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Buoyancy, Temperature, and Core Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 31, 2011

Core

Geologists cannot collect core samples and study the Earth’s interior directly, so much of their information has been gathered from observations and clues from other sources. When the Earth’s magnetism is measured, a variety of readings at different locations around the globe show a mixture of mass types within the planet.

The composition of meteorites gives scientists even more clues to the inner earth. These chunks of original matter from which the galaxy and solar system were formed continue to fall from space. Most burn up in the atmosphere because of the intense heat and friction, but a few larger chunks make it to Earth in one piece. There are two main types of meteorites: stony meteorites and iron meteorites. The stony meteorites are a lot like the mantle of the Earth, while the iron meteorites are more like the core of the planet.

The way seismic waves travel through the Earth are probably used the most to figure out how the Earth is put together to its core.

Seismic tremors or waves are made or related to the vibrations of the Earth. They are caused by earthquakes and other activities going on in the Earth’s interior.

Geologists report that seismic waves show a major change in the way they travel and the material they travel through at a depth of 2900 km (1800 miles). The sudden shift points to the fact that the makeup of the Earth’s inside changes at that depth. This is called the core–mantle boundary. Think of it like a peach with an outer skin, the fleshy fruit, and the woody pit. The fruit and the pit don’t slowly morph into each other. The fruit doesn’t gradually get tougher and harder until you reach the center, but changes abruptly from soft to hard.

But scientists don’t have to wait around for an earthquake to test seismic activity, they can produce seismic waves with explosions or large vibrating machines on carrier vehicles. Then when the explosions or vibrations begin, they measure the shock waves with special recording equipment called geophones , and then analyze the waves with computers to give complex pictures of how the wave patterns act. Results show how shock waves bounce off different layers within the crust and give geologists an idea of what a particular layer might be made of. For example, the speed of the waves would reveal whether a layer was solid or molten.

Seismic waves are known to travel slower through liquid than solid matter. Just as it is harder to drag your hand through water, compared to air, seismic waves go more slowly when traveling through liquid rock, compared to solid rock.

Using this knowledge, scientists found that seismic waves slow down when passing through the outer core, but speed back up when passing through the inner core. In fact, waves that don’t normally pass through liquids at all are also blocked by the outer layer of the core. So scientists became fairly sure that the outer core is liquid or molten , rather than solid.

Molten rock is found at the innermost core of the Earth and in “hot spots” around the globe where internal pressures force it to the surface.

Waves change in strength according to the distance from their source and the types of matter they pass through. Seismic wave strength and behavior show density, movement, location, fluidity, and boundaries of different Earth layers.

Further evidence of a molten outer core is gathered from temperature readings. Miners found that rocks buried below the surface in the deepest shafts were hotter than those nearer the surface. When tested, shaft temperatures increased as depth increased. This excited scientists who had been puzzling over volcanoes, hot springs, and other geothermal sites for centuries.

We will study earthquakes more thoroughly, but data shows that the core is made up of two major parts, the outer core, thought to be liquid, and the inner core, thought to be solid. The solid inner core is estimated at roughly 85% iron with small amounts of nickel, silicon, and cobalt. No one knows for sure how the Earth’s core is layered because there is no way to drill to the center of the planet, but scientists continue to investigate with seismic testing. We saw in Table 3-1 that there are many different elements present in the continental versus oceanic crust.

Practice problems of this concept can be found at: Earth's Structure Practice Test

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