Identifying Mafic and Ultramafic Igneous Rocks

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Mafic Igneous Rocks

Should be dark in colour - due to ferromagnesian minerals (pyroxne augite, and sometimes olivine with Ca-rich plagioclase but no quartz)

Low silica magma only 45-52% silica

Low viscosity, so reaches surface, making basalt the most common mafic igneous rock.

Solidifies at high temperatures above 1000 degrees.

Most low silica magma is produced by melting of the Earth's mantle.

Then look at the crystal grain size

fine, vesicular or amygdaloidal or porphyritic or equigranular - basalt

medium, porphyritic or equigranular - dolerite

coarse, equigranular - gabbro

Ultramafic Rocks

Less than 45% silica and consist almost entirely of ferromagnesian minerals.

Most common ultramafic rock is peridotite, forms most of upper mantle

Mainly olivine with some pyroxene and a small amount of Ca-rich feldspar.

Some ultramafic rocks are monomineralic - only consist of one mineral

Have extremely high melting points.

Ultramafic magma does not exist in magma on Earth at the moment, although in the Precambrian ultramafic lavas did exist, indicating that the Earth was hotter than at present. We can find ultramafic rocks from the mantle, where fragments have been brought up to the surface by volcanoes.

The most famous of these are the kimberlite pipes of South Africa. Other peridotites are found as ophiolites in mountain ranges where a slab of oceanic crust has broken off at a subduction zone.

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