How do We Classifiy Igneous Rocks?
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
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.
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.
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.
- 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
- 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.
Let's Practice: Identify the rock based on its description.
- Felsic composition
- Coarse texture
- Glassy non-vessicular texture
- Dark Color
- High density
- Fine vesicular texture
Rock 1 = Granite, Rock 2 = Basaltic Glass, Rock 3 = Vessicular Basalt