Earth Materials

Igneous Rocks

I. Rocks are aggregates (mixtures) of minerals or simply large samples of one mineral.

1. Usually, rocks are polymineralic (have more than one mineral), such as granite.

2. Rocks are NOT identified using the tests you used for mineral identification.

3. The minerals in rock help determine the name of the rock.

4. The origin of the rock determines which TYPE of rock it is.

II. There are 3 main categories (types) of rocks:

• Igneous: (‘fire’ rock) • Metamorphic: (have ‘changed form’)• Sedimentary:(from particles of other rocks)

III. Igneous Rocks1. Igneous rocks are called ‘fire rocks’ because

they form from lava (surface) or magma (below ground).

2. Magma is called ‘mineral soup’: the elements come together that will form minerals as magma cools.

3. Igneous rocks are called the “parent rock” of all others.

4. All igneous rocks have intergrown crystals, because they form as the magma cools.

_each crystal grows into the ones around them. Some crystals are too small to see.

5. Igneous rocks are classified (see page 6 of ESRT) according to:

a. the mineral composition of the rock, which is determined by the magma or lava that forms the rock.

The different colors aid in identifying the type of igneous rock.

b. The texture, or grain size, of the rock is used to determine the environment of formation.

This is how quickly or slowly the magma cools to form rock.

c. Together, mineral composition and texture identify the rock.

• See page 6 ESRT

• In the Bowen Reaction series, the melting points for common igneous minerals are shown. Minerals with high melting points form crystals first, while those with low (cool) melting points (typical of crust closer to the surface) forms crystals last.

6. Environment of formation and crystal size

a. When the magma cools deep below the ground, it is called intrusive or plutonic, and the minerals formed will be coarse-grained.

b. When the magma cools NEAR the surface, quickly, the crystals don’t have much time to form and the rock is called extrusive or volcanic, and the minerals will be fine-grained

c. Glassy minerals form at the surface and so quickly no visible minerals form. Vesicular minerals have gas

pockets from gas exploding out.

7. Mineral Composition Three main igneous families have different

minerals from the lava or magma from which they formed. (note colors)

a. The basalt family, formed from mafic magma, makes up the ocean crust. Dark rocks.

• Its density explains why the oceans form basins. (this crust pushes deeper into earth)

b. The granite family, formed from felsic magma, makes up the contintental crust.

Light in color.• This crust is low density (2.7 g/cm3) compared

with the ocean crust (3.0g/cm3).

c. The third family is the diorite family, from magma that is a mixture of both mafic and felsic minerals.

These are ‘salt and pepper’ rocks and formed at margins between ocean and continent.

There are also ‘ultra-mafic’ rocks from deep in the mantle.

• These don’t last too long at surface.

8. In summation, Igneous rocks are identified in the lab by comparing their:

• composition, which tells you what minerals are in it, and therefore what magma it came from, and;

• the texture, which indicates whether the rock cooled quickly or slowly (extrusive or intrusive).