

...forming one large flaming ball of molten matter, called Earth.

Over billions of years, the Earth has cooled down leaving the surface like it is today.

But inside it is still extremely hot.

The inner most layer is called the core of the Earth, and it's even hotter: over 5,000 degrees celsius. That's almost as hot as the surface of the sun. 

Past scientific studies suggest that the inner most part of the core is solid, because it's made mostly of iron, under intense pressure of all the rock above it. 

So the inside of the Earth being extremely hot rock, and under tons of pressure, is what causes the land to move. This  process is called convection.  

Convection is movement caused by warm fluids rising, and then cooler fluids sinking. Like the lava lamp pictured below.

The light from the lava lamp causes the wax to heat up and rise as it becomes less dense. Then, when it becomes more dense, it cools down and sinks again. And the process continues as long as the bottom stays hot. Just like the convection process on Earth.

On Earth, convection is powered by heat from Earth's core and mantle. And that process moves large pieces of the Earth's crust, called tectonic plates.

Geologists label these plate boundaries into either seven or fifteen major tectonic plates [and there's a bunch more that are smaller]. Vasquez Rocks is marked by the red pin below.

About 20 million years ago, these two plates were crashing together, pushing up rock layers, like the ones seen in Vasquez Rocks, pictured below.

But now those two plates, the Pacific Plate and the North American Plate, are grinding past each other. 

Do these kind of rocks form everywhere their plates meet?

What forms mostly depends on what kind of plate boundary it is.

The one in the Vasquez Rocks area is called the San Andreas Fault, and it has a history of having major earthquakes. It's a transform plate boundary where the Pacific and North American Plates meet. Here, the two plates have been sliding past one another a few millimeters a year.

The map below shows large earthquakes over the last 100 years. Do you notice a pattern to it?

The earthquake locations are forming lines. And if you connect the lines, it looks a lot like the map of the tectonic plates. Most earthquakes happen where plates meet.

As plates move their rough edges lock together, pressure builds and then boom! It jolts forward, causing the ground to shake, which is an earthquake.

And if you look at the map above showing major volcanic eruptions, they also occur mostly on plate boundaries.  

The next type of plate boundary is the convergent plate boundary. That is where tectonic plates crash together. The best example of this is the Himalayan Mountains.

The Himalayan Mountains are formed due to the Indian Plate and the Eurasian Plate converging together for over 50 million years. Scientists call this uplift, and it's still continuing today. 

When a continental plate and an oceanic plate converge together, something interesting happens. Because the oceanic plate is more dense, it sinks down, below the land. We call this: Subduction.

Subduction causes deep trenches to form along the boundary, and a lot of volcanic activity over the land. The wet rock from the oceanic plate is heated in the mantle, and it melts. Then, when this molten rock rises, it erupts into volcanoes. 

We can see this on the coast of South America, in countries like Chile, where lots of volcanic mountains form on the coast because there's a subduction zone.

When two oceanic plates come together, one of them usually subducts below the other. The resulting volcanoes that form, can create chains of volcanic islands, like the Aleutian Islands, off the coast of Alaska, pictured below:

The Divergent Boundary Plates

The divergent boundary plates are when two plates move apart. When this happens underwater, it forms new ocean floor, and takes the shape of a ridge.  Scientists call it a mid-ocean ridge. 

There is a huge mid-ocean ridge in the middle of the Atlantic Ocean, As the plates diverge apart, new ocean floor is forming. The Atlantic Ocean is literally getting bigger. 

When this happens on land, continents can be split apart. The new land forming in between is at sea level, so the ocean is filling in that space.

An example of this is in the northeastern part of Africa, two plates are currently diverging apart. What was once one continent is slowly diverging into two. If all of this is happening, then millions of years ago the continents would have been in different places. 

Why do scientists believe this? Scientists have some impressive evidence to suggest that the current position of the continents was way different millions of years ago, then the way they are portrayed on the map today.

There are species of land animals found only in the two continents that look like puzzle pieces on today's map: South America and Africa. These land animals could have swum across the vast Atlantic Ocean, so scientists believe this to be evidence to suggest that the two continents were once connected. Other evidence resulted from studying rock formations from the two continents, whose similarities were astounding.

They discovered matching layers of rock samples, dated from the same time period. This also suggested that the continents were connected some time in the past. Scientists then needed to show how these continents moved, and this was explained through tectonic plates.

In the ocean between Africa and South America is a divergent boundary which is what pushed the continents apart. 

Scientists knew that the rock furthest from the plate boundary is millions of years old. But the rock very close to the boundary is only thousands of years old. That's evidence that the boundary is producing new rock that is forcing  the continents apart.

Scientists repeated this analysis on all continents and built a model that looks like this:

This coming together and breaking apart of continents seems to repeat in a cycle every 500,000,000 years. So millions of years from now, the map may look completely different than it does now.

What are some careers that you can explore studying tectonic plates? 

Seismologists, volcanologists,