Igneous Rocks-(Extrusive-Intrusive-Mafic-Felsic

Friday, March 13, 2020

Igneous rocks are created from molten rock. In fact, the name is derived from a Greek word for fire.
Igneous rocks form from hot, molten rock that cools and then solidifies. Igneous rocks may form from magma which is found inside the Earth or Lava which is the molten rock on the surface of the Earth.
There are two major types of igneous rocks.

Intrusive and extrusive
Let’s take a look at how intrusive and extrusive rocks differ in terms of the following.
  • What the rocks form from
  • Where they are created
  • The rate of cooling
  • The size of crystals
  • The texture of the rock
Extrusive igneous rocks are created from lava.
Intrusive rocks are created magma.

Extrusive rocks are created on the surface where molten rock has flowed.
Intrusive rocks are created inside the earth from hot magma.

Extrusive rocks cool quickly because the lava flows on the surface of the Earth or may even come in contact with water.
Intrusive rocks cool slowly. Intrusive rocks cool inside the Earth which acts a little like an oven which slows the rate of cooling of these rocks.

Extrusive rocks may have really small crystals. These crystals are smaller than 1 mm or they have no crystals at all.
Many times they may be vesicular which means the rock may have air bubbles.
Intrusive rocks have larger crystals of 1mm or larger.

Extrusive rocks have a fine texture. The texture is related to the size of crystals. Small crystals or no crystals at all are described as fine. Some intrusive rocks like obsidian may even be smooth.
Intrusive rocks may have a coarse texture which can be described as having larger crystals. Because intrusive rocks have larger crystals their texture is described as coarse.

Another way you can classify igneous rocks is to determine if the rock is mafic or felsic.
Classifying an igneous rock as mafic or felsic is actually a scale of the percent of silica found in the rock.
  • 0% to 45% silica is considered mafic 
  • 65% or greater silica is felsic
  • 45 to 65 is considered intermediate
Mafic rocks are dark in color and felsic rocks are lighter in color.
The names give you a clue as to what other minerals make up these rocks.
Mafic can be broken in MA for magnesium and fic which is Latin for iron.

Felsic is broken into fel for feldspar and sic for silica.


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