Tectonic history over 600 million years
Much of the interior of our planet is liquid rock called magma. It is extremely hot, and the magma is in constant motion.
Earth's crust moves around on the magma and has for hundreds of millions of years. The animation to the right shows how tectonic plates have moved around over 600 million years to the present day. Notice that the continents were in different regions at different times. They came together, moved apart, and came together again. Challenge question: How do you think scientists figured this out? |
|
Tectonic Plates
Our planet is made up of several layers, and Earth’s surface is divided into plates.
The top layer is the crust, and the center layer is the core. Between the crust and core is a thick layer of partly molten rock called the mantle. Earth’s crust and the uppermost part of the mantle together make up a layer of solid rock called the lithosphere. The lithosphere is the cool, rigid, outermost layer of Earth. Earth’s lithosphere consists of the crust and upper mantle. The lithosphere is broken into large, thick pieces called tectonic plates. Tectonic plates are massive, irregularly shaped slabs of solid rock. Plate size can vary greatly, from a few hundred to thousands of kilometers across. Plate thickness varies greatly, ranging from less than 15 km for young, oceanic lithosphere, to about 200 km or more for ancient, continental lithosphere (for example, the interior parts of North and South America). |
The map to the right shows the major tectonic plates on the planet.
Throughout history, Earth’s plates have moved great distances, collided, and spread apart. They are still moving today! It is an extremely slow process. Even the fastest plates move about 10 cm per year. Earth’s major tectonic plates include the African plate, Antarctic plate, Eurasian plate, Indian plate, Australian plate, North American plate, South American plate, and Pacific plate. We are going to focus on the North American plate and Pacific plate because those are colliding to form the San Andreas Fault. This is the fault that Los Angeles sits on top of. Can you find them? What makes these plates move? Convection! |
Convection
Convection is the process by which hot material rises and cooler material sinks.
Beneath Earth’s surface, temperature increases with depth. Deep within Earth, the soft material of the mantle heats up and rises toward the crust. As the material gets close to the crust, it cools down. As it cools, it sinks back toward the core. Eventually, the sinking material heats up and rises again. This produces convection currents. How do these massive slabs of solid rock float despite their tremendous weight? The answer lies in the composition of the rocks, or what the rocks are made of. Continental crust is composed of granitic rocks (igneous) which are made up of relatively lightweight minerals such as quartz and feldspar. By contrast, oceanic crust is composed of basaltic rocks, which are much denser and heavier. |
Because continental rocks are much lighter, the crust under the continents is much thicker (as much as 100 km) whereas the crust under the oceans is generally only about 5 km thick. Like icebergs, only the tips of which are visible above water, continents have deep "roots" to support their elevations.
Plate Boundaries
Most of the boundaries between individual plates cannot be seen, because they are hidden beneath the oceans. Oceanic plate boundaries can be mapped from outer space by satellites.
In the map to the right, you can see a line down the middle of the Atlantic Ocean. This is where two plates are moving away from each other, and it's called the "Mid Atlantic Ridge." It is lined with mountain ranges on both sides of the boundary. It is important to know that earthquakes and volcanoes are concentrated near the boundaries of plates. Tectonic plates probably developed very early in Earth's 4.5-billion-year history, and they have been drifting on the surface ever since. |
The plates are moving in different directions and speeds, so when they collide, different things can happen. Because of this, there are 3 types of boundaries you need to know:
Look at the image below and find the different boundaries. What do you notice forms at the convergent boundaries? What happens to the ocean seafloor at a divergent boundary?
- Convergent Plate Boundaries: At convergent plate boundaries, tectonic plates move toward each other.
- Divergent Plate Boundaries: At divergent plate boundaries, tectonic plates move away from each other.
- Transform Plate Boundaries: At transform plate boundaries, tectonic plates slide horizontally past each other in opposite directions.
Look at the image below and find the different boundaries. What do you notice forms at the convergent boundaries? What happens to the ocean seafloor at a divergent boundary?
Transform Boundary: San Andreas Fault
Convergent Boundary: Appalachians & Himalayas
The Epic Himalayas Forming |
The Appalachians & Himalayas Forming |
|
|
Divergent Boundary: The Mid Atlantic Ridge |
How do we know the seafloor spreads? |
|
|