Igneous cheat sheet

HideShow resource information
Preview of Igneous cheat sheet

First 252 words of the document:

Igneous Rocks
Formed in molten lava or magma. As it cools crystals begin to grow in the liquid. All the crystals
are touching. If not this would make it a sedimentary. If the magma cools slowing then the crystals
will be large as they have plenty of time to grow. However if the magma cools quickly then the
crystals will be small. ­ Little time to grow.
Because the crystals grown in a liquid they can grow easily in any direction. Hence igneous rocks
have no preferred direction of crystal but form an interlocking network. In other words igneous
rocks show no preferred orientation.
We can decide its classification in two ways
1) Grain size
2) Chemical composition
Grain size (average size of grains)
Coarse Grained = Greater than 5mm
Medium Grained= between 1mm 5mm
Fine grained= less than 1mm
Glass= no visible crystals
Chemical Composition
The most important feature of the composition is the amount of Silicon or silica. Silica is contained
in all the Silicate Minerals above but most is contained in Quartz. If Silica is dissolved in water it
makes a weak acid (Silicic Acid). This means the rocks can be described in a similar way.
Colour index
ACIDIC igneous rocks are light coloured called leucocratic
INTERMEDIATE rocks are medium coloured called mesocratic
BASIC rocks are dark coloured called melanocratic
ULTRABASIC (peridotite) is green due to Olivine

Other pages in this set

Page 2

Preview of page 2

Here's a taster:

Identify other aspects of texture:
· Porphyritic: Large phenocrysts in a finer groundmass. In fine grained rocks this may result
from two stages of cooling.
· Flow banding: In finegrained lavas, light and dark bands sometimes contorted by plastic
flow. Typical of viscous lavas.
· Flow alignment: In finegrained porphyritic rocks, the phenocrysts may show a preferred
orientation resulting from flow.
· Vesicular: Gas bubbles. These can only develop under low pressure. They may taper
downwards.…read more

Page 3

Preview of page 3

Here's a taster:

Columnar joints
Columns can vary from 3 meters to a few centimetres in
diameter. They are typically parallel and straight but can
also be curved. The number of sides of the individuals can
vary from 3 to 8. 6 sides are the most common.
Liquid contracts around central cooling nucleus (C)
Sill Concordant
A metamorphic event due to hot lava, when the pressure is so great that the
underlying magma actually lifts the overlying rocks.…read more

Page 4

Preview of page 4

Here's a taster:

Dyke ­ Disconcordant
Moves along a fault. Lava flow is not
concordonot with beddind planes.
Baked margins either side.…read more


No comments have yet been made

Similar Geology resources:

See all Geology resources »See all resources »