Internal ForcesInternal ForcesShaping the EarthShaping the Earth

Plate Tectonics

• As we talked about before, the continents are on a slow ride. This is because they’re riding on the tectonic plates.

• The tectonic plates are the sections of the Earth’s lithosphere (the Earth’s crust and upper mantle) that float around on the molten activities of the mantle

It’s the movement of these plates that causes things like volcanoes, some mountains, and especially earthquakes. There are three types of boundaries that mark plate movements.

1. Divergent: the plates move apart, spreading horizontally. This causes an expansion as the plates move apart and magma comes to the surface

2. Convergent boundary: the plates collide (2 types)

a. Subduction: the plates collide and one plate is forced under the other. Often, the plate that is forced under will melt when it gets deep and hot enough and the resulting magma forces its way back to the surface as a volcano.

b. Collision: The plates collide and buckle and fold. Mountains may result. Take India, for example.

Remember this?

That’s India.

Note that it’s on its own

tectonic plate

Millions of years ago, the thing rampaged north and crashed into Asia. It created the Himalaya Mountains, including Mt. Everest.Even today, the mountains are still growing by a few millimeters per year. They’ve also found ancient ocean fossils in these, the highest points on Earth.

• You get collision instead of subduction when the crust densities of the plates are about the same.

• Subduction happens when the density of one plate is less than the other (usually the oceanic crust subducting under a continental crust)

3. Transform Boundary: When the plates slide past one another

• This one most commonly causes earthquakes (although the other types can as well)

Earthquakes

• The fault is the line at which the plates rub past each other.

• Normally (hopefully), the plates slide past each other steadily and gradually and we don’t notice it.

• Sometimes, though, the crust at the fault sticks, but the plates keep moving. This builds up tremendous pressure on the stuck crusts until they finally unstick and they suddenly move a lot at once. This is when the earthquake happens.

• All that kinetic energy is released at once instead of gradually.

This was a fence crossing over the San Andreas fault in California.

• It used to be in a straight line. After the 1906 quake struck, it offset the fence by 8.5 feet.

Here’s a current fence crossing over the San Andreas fault.

• That bend you see isn’t from a quake. It’s just the natural gradual sliding movement of the fault.

The earthquake’s focus is the point underground at which the quake originates.

• The point on the surface directly above the focus is the epicenter.

Seismographs measure seismic activity (geologic vibrations)

The Richter Scale determines the relative strength of an earthquake.

• To give you an idea of the energy released by an earthquake, the largest thermonuclear device ever detonated measured 7.0 on the Richter and released energy equivalent to 50 megatons (50 million tons) of TNT.

• The 2004 earthquake in the Indian Ocean that caused the tsunami measured 9.0 and released energy equivalent to 30 gigatons (30 BILLION tons) of TNT.

Tsunami

• You should be familiar with this. It’s what happened on 12/26/04.

• An earthquake occurs on the ocean floor or along the coast. The energy travels as waves through the water and across the ocean until it gets to a coast. The shallow region causes the wave to rush over land and flood the place.

This shows the progression of the 2004 tsunami

World here.

• It can also happen from landslides – one in Lituya Bay in Alaska in 1958 caused the largest tsunami on record (despite what your book says). It was 524 meters high. That’s 1,742 feet.

• The Empire State Building is 1,250 feet tall.

• That’s a big wave.

• Some are worried that a massive landslide (some 500 billion tons of rock slipping into the ocean) from the Cumbre Vieja volcano off the west coast of Africa could cause a massive tsunami that will devastate the East Coast of the U.S. Others think the concern is overblown and unwarranted.

• Tsunamis can also be set off by volcanic eruptions, such as the 1883 eruption of Krakatau. Basically, anything that causes massive energy waves through the ocean waters

Volcanoes

• Form from magma beneath the crust coming out.

• Sometimes it’s a gradual flow. Other times, it erupts in a violent explosion, sometimes releasing more energy than nuclear weapons.

• Usually found along tectonic plate boundaries. Most are along the “Ring of Fire” seen on p.37. It follows the plate boundaries.