Igneous RocksChapter 3.2

Igneous Rocks The word igneous comes

from the Latin word ignis which means “fire”.

The reason why fire is a fitting name for igneous rocks is because igneous rocks come from hot molten magma and lava.

The Two Types of Igneous Rocks Intrusive Igneous Rocks –

Also called Plutonic Igneous Rocks.

Intrusive Igneous Rocks are rocks that form under the ground when magma cools and crystallizes.

Magma is what we call rock when it is melted.

Extrusive Igneous Rocks- Also called Volcanic Igneous Rocks.

Extrusive Igneous Rocks are rocks that form above the ground when magma comes to the surface.

When magma reaches the surface it is called lava.

Igneous Rocks

Classification of Igneous Rocks The way Geologists classify

Igneous Rocks is by their texture and composition.

First let’s explain texture. The texture of an igneous

rock is determined by how quickly the rock cooled.

If an igneous rock cooled quickly, then it did not have time to form large crystals.

If an igneous rock formed more slowly, then it usually has larger crystals.

So to summarize, the slower the rock forms, the larger the crystal size.

Bigger crystals means more time. Smaller crystals means less time.

Igneous Rock Textures Coarse-Grained Texture –

the term Geologists use to describe Igneous Rocks with a Coarse-Grained Texture is Phaneritic.

Fine-Grained Texture – the term Geologists use to describe Igneous Rocks with a Fine-Grained Texture is Aphanitic.

Vesicular and Glassy – Some Igneous Rocks form so quickly they don’t form crystals at all. Sometimes they form bubbles or air pockets. Other times they form a smooth glass.

Porphyritic Texture – If an Igneous Rock has a Fine-Grained Texture with some large crystals mixed in.

Igneous Rock Textures

Phaneritic (Coarse-Grained) Texture Aphanitic (Fine-Grained) Texture

Porphyritic TextureVesicular and Glassy Texture

Igneous Rock Compositions Generally speaking, you

can tell an Igneous Rock’s Composition, or what it is made of by its color.

Almost all of Igneous Rocks are made up of Silicates.

The lighter color Silicates are referred to as Felsic or Granitic.

The darker color Silicates are referred to as Mafic or Basaltic.

Also as a general rule, the lighter the color, the lighter the minerals in the rock.

The darker the minerals, the heavier the minerals in the rock.

Felsic (Granitic) vs. Mafic (Basaltic)

Light-Colored Felsic/Granitic Rock Dark-Colored Mafic/Basaltic Rock

Igneous Rock Compositions You can also have an

Igneous rock that is an intermediate or in between a mafic and felsic composition.

We refer to these rocks as andesitic, dioritic, or intermediate composition.

You can also have an Igneous Rock that is extremely Mafic.

We refer to these rocks as Ultramafic.

Ultramafic rocks often have an olive/green coloration.

The reason why they are that color is because they contain the mineral Olivine.

Also, some Felsic/Granitic rocks are colored pink.

The reason why is because they contain the mineral orthoclase feldspar.

Other Compositions

Andesitic/Dioritic- Intermediate ColorNotice it is Gray. A mix between the two compositions

Ultramafic- Notice the Olive/Green Coloration that comes from the mineral Olivine

Felsic/Granitic- NoticePink Granite is also Felsic.White is not always the color of rocks with a Felsic Composition.

Igneous Rock Compositions And to make matters even more complicated and confusing,

the rocks that cool so quickly and don’t have time to form crystals are usually not classified by composition.

You can not determine their composition by color alone. For example, Obsidian which is extremely dark black would

seem to be mafic because it is dark. But actually, Obsidian is considered to be a felsic rock.

Other Igneous Compositions:None of these rocks have a determined composition

Classification of Igneous Rocks