How do We Classifiy Igneous Rocks?

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To classify and identify igneous rocks we must examine their physical properties and unique characteristics.  Although all of the rocks above are igneous and formed from magma / lava, they appear very different from one another.  The differences between the igneous rocks shown above can be described largely by differences in their formation. 

Environment of Formation

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THE ENVIRONMENT OF FORMATION REFERS TO WHERE A ROCK FORMS.  Since igneous rocks form from magma or lava, they can form either inside the Earth or on the Earth's surface.  

Intrusive Igneous Rocks

  • FORM FROM MAGMA COOLING AND HARDENING DEEP UNDERGROUND

Extrusive Igneous Rocks

  • FORM FROM LAVA COOLING AND HARDENING ON EARTH'S SURFACE

THE ENVIRONMENT OF FORMATION IS IMPORTANT BECAUSE IT EFFECTS HOW LONG THE ROCK TAKES TO COOL AND HARDEN.

 

Texture

There is a wide range of possible textures for igneous rocks, from very coarse rough rocks, to very fine smooth rocks.  There are even some textures that are unique to igneous rocks.  For example igneous rocks are the only type of rock that can have a glassy texture.  Similarly, they are the only type of rock that can have vesicular air pockets inside the rock.  

Obsidian is and example of an igneous rock with a glassy texture.

Obsidian is and example of an igneous rock with a glassy texture.

Basalt is an example of an igneous rock with a vessicular (air pockets) texture

Basalt is an example of an igneous rock with a vessicular (air pockets) texture

 The precise texture of an igneous rock depends on the size of the mineral crystals that make it up.   

LARGE CRYSTALS = COARSE TEXTURES

SMALL CRYSTAL = GLASSY SMOOTH TEXTURE

If only we knew what determined the size of crystals...oh wait...we do.  The size of the crystal depends on the cooling time!   So, if we think about it we should be able to put this all together.

Intrusive rocks form inside the Earth.

  • This means they COOL SLOWLY.  
  • Which means they have LARGE CRYSTALS
  • Which means they have a COARSE TEXTURE!

 

Extrusive rocks form on the Earth's surface. 

  • This means they COOL QUICKLY
  • Which means they have SMALL CRYSTALS
  • Which means they have a GLASSY / FINE TEXTURE!

 

Type of Magma / Lava

The chemical characteristics of the magma or lava that forms an igneous rock also effect the properties of the rock.  There are two general types of magma / lava:  Mafic or Felsic. 

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Mafic Magma

  • Mafic Magma contains more Calcium and Iron, and less Silicon.
  • This makes it thick and slow moving.
  • PRODUCES HIGH DENSITY ROCKS
  • PRODUCES DARK COLORED ROCKS
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Felsic Magma

  • Felsic Magma contains more Silicon and Aluminum.  
  • This makes it thinner, more fluid, and faster moving.
  • PRODUCES LOW DENSITY ROCKS
  • PRODUCES LIGHT COLORED ROCKS

 

 

If we know several of the properties of an igneous rock we can use the ESRT Igneous Rock ID chart to figure out the name of the rock. 

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Let's Practice:  Identify the rock based on its description. 

Rock 1: 

  • Felsic composition
  • Coarse texture

Rock 2:

  • Glassy non-vessicular texture
  • Dark Color

Rock 3: 

  • High density
  • Fine vesicular texture

 

 

Rock 1 = Granite, Rock 2 = Basaltic Glass, Rock 3 = Vessicular Basalt