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The Dynamic Earth: Internal & External Forces that Shape Earth's Surface

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  • 1:04 Earthquakes
  • 1:56 Earth's Inner Layers
  • 3:14 Tsunamis
  • 4:55 Volcanoes and Hot Spots
  • 6:40 Weathering and Deposition
  • 7:37 Lesson Summary
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Lesson Transcript
Instructor: Peter Jaeger

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

There are many geologic processes that shape the surface of the Earth. Not only do they build and destroy a variety of landscapes, but they also give us clues to the interior of the Earth and the forces that exist there.

The Dynamic Earth

If you have ever been to Hawai'i, you may have had the opportunity to snorkel. People are attracted to this activity because it gives you a small glimpse of the interesting features of the Earth that we often miss out on by living above water. Identifying these features both underwater and on land can give us some clues as to how the lithosphere, or the Earth's crust, changes.

Hawai'i is a great place to observe these changes because, unlike many places, they all can occur there. Just like snorkeling gives us a glimpse of what happens under the waves, volcanoes and earthquakes can give us glimpses of what is happening under the lithosphere, inside the Earth. Volcanoes and earthquakes are natural disasters that can radically transform the landscape of the Earth in just a few moments. This, along with the ongoing breakdown of the surface of the Earth by weather, produces profound changes over time over the world.

When the Earth cracks from stress, energy is released and causes ground shaking.
Earth Cracking Releases Energy

Earthquakes

Earthquakes occur when stress builds up in the Earth's crust and causes it to break in a line known as a fault. A fault is a crack in the Earth's crust caused by stress in the surrounding rocks. Stress builds up in the crust for several reasons. Most commonly, pieces of the Earth push and grind past each other, and stress builds up as the pieces get stuck on each other. Rocks store energy when the pressure changes as new sediment is deposited in layers on the Earth's crust.

The Earth's surface also may store energy because of contraction and expansion as the crust heats and cools. As the stress builds, eventually the rock reaches its breaking point. When it fractures, energy is released in waves over the crust's surface and down into the Earth. These waves are what cause the shaking of the ground, much like the waves felt on a waterbed.

Earth's Inner Layers

Earthquakes show us how the surface of the Earth is moving, but did you know that measuring earthquake waves can give us information about the interior of the Earth? Seismologists, scientists who study earthquakes and related phenomenon, have discovered a way to use earthquake waves to deduce the structure of the Earth's interior. Geologists cannot directly study anything more than the outermost 5-8 miles of the crust, leaving a vast majority of the Earth unobserved.

Scientists have defined the five layers of the inner Earth by studying earthquake waves.
Earth Inner Layers Diagram

However, they have found that after an earthquake, waves of energy travel not only through the rocks of the Earth's surface but also through the center of the Earth. The waves that pass through the Earth travel at different speeds depending on the rock type, its temperature, and its pressure. Following an earthquake, scientists at stations all around the Earth record the time and intensity of the waves that arrive at their location. Using information about the arrival and behavior of the waves, scientists can deduce a picture of the different layers of the Earth's interior. This is how scientists have developed a 5-layer model of the Earth consisting of a solid inner core, a liquid outer core, a liquid mantle, a taffy-like upper layer, all underneath the outer crust.

Tsunamis

Tidal waves, or tsunamis (as shown in this animation), are also the result of earthquakes, and they can affect land, including the islands of Hawai'i. Tsunamis can form in a couple of ways. One, an earthquake could trigger a massive landslide near a body of water. This landslide entering the water could trigger a large wave similar to the waves made when someone jumps in the pool. The mass of the material entering the water triggers the wave.

Tsunamis are created by earthquakes.
Image of earthquake creating tsunami

Underwater landslides or volcanic eruptions can also cause tsunamis. Tsunamis can also be created by earthquakes that happen underwater. Fault lines exist under the ocean, just like on land. Sometimes, earthquakes cause one of the plates to move violently upward. This also causes all the water above that fault line to push up, triggering the start of a tsunami. You might think that it seems unlikely that this can cause a gigantic wave, but imagine a 100-mile-long piece of crust being suddenly forced up 10 feet. The amount of water displaced by this event is easily enough to begin the wave.

Earthquakes that occur underwater can often cause tsunami waves.
Earth Movement Causes Tsunamis

Out in the open ocean, the tsunami may not look like much, usually having a long, low crest, but these waves can move quite fast, up to 500 miles per hour. As the wave reaches the shoreline, it begins to slow down and pile up on itself. Initially, the shoreline water of the ocean may recede from the shore as it gets pulled into the wave itself. Tsunamis also may occur in several waves, increasing the danger of the event. Tsunamis have so much energy that they push up on land like a bulldozer, causing massive destruction.

Volcanoes and Hot Spots

Volcanoes are formed one of three ways. First, magma can ooze out of the ground when the lithosphere pulls apart. These are often non-explosive volcanoes that form where two pieces of the Earth are pulling apart. Second, volcanoes can erupt explosively, usually the result of a piece of the Earth's crust being forced back into the mantle. The piece of crust re-melts and moves to the surface to cause the eruption. The third way a volcano can be formed is called a hot spot.

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