Igneous Rocks

Igneous rocks form when the molten magna inside the Earth solidifies or crystallizes. Igneous rocks are made up of silicates. The type of rock formed depends mainly  on two things: where the magma originates and its rate of cooling.


Origin of the Magma

There is more silica near the surface of the Earth, so rocks which are formed from surface magma tend to contain more silica and to be lighter in color.

Rocks which are formed from magma which originates deep within the Earth tend to contain less silica and to be darker in color.


Rate of Cooling

Magma may cool on the Earth's surface when when lava is ejected from a volcano. The lava cools quickly and forms fine-grained rocks. 

Sometimes the cooling is so rapid that no grains at all form.

Alternatively, the magma may cool slowly inside the Earth producing coarse-grained rocks.

Sometimes gas is trapped within the magma. as it cools  This can produce a light "frothy" type of rock. These rocks are described as "vesicular".

 

Main Types of  Igneous Rocks

  COLOR
Light Intermediate  Dark
G
R
A
I
N
Coarse Granite Diorite Gabbro
Fine Rhyolite Andesite Basalt
None (glassy) Obsidian    
Vesicular Pumice   Scoria

 

Pumice Rhyolite Basalt

 

Igneous Rocks
Ash
Basalt
Dolerite
Eclogite
Gabbro
Granite
Granodiorite
Ignimbrite
Microsyenite
Peridotite
Pitchstone
Pumice
Pyroclastic
Rhyolite
Syenite
 

Rock specimens now available (at eBay):

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