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Plate Tectonics: A Unified Theory for Change of the Earth's Surface

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  1. 0:07 Background on the Ground
  2. 0:32 Definition of Plate Tectonics
  3. 1:39 Causes of Plate Movement
  4. 4:10 More Causes of Plate Movement
  5. 5:21 Lesson Summary
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Taught by

Peter Jaeger

Pete currently teaches middle school Science, college level introductory Science, and has a master's degree in Environmental Education.

After many years of trying to solve the mystery of the moving continents, enough data and evidence was collected to develop a unifying theory of how the surface of the earth changes. It's called plate tectonics.

Background on the Ground

Have you ever been babysitting and decided it would be fun to play Nerf Gun Wars with the child, only to have a misaimed dart hit a rare plate from Israel and fall on the brick fireplace hearth, breaking into many pieces? Not that I have ever done this or anything... The only good thing is that the pieces of the plate fit together nicely and allowed a stressed-out babysitter to glue it back together. But what does this have to with the earth?

Definition of Plate Tectonics

In the middle of the 20th century, just a few decades ago, a new theory about the Earth and how it changed was proposed, and the mystery of drifting continents was solved. Geologists, scientists who study the earth, now had an all-encompassing theory to tie all of its observations and data together. This new theory of large-scale change on the earth is known as plate tectonics.

Plate tectonics is the theory that the surface of the earth is broken into larger pieces of crust called plates that ride along on a softer layer of the earth, known as the asthenosphere, which is the upper part of the mantle.

A diagram of the plates moving along the surface of the earth
Plate Techtonics Diagram

Each plate's thickness depends on the type of crust it is made of and the location of the plate. Continental plates can be up to 150 km thick, whereas oceanic plates usually average about 5-10 km thick. Also, plate boundaries are not the same as continental boundaries, as the edges of the plates can be underwater. For example, the North American plate extends from the middle of the Atlantic Ocean to the west coast of North America.

Causes of Plate Movement

So, the big question that remains is: how do the continents move? The answer is related to the layers of the earth and how they interact with each other. So, first, we need to understand how the properties determine the layers of the earth. Let's use an apple as an example. An apple has 3 main layers to it - the peel, the fruit, and the core.

The crust, or lithosphere, is the hard outer layer of the earth and would be the peel of our apple. This is the part where we reside and about which we have the most data.

The mantle is the liquid middle layer of the earth, consisting mostly of oxygen, silicon, magnesium, and iron. This would be represented by the fruit of the apple between the peel and the core. The outermost part of the mantle is slightly solid, like taffy, and is known as the asthenosphere.

The core is the center of the earth that is represented by the core of the apple. The earth's core consists of two parts - a solid center about 1200 km thick, surrounded by a liquid layer consisting of iron and nickel that is about 2300 km thick.

The main theory of plate movement states that heat from the core causes convection cells in the mantle that move the plates as they ride on the mantle. The main source of heat that drives this process is thought to be the radioactive decay of uranium and other elements that give up their energy as heat as they break down. All this heat softens the rock enough that it will begin to flow. But how does this happen? When you put a pot of water on the stove to boil, the water nearest the stove heats up faster than the water on the surface of the pot.

The plates move when the mantle is heated by the core of the earth.
Location of Mantle

The hotter water tends to move toward cooler areas and, as it does this, a current of hot water rising and cooler water sinking begins to form. This is known as a convection current. Using this model, the stove is like the core and the water is the liquid mantle that rotates. Geologists think that this same phenomenon is what is happening inside the Earth. Liquid rock near the mantle is heated and rises toward the crust. The rock near the surface is cooler and sinks back down toward the core. This forms the same type of convection current which causes the plates to move. Scientists believe that this cycle takes thousands of years to complete.

More Causes of Plate Movement

At the top of the mantle, the convection currents encounter the thin crust, causing it to move. As this movement occurs, the plates smash into each other, slide past each other, or are pushed under another plate. This may also produce secondary events that cause the plates to move.

In ridge push, plates that are higher at the spreading center flow downhill and eventually flatten out to the ocean floor. Gravity causes this flowing down the ridge and may give the plate a slight push along as new crust is forced up at the fault, causing the rest to move out of the way. This is like those coin bulldozers at the fair where one new coin may push forward and knock others out of the way!

As the plate is pushed along, it may run into another plate. Oceanic crust is easily forced under another plate and back into the mantle. These converging boundaries can be identified by deep ocean trenches that mark the location where one plate is sliding under another one. As it slides under the other plate and is forced back into the mantle, gravity again works to pull it along, giving the plate another force to keep it moving.

Ocean trenches are created when oceanic plates slide under other plates.
Oceanic Plates Go Under Plates

Lesson Summary

Plate tectonics is the theory that unified all the puzzling data from early observations, accounted for the movement of the plates of the earth, and gave a mechanism for how the plates actually moved around. The theory stated that the plates moved around on top of the mantle driven by convection currents caused by heat from the earth's core. This theory has been supported since then by the advance of technologies that allow scientists to actually measure the movement of the plates in centimeters per year. Plate tectonics is the current accepted theory that describes how the face of the earth can change over time.

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