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Buoyancy, Temperature, and Core Help

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

Buoyancy

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.

On the Inside

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 On the Inside 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.

Temperature

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.

On the Inside

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.

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