Volcanoes Bringing Magma from Depth
The 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.

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Undergraduate Geology - Part 1 (Earth Structure) Notiz am Direct Evidence for the Structure of the Earth, erstellt von siobhan.quirk am 10/05/2013.