Peridotites and other ultramafic rocks

Abstract

Peridotites are a group within the category of ultramafic rocks. These are usually dark-coloured rocks rich in magnesium, poor in silica and lacking feldspars. Earth's mantle, which makes up 83 percent by volume and 67 percent by weight of the planet, consists largely of peridotite, which, although relatively sparse at the surface, therefore, is Earth's most abundant rock type. Unfortunately, the mantle, which lies on average at >7 km under the oceans and >35 km under the continents, is not directly accessible for observation and our information comes largely from seismic studies, inclusions in volcanic rocks and sections of the ocean floor which have been tectonically emplaced into the crust: the ophiolite complexes and abyssal peridotites of the oceans. Such rocks also occur when mafic minerals, such as olivine, pyroxene and spinel, accumulate in magma chambers. An unusual occurrence of ultramafic rocks is the lava called komatiite, representing ultramafic liquids, which are largely restricted to Precambrian environments.