Buoyancy, Temperature, and Core Help (page 2)
When geologists studied earthquake data further, they found that the crust was thinnest under the oceans and thickest below high mountain ranges. It was the thickest at the highest elevations and thinnest at low elevations. In other words, the Moho crustal boundary provides a mirror image of the crust above it. Figure 3-4 shows this mirroring of surface features.
Fig. 3-4. Granite and basalt densities in the continental and oceanic crusts are different.
This mirroring is based on buoyancy. The less-dense crust is floating on the pliable asthenosphere layer. Since buoyancy depends on thickness and density, the Moho boundary effect is a lot like that of an iceberg floating above the surface of the ocean. Icebergs float with only 10% of their volume showing above the waves. The density of water is 1.0, while the density of ice is 0.9 (because of the air trapped in the frozen water). The “tip of the iceberg” happens because ice is 10% less dense than water and 90% then, of the iceberg’s volume is below the surface.
The continental crust is thicker under high mountain ranges to balance the floating “tip” above the land surface. Some geologists estimate that the depth to which a mountain’s “foot” descends into the denser mantle is about times the elevation of the mountain above. If this is true, then Mount Everest which stands about 8 km high must be supported by a crustal foot that reaches nearly 36 km, in addition to the 35 km of existing continental crust.
The oceanic crust is much thinner with few thickened spots. This is because it is made up mostly of mafic minerals that are heavy in iron and magnesium compared to the continental crust made up of mostly felsic minerals, richer in lighter, aluminum-bearing silicates.
Much of the Earth is made up of two pairs of elevations. One pairing is between 1000 m and sea level and the other pairing drops from sea level to 4000–5000 m below sea level. The first pairing includes the crustal continental platforms while the second pairing describes the abyssal oceanic plains. The balance between these two layers overlying the mantle allows density equilibrium to be maintained. The thickness gradient then allows for continental mountains and ocean basins. Ocean basins are low spots where water gathers, but flows across the lithosphere. Continental shelves create a gradual boundary into the oceans.
Depth tests of mine shafts found that for every 60 feet drilled deeper into the Earth’s surface, the temperature increased by one degree Fahrenheit.
1° Fahrenheit ↑ /60 feet ↓ in depth
The deepest shafts drilled into the Earth have been to a depth of about 13 km, but this is just a tiny prick compared to the total depth of the mantle. The entire mantle of the Earth is about 2897 kilometers (1800 miles) thick. Figure 3-5 gives an idea of the size comparison of different layers of the Earth and their incredible depths.
Fig. 3-5. The different layers of the Earth are hundreds of meters thick.
Core samples are rock layer samples taken by drilling or boring at different depths of the mantle and bringing long cylinders of rock.
Bore holes and core samples are important in other ways. They give us information on the layering of the mantle as well as its makeup. Samples can be analyzed for their content and percentages of different elements. Just like a core sample from a tree, a rock core sample shows different growth (or sedimentation) and composition patterns.
Electrical instruments that measure conductivity can also be used to take a look at the electrical properties of different layers of core samples. A sonic generator can be eased into a bore hole to provide a sound source to measure acoustic variations. Other sensors can be used to detect naturally occurring radioactivity levels of different elements in the layers of the crust. A combination of research tools are used individually and in combination with others to tell the overall picture of an area’s geological profile.
For every kilometer drilled into the Earth, the temperature increases along a thermal gradient between 15 and 75°C depending on location. Just as the temperatures at the center of the Earth are extreme, the pressures are equally as intense. Temperatures have been estimated to be as great as 6000°C, with crushing pressures of 300 million kilonewtons per square meter or about three million atmospheres within the core. The size of the Earth allows a huge amount of energy to be stored within it as heat. The original heat of the planet is maintained by the constantly produced transformation, generation, and release of energy from radioactive elements.
Anyone traveling to the Earth’s core would require vehicles found in science fiction that could withstand the intense heat and pressure. Otherwise, they would be fried and flattened like pancakes, not a good end for someone wanting to satisfy their scientific curiosity.
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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