ROCKS
- a rock is a mixture of minerals. The minerals can be all of one type or can include tens or hundreds of different mineral types.

3 Types of Rocks
- Igneous Rocks
- Sedimentary Rocks
- Metamorphic Rocks

LAB 3 - IGNEOUS ROCK IDENTIFICATION

What is an igneous rock?

- formed when magma, or melted rock, cools and crystallizes at or below the Earth’s surface.

- How, when and where an rock cools determines the characteristics of the rock as well as the chemical composition of the melt.

- In order to identify a igneous rock we must describe 2 things; the rocks COMPOSITION and the rock’s TEXTURE.

But first we must know something about the melt from which a rock is derived.

IGNEOUS ROCK COMPOSITION
- Magma can have a large range of compositions but primarily depends on the amount of silica in the melt. When a melt of a particular composition and a certain amount of silica cools, we can predict what the rock will look like.




- Minerals with higher melting temperatures cool faster than others. Olivine, with the highest melting temperature cools FIRST.
- Minerals with low melting temperatures cool LAST. Quartz and Mica minerals cool last.


Ultrabasic minerals such as olivine are the first to crystallize as magma slowly cools deep within the surface.

Dark-colored ferromagnesian, or MAFIC minerals (those containing iron and magnesium) tend to crystallize next as magma cools.
These include Pyroxene and Ca-Rich Plagioclase.

Intermediate minerals such as amphibole, biotite appear next.

Light colored minerals, called FELSIC minerals, are the last minerals to crystallize in the magma. These include quartz, muscovite, and K-Feldspar.

Once quartz crystallizes, there is no more magma left, it has all been solidified.

By looking at the relative abundance of dark-colored minerals (mafic) to light-colored minerals (felsic), we can classify the rock’s composition as
Ultramafic (contains green olivine)
Mafic (dark)
Intermediate (salt-and-pepper colored)
Felsic (light)
and we can use the table to figure out what minerals make up each.

IGNEOUS TEXTURES

Igneous rocks can be classified in two categories based on texture regardless of composition;
a) VOLCANIC, or EXTRUSIVE (surface)
b) PLUTONIC, or INTRUSIVE (deep interior)

Texture describes the outward appearance of the rock. The features we want to describe are

a) Crystal Size - the larger the crystals, the slower the rock cooled, and the deeper in the crust it cooled.
A rock with small crystals (too small to see) cooled rapidly at or near the surface.

To describe crystal size we use a few terms,

Phaneritic - rocks in which the crystals are easy to see with the naked eye

Aphanitic - rocks i which the crystals are NOT easy to see with the naked eye, or one in which ` you cannot see individual crystals

Porphyritic - a rock which has cooled more than once in its lifetime may have two different sets of crystals each a different size. A rock with some large crystals and many small crystals would deserve this term. The term is used as an adjective, i.e. “porphyritic rhyolite”. The large crystals are called PHENOCRYSTS.



Pegmatitic - texture with unusually large crystals which form very slowly

Glassy - texture formed when magma cools near instantaneously, and no crystals have time to form.

Scoriaceous - texture formed when volatile gas bubbles up through the cooling rock and causes vesicles, or bubble hols, to form on the rock’s surface. typically dark in color.

Pumiceous - rock is light in weight due to small gas cavities throughout the rock. typically light in color. Pumice floats on water.