Igneous Textures

Igneous rocks generally consist of interlocking crystals, which gives them a crystalline texture. Pyroclastic rocks will have a fragmental texture as they consist of fragments of glass, crystal and rock, as a result of explosive volcanic activity. Equigranular TextureAn equigranular texture simply means that all the crystals are of equal size but may all be fine, medium or coarse. The relationship of crystal size to cooling is the larger the crystal, the longer the time of cooling: Fine - rapid cooling at surface Medium - slow cooling below surface - hypabyssal Coarse - very slow cooling deep below the surface - plutonic Usually crystal grain size correlates to the depth of cooling but there are exceptions: an intrusion that cooled 2km below the surface will have medium grained crystals but will also have fine grained chilled margins at the edges which cooled rapidly a lava flow that cooled at the surface will have fine grained crystals but if it is very thick then the centre of the flow may be medium grained Vesicular and AmygdaloidalVesicular texture is where gas bubbles are trapped in lava as it cools rapidly, leaving holes where the gas was present. The holes, called vesicles, are usually oval or ellipsoid in shape and are also elongated parallel to direction of flow. The gases rise so most vesicles are found near the top of the lava or sometimes at the edge of an intrusion. Vesicular texture is common in basalt and pumice.Amygdaloidal texture is where vesicles are later infilled by minerals deposited from groundwater. A vesicular rock will be very porous so that groundwater can flow through it. The groundwater contains dissolved minerals such as calcium carbonate, so calcite will be precipitated into the holes. This can happen millions of years after the vesicular rock formed. The most common minerals are calcite or quartz but other minerals can be found. Each infilled hole is called an amygdale, though large ones, partially filled with crystals growing in towards the centre, are called geodes. It is common in basalt.Flow BandingFlow banding occurs where layers of dark and light minerals form due to the separation of minerals within a silicic lava flow. They will be aligned parallel to the flow direction, though they are often contorted because the lava was viscous so flowed slowly. Flow banding is usually seen in rhyolite.PorphyriticA porphyritic texture forms where a rock has two stages of cooling and results in two distinct sizes of crystals. Large crystals are called phenocrysts and form first by cooling slowly. They are surrounded by a finer grained groundmass, which cooled more quickly. Porphyritic texture can be found in any igneous but it is common in porphyritic basalt and porphyritic granite, though each has formed at different depths.In a porphyritic basalt the phenocrysts, often plagioclase feldspar, start to form very slowly at great depths - perhaps 30km. The magma and feldspar crystals move up within the crust by diapiric action to form a batholith at a depth of about 15km. The rest of the magma then cools very slowly to form the coarse grained crystals of the groundmass.

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