Introduction
Basalt is a fine-grained (mineral crystals cannot be seen in a hand lens, minerals are less than 1mm in diameter) extrusive (crystalises on the earth's surface) igneous rock. The rock has a mafic composition (46%-52% silica) giving it its dark colour. Basalt is also the most abundant rock in the oceanic crust.
Composition
Basalt is a mafic rock and as a result, is mainly composed of dark ferromagnesian minerals which are dense, as a result, basalt has a high density and dark colour. The main minerals are sodium plagioclase, pyroxene and olivine.
Origin and formation
Basalt is often formed during effusive eruptions of shield volcanoes and fissures, which occur on constructive plate margins (tectonic plates move apart) or on hotspots. in an effusive eruption, the lithosphere stretches and thins allowing the asthenosphere to upwell this reduces pressure on ultramafic periodite in the lower lithosphere, consequently, the periodite partially melts forming mafic magmas. This mafic magma rises to the surface and extrudes/erupts and cools very quickly, therefore not giving mineral crystals time to grow this creates a fine-grained texture.
Often basalt is exuded as pillow lava underwater this occurs when the magma's exposed exterior cools faster than the interior, as a result, there is solid skin and a molten interior. When this crystalises there is a glassy or finer exterior and a basalt interior. This often has a vesicular texture which occurs when escaping gas bubbles are preserved during crystallisation. Vesicles can be infilled with ions which exsolve from permeating the water, these build up in layers forming white calcite or quartz crystals, thus forming amygdales and amygdaloidal basalt.
Collumnar jointing
This is very common in basaltic lavas. It occurs when lava flows are over 3m thick wherein the outer parts of flows cool much faster than the centre. Cooling originates at equally spaced ceentres and spreads in all directions from the centres. Contraction then causes tension cracks halfway between the cooling centres. This forms hexagonal columns such as those at giant's causway.
Bibliography
Rocks and minerals the definitive visual guide
Illuminate publishing's OCR geology for A level and AS
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