Igneous Rocks

The Syllabus and the Details...

There are 3 igneous rocks [gabbro, basalt, granite] that you have to be able to recognise...

Crystal Size in igneous rocks is related to the cooling rate of the magma they were formed from. The faster they cool, the smaller the crystals. Compare the pictures of basalt (top) which cooled quickly, and gabbro (below) which cooled slowly.

Volcano Types

You need to know how magma viscosity affects the type of volcanic activity and the shape of the volcano - the differences between relatively passive [fissure] volcanoes...

and violent eruptions [central vent] volcanoes...

You need to be able to recognise these volcano shapes from diagrams and photographs. You also need to be able to explain them; less viscous (basalt) lava from fissure volcanoes runs easily so it makes a flatter shape, more viscous (rhyolite) lava doesn't run well so it piles up where it is erupted.

Intrusive Igneous Features

You need to know how the following igneous bodies can be distinguished by their structures and field relationships...

Lava Flows [tabular form, columnar jointing, pillow lava (see this link for a great film looking at how pillow lavas form).

You'll need to know that lavas can be either...

Conformable - a lava flow on top of sedimentary beds followed by another period of sedimentation or...

Unconformable - a lava flow cutting across other beds that have been tilted and/or eroded.

The rock under a lava flow will have a baked margin where the heat from the lava caused local metamorphism. The rock above the lava won't have this - it wasn't there when the lava was erupted.

Sills are intrusions of magma between the beds of previously laid down rock. The dark grey strip of dolerite in the picture below is a classic example...

Both the rock above and below a sill will have a baked margin as both were there when the rock was intruded. The edges of the sill itself will show chilled margins, areas of smaller crystals caused by the magma cooling quickly on contact with the surrounding rock.

Dykes are formed when magma is intruded into a crack that cuts across the beds of other rocks. The dark rock of the basalt dykes in the picture below can be seen to be at right angles to the surrounding sedimentary beds. The rock on both sides of a dyke will show baked margins. The edges of the dyke will have chilled margins.

Plutons are large bodies that form from cooling magma chambers deep underground. Their edges may show chilled margins, the surrounding rock becomes metamorphosed to show contact metamorphism as opposed to just a baked margin.

The diagram below shows how plutons, dykes and sills occur in the ground.

You need to be able to identify the characteristics of these igneous bodies from their; structures and field relationships as seen in rock exposures, diagrams/photographs and by analysis of simple geological maps and cross sections to interpret their contrasting modes of formation.