Geology GCSE Notes: Igneous processes

My Geology GCSE Notes: Igneous processes

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Igneous processes
Igneous rocks
Igneous rocks are rocks that form when magma cools and solidifies.
When the magma cools mineral crystals begin to grow
The texture of the rock can be:
Coarse - 7 - 2mm
Medium - 2 - 0.5mm
Fine - <0.5mm
Silicic Intermediate Basic
Fine grained Ryolite Andesite Basalt
Medium grained jjfdfdgergegee Dolerite
sdssvvd ff
Coarse grained Granite sdvsdvfufufufuf Gabbro
Colour Light Dark
Density Low High
Silica content High Low
Some rocks contain two distinct crystal sizes. This is called porphyritic texture.
The large crystals are called the phenocryst.
The smaller crystals are called the groundmass.
The porphyritic texture is formed from a two stage cooling
Big crystals form deep underground as magma cools slowly.
The magma then suddenly rises faster and crystals don't have as much time to form, these
crystals are called the groundmass.
In some rocks the grain size is the same. This is called equigranular texture.

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Lava viscosity
Viscosity is how thick a liquid is.
A lava with a high viscosity is thick and will flow slowly and a lava with a low viscosity is runny
and will flow slowly.…read more

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Sills and Dykes
A sill occurs when magma intrudes between layers of sedimentary rocks. Sill are always parallel
to the rock beds.
A dyke is formed when magma has intruded into a fracture or fissure through the original
country rock. Dykes tend to be vertical and don't follow the rock beds.
Chilled and baked margins
The heat from the magma, through metamorphism, changes the surrounding rock.…read more

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Volcanic textures
The texture is the size, shape and the arrangement of the crystals.
Columnar jointing
Columnar jointing is where the lava cools and contracts quickly forming hexagonal joints. The joints
are usually perpendicular to the lava flow.…read more

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Vesicular texture
Vesicular texture are rocks that contain small vesicles (gas bubbles)
The vesicles are formed by the expansion of gas bubbles
The two rocks that have vesicular texture are pumice and scoria.
Amygdaloidal texture
Amygdaloidal rocks have lots of different minerals in them.
They are formed from the vesicles in rocks being filled up with water with dissolved minerals and the
water evaporating leaving behind minerals in the rock.…read more

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