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Earthquakes and Volcanoes: Evidence of Earth's Inner Layers

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  • 0:57 Volcanoes
  • 3:08 S Waves
  • 4:02 P Waves
  • 5:04 Shadow Zone
  • 6:08 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.

The interior layers of the earth are not able to be observed directly, so scientists need to rely on other information to learn about it. S waves, P waves, and magma from volcanoes and earthquakes give a glimpse at the layers of the earth by providing data that can be built into a model of the earth.

What's on the Inside?

Have you ever been on a quest for that perfect melon? We tap it, push on the ends, listen to it, shake it, smell it, and all kinds of things. Why do we do this? We do it because we try to find out what's on the inside. Is it ripe fruit - or something not good or overripe? We try to use our senses to gain as much information as possible about this piece of fruit to determine what it's like on the inside. And hopefully, we are successful. Yum!

One of the first questions that gets asked when people start learning about the five layers of the earth is, 'How do people know what the interior layers of the earth are like if we can't get there to see?' This answer is very similar to searching for the perfect piece of fruit and is the subject of this lesson.

Volcanoes

The amount of silica in the magma affects how volcanoes erupt.
Volcano Eruption Types

While we can't pick up the earth and shake it, and smelling it probably wouldn't help us much, tapping on the earth and listening to the sounds it makes does have some possibility. We can do this by exploring earthquakes and volcanoes! We hear about them and the destruction that they can cause, like Mt. St. Helens, Vesuvius, and Pinatubo.

In a volcano, melted rock called magma breaks through the crust and begins to pile up on the surface, forming any one of a variety of cones. Volcanoes can erupt in one of two ways; either suddenly and violently, like Mt. St. Helens or Pinatubo, or slowly and gradually, like the Hawaiian volcanoes.

One of the factors that determines whether they explode or not is the amount of silica in the magma. The more silica present, the more solid it can become, plugging up the chambers under the ground. Pressure then builds up, causing the catastrophic eruptions seen in Mt. St. Helens, Pinatubo, or Krakatoa. Magma with relatively little silica is much more fluid and will flow more like water as the volcanoes in Hawaii do. Either way, scientists can get a glimpse of the contents of the layers under the earth's crust by studying the debris that erupts from these landforms.

Another theory about why some volcanoes erupt violently and others do not gives us some understanding about the content of gases in the earth. Sometimes, the magma rising to the surface of the earth happens quite fast. When it does, the gas bubbles in the magma begin to expand as the pressure decreases. When the magma reaches the surface, there is a large release of the pent-up gases, causing the volcano to erupt - much like opening a soft drink that has been shaken.

S waves are transverse waves and travel through solids.
S Earthquake Waves

In an eruption that is nonexplosive, scientists think most of the gas has had a chance to be released, as the magma has risen to the surface more slowly. This leaves little gas to be released, and just the magma is emitted from the earth.

S Waves

EarthquakesS wavesshear waves

This would be like shaking the end of a Slinky up and down and seeing the waves travel along it. These waves travel only through solids, so scientists can track where these types of waves show up - and how they are absent in other places. From this information, they can figure out which layers of the earth are solid and which are liquid.

P Waves

P waves are compression waves traveling through solids and liquids.
P Earthquake Waves

Another type of wave is a compression wave, and are also known as P waves, and they are the faster of the two types. P Waves are waves that travel through the layers of the earth by compressing the matter they travel through.

This is similar to how sound travels through the air and would be like taking the end of the stretched Slinky and pushing it towards the other end. The resulting wave is a compression wave that travels through the spring. Compression waves travel between five and eight kilometers per second, or three to five miles per second. They are able to travel through solids and liquids along the surface and through the earth's interior.

These waves actually can travel to the opposite side of the earth by traveling through the earth's layers and be detected by instruments there. Scientists measure how long it takes to reach different places around the earth, and since these waves travel at different speeds through solids and liquids, the structure of the layers can be determined by analyzing the arrival time and location.

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