Reading_ Inside Earth (2023)

pdf

School

Harvard University *

*We aren’t endorsed by this school

Course

180

Subject

Geology

Date

Oct 30, 2023

Type

pdf

Pages

Uploaded by AmbassadorGiraffeMaster852

IS2T3 Inside Earth Reading [Figure 1] Vasquez Rocks have taken shape over 25 million years, erected through the violent, but slow, tectonic forces of two continental plates crashing into one another. Introduction Before you can learn about plate tectonics, you need to know something about the layers that are found inside Earth. These layers are divided by composition into core, mantle, and crust or by mechanical properties into lithosphere and asthenosphere. Scientists use information from earthquakes and computer modeling to learn about Earth’s interior. Exploring Earth’s Interior How deep can we go into Earth's interior? Not very deep, that's for sure! The deepest a drill hole has gotten was the Kola Superdeep Borehole. That hole got to 40,230 feet (12,262 m), about one-third of the way into the crust in that area. So learning about what's deeper requires less direct methods. A few of these methods will be described in this concept. How do scientists know what is inside the Earth? We don't have direct evidence! Rocks yield some clues, but they only reveal information about the outer crust. In rare instances, a mineral, such as a diamond, comes to the surface from deeper down in the crust or the mantle. To learn about Earth's interior, scientists use energy to “see” the different layers of the Earth, just like doctors can use an MRI, CT scan, or x-ray to see inside our bodies. Seismic Waves One ingenious way scientists learn about Earth’s interior is by looking at how energy travels from the point of an earthquake. These are seismic waves ( Figure below). Seismic waves travel outward in all directions from where the ground breaks at an earthquake. These waves are picked up by seismographs around the world. Two types of seismic waves are most useful for learning about Earth’s interior. Geologists study earthquake waves to “see” Earth's interior. Waves of energy radiate out from an earthquake’s focus. These waves are called seismic waves ( Figure 2). Seismic waves go different speeds through different materials. They change speed when they go from one type of material to another. This causes them to bend. Some seismic waves do not travel through liquids or gasses. They just stop. Scientists use information from seismic waves to understand what makes up the Earth’s interior.

● P-waves (primary waves) are fastest, traveling at about 6 to 7 kilometers (about 4 miles) per second, so they arrive first at the seismometer. P-waves move in a compression/expansion type motion, squeezing and unsqueezing earth materials as they travel. This produces a change in volume for the material. P-waves bend slightly when they travel from one layer into another. Seismic waves move faster through denser or more rigid material. As P-waves encounter the liquid outer core, which is less rigid than the mantle, they slow down. This makes the P-waves arrive later and further away than would be expected. The result is a P-wave shadow zone. No P-waves are picked up at seismographs 104° to 140° from the earthquake's focus. [Figure 1] How P-waves travel through Earth’s interior. ● S-waves (secondary waves) are about half as fast as P-waves, traveling at about 3.5 km (2 miles) per second, and arrive second at seismographs. S-waves move in an up and down motion perpendicular to the direction of wave travel. This produces a change in shape for the earth materials they move through. Only solids resist a change in shape, so S-waves are only able to propagate through solids. S-waves cannot travel through liquid. By tracking seismic waves, scientists have learned what makes up the planet’s interior ( Figure 2 below). ● P-waves slow down at the mantle core boundary, so we know the outer core is less rigid than the mantle. ● S-waves disappear at the mantle core boundary, so the outer core is liquid.

[Figure 2] Letters describe the path of an individual P-wave or S-wave. Waves traveling through the core take on the letter K. Other Clues about Earth’s Interior 1. Earth’s overall density is higher than the density of crustal rocks, so the core must be made of something dense, like metal. 2. For there to be a magnetic field, there must be liquid metal. The metal must be convecting. If the core did not have convecting metal, there would be no magnetic field. Iron and nickel are both magnetic. 3. Scientists study meteorites to learn about Earth’s interior. Meteorites formed in the early solar system. These objects represent early solar system materials ( Figure below). Some meteorites are made of iron and nickel. They are thought to be very similar to Earth's core. An iron meteorite is the closest thing to a sample of the core that scientists can hold in their hands! [Figure 3] This meteorite contains silica minerals and iron-nickel. The material is like the boundary between Earth's core and mantle. The meteorite is 4.2 billion years old.

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version