Direct Evidence for the Structure of the Earth

Mines and BoreholesWe have direct access to higher levels of the crust by means of mines for coal, metal ores and diamonds. Problems of drainage, ventilation, high working temperatures and lifting the ore to the suface limit the depth of a mine. The deepest mines, for gold in South Africa, penetrate nearly 4km of the crust.Boreholes go deeper. A Russian research borehole in Siberia reached a depth of almost 13km. The deepest oil well penetrates 10km into the crust, the depth below which oil is not found. Samples of rocks and microfossils can be brought up from boreholes and remote sensing undertaken.

Geothermal GradientThe geothermal gradient is the rate of increase in temperature per unit depth in the Earth.In deep horeholes and mines the temperature of rock increases with depth and can cause serious problems. The average geothermal gradient of the Earth is about 25C/km.However in volcanically active areas, the temperature may increase by about 30 to 50C/km. If the rate of temperature change were constant, temperatures deep in the Earth would soon reach the point where all known rocks would melt. However, we know that the Earth's mantle is solid because S waves pass through it. The temperature gradient is lower in the mantle for two reasons: radioactive heat production is concentrated within the crust of the Earth, where radioactive elements such as uranium are found the mechanism of thermal transport changes from conduction within the rigid lithosphere plates, to convection in the asthenosphere and upper mantle, and this transports the heat differently

Volcanoes Bringing Magma from DepthThe magma that feeds volcanoes through vents originates in the lower crust or upper mantle and so carries up samples of the rocks from these layers. Basalt lavas which erupt at mid-ocean ridges are formed by partial melting of the upper mantle. We can estimate the composition of the upper mantly from the chemical analysis of these basalts and their volatiles.Occasionally, the igneous material in a volcanic pipe includes diamonds and other minerals, which have a compact crystal structure. Diamonds crystallise under the high pressure conditions of the upper mantle, at depth of up to 250km. The enclosing igneous rock called kimberlite after the Kimberley diamond mine in South Africa, also includes xenoliths of peridotite. The magma carries up fragments of country rock torn from the walls of these very deep volcanic vents. Kimberlite pipes are the result of explosive volcanism from deep mantle sources. Within 2km of the surface, the highly pressured magma explodes upwards and expands to form a concical vent a few hundred metres to a kilometre across.

Ophiolite Suites - Ocean Crust on LandDuring the collisions of plates, sections of oceanic crust may be broken off a descending oceanic plate and thrust onto the edge of the continental plate, instead of being carried down into the mantle. They may then be exposed by erosion. The section of ancient oceanic crust can be examined on land without the need to drill a borehole through the ocean floor. The peridotite at the base of an ophiolite sequence is returned to its original underformed orientation, its total thickness is usally about 7km, the thickness of the oceanic crust.Ophiolites can be seen in the Lizard Peninsula in Southwest England and the Troodoes Massif in Cyprus. These ophiolite sections are millions of years old, but they are similar to the modern-day oceanic crust and upper mantle.

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