The basic idea behind plate tectonics is that there are eight major plates on the surface of the Earth. There are also bunches of minor plates. The plates are like the skin of the planet. They constantly move around the planet.
- When we say constantly moving, we're talking centimeters each year. You couldn't sit down and watch it happen. Or can you? You could watch it happen if you watched an earthquake.
As new crust is created in a particular location on Earth, it forms what resembles giant plates. One side of the plate is where new crust is being created, while the other side is where older crust is being destroyed.
Using
modern equipment, scientists known as oceanographers have been able to measure
and map out the ocean floor. What these scientists have discovered has helped
explain how it is that continents are able to move around on the Earth’s crust.
Located
deep beneath the waves on the ocean floor almost exactly halfway between the
continents are raised areas known as ridges. These ridges are similar to
under-water mountain ranges. At other locations we find extremely deep
trenches, some reaching many thousands of feet in depth.
The
amount of time that it takes for crust to be created and later destroyed is
approximately 100 million years. Thus, most crust has a lifetime of around 100
million years.
Because
continents do not fall back into the Earth’s mantle, they survive much longer.
Many parts of the continents we see today are almost as old as the Earth
itself.
Geologists
refer to this process as plate tectonics. As we study plate tectonics, a
picture emerges of very old continents riding on top of much younger and ever
moving plates. These plates move extremely slowly, at a rate of only about 10
cm per year.
PLATE TECTONICS
| Type of Crust | Average Thickness | Average Age | Major Component |
|---|---|---|---|
| Continental Crust | 20-80 kilometers | 3 billion years | Granite |
| Oceanic Crust | 10 kilometers | Generally 70 to 100 million years old | Basalt |
The theory of plate tectonics (meaning "plate structure") was developed in the 1960's. This theory explains the movement of the Earth's plates (which has since been documented scientifically) and also explains the cause of earthquakes, volcanoes, oceanic trenches, mountain range formation, and many other geologic phenomenon.
The plates are moving at a speed that has been estimated at 1 to 10 cm per year. Most of the Earth's seismic activity (volcanoes and earthquakes) occurs at the plate boundaries as they interact.
UPPER MANTLE
- Lithosphere (Sial and Sima)
- Asthenosphere (Molten Rock River)
- Mesosphere
LOWER MANTLE
OUTER CORE (Liquid Iron and Nickel)
INNER CORE (Solid Iron Compounds)
The top layer of the Earth's surface is called the crust (it lies on top of the plates). Oceanic crust (the thin crust under the oceans) is thinner and denser than continental crust.
Crust is constantly being created and destroyed; oceanic crust is more active than continental crust.
The CRUST
- It is very thin in comparison to the other three layers. The crust is only about 3-5 miles (8 kilometers) thick under the oceans(oceanic crust) and about 25 miles (32 kilometers) thick under the continents (continental crust).
- The crust of the Earth is broken into many pieces called plates. The plates "float" on the soft, plastic mantle which is located below the crust.
- These plates usually move along smoothly but sometimes they stick and build up pressure. The pressure builds and the rock bends until it snaps. When this occurs an Earthquake is the result!
Under the crust is the rocky mantle, which is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium.
- Think of the molten rock in the
- The upper mantle is rigid and is part of the lithosphere - These plates make up the top layer of the Earth
- Directly under that layer is the asthenosphere.(together with the crust). The lower mantle flows slowly, at a rate of a few centimeters per year. The asthenosphere is a part of the upper mantle that exhibits plastic properties. It is located below the lithosphere (the crust and upper mantle), between about 100 and 250 kilometers deep.
- It's a flowing area of molten rock. There is constant heat and radiation given off from the center of the Earth. That energy is what constantly heats the rocks and melts them. The tectonic plates are floating on top of the molten rock and moving around the planet. Think of it as ice floating at the top of your soda. When the continents and plates move it's called continental drift.
- asthenosphere, not as rock, but as a liquid. It has currents and it flows just like any other liquid. When the floating plates spread apart, it's called a spreading center.
- When they are moving together, it's called a subduction zone.
- When they are forced together, it is called a zone of convergence. One of the plates usually moves under the other in a zone of convergence. As the plate moves down into the asthenosphere it begins to melt. The place where they meet has a crack or a trench. Some of the deepest parts of the oceans are these trenches. .
TYPES OF PLATE MOVEMENT:
Divergence, Convergence, and Lateral Slipping
At the boundaries of the plates, various deformations occur as the plates interact; they separate from one another (seafloor spreading), collide (forming mountain ranges), slip past one another (subduction zones, in which plates undergo destruction and remelting), and slip laterally.
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PLATE TECTONICS - LAUNCH INTERACTIVE #2
- complete the module
- click on the following topics in the box on the left side of the module "DEEPER DISCOVERY",
- PLATE TECTONICS > INTERACTIVE
- BREAK UP OF PANGAEA
- PLATE BOUNDARIES
- PLATE MOVEMENTS
- PLATE TECTONICS QUIZ #2 - add score to study guide notebook paper page.
Scientific Evidence
How do we back up these ideas? Scientists have traveled all over the Earth and found evidence that supports the ideas of plate tectonics. First, they looked at the continents. Ever notice how Africa and South America look like they could fit together? Scientists did. They cut up a map, moved the continents close together, and came up with a huge landmass called Pangaea (one super-continent).Scientists also looked at the fossils (long-dead animal bones and plants) on the different continents. They found that fossils on Australia were similar to the ones in Southern Asia. They think the same plants once lived on the continents, but when they split apart, new plants developed. When they were digging, they also looked at the types of rocks. The West Coast of Africa has very similar rock formations to those on the East Coast of South America. They are too similar to be a coincidence.
PLATE & EARTHQUAKE INTERACTIVE #3
WORDS TO KNOW:
- oceanographers
- ridges
- lithosphere
- asthenosphere
- continental crust
- oceanic crust
- transform plate boundary
- plate tectonic
- seafloor spreading
- lateral slipping plate
- convergent plate
- subduction zones
- zone of divergence
- lateral slipping plate movement
- crust
