1. How Earth’s Rocks Were Formed

2. Three Groups of Rocks
A rock is a group of minerals bound together in some way
Uniformitarianism explains the origin of rocks: all rocks of the crust form in one of three general ways.
Igneous rocks are formed by the cooling and hardening of hot molten rock (magma) from inside Earth
Sedimentary rocks are formed by the hardening and cementing of layers of sediments
Metamorphic rocks are formed when rocks that already exist are changed by heat and pressure into new kinds of rocks

3. Igneous Rocks
Plutonic or intrusive rocks form underground from cooled magma eg. Granite
Volcanic or extrusive rocks forms when lava cools on Earth’s surface
Volcanic rocks lack distinct mineral grains
Granite
Basalt

4. Magma Two general kinds of magma, both are silicates
Temperatures range from 600°C to 1200°C
One has a high % of silica (SiO2), and not much Ca, Fe, or Mg
This high silica magma is thick and slow flowing, and forms light coloured minerals, called felsic rocks  granite
The second kind of magma has a lower % of silica, and a higher % of Ca, Fe, and Mg.
This low silica magma is hotter,thinner, and more fluid, it forms rocks that are dark ferromagnesian minerals, called mafic  basalt

5. Texture of Igneous Rocks
Texture depends on the size, shape, and arrangement of its mineral crystals
Igneous rock range from glassy smooth to coarse grained
Crystal size is the most important factor affecting texture, and it depends upon how fast the magma hardens
The longer the magma stays liquid, the larger the crystals become
A high % of dissolved gases helps crystals grow faster
Magmas trapped in the crust grow slowly, such plutonic rocks have large mineral grains, and a coarse grained texture

6. Granite

7. Magmas that reach Earth’s surface as lava harden rapidly
Volcanic rock that forms has tiny crystals
Basalt is a good example
Some rocks harden so rapidly that no crystals form at all and are glassy eg. obsidian

8. Porphyritic Texture
Some igneous rocks have two distinctly different textures
Large crystals are surrounded by a fine-grained mass of rock
Such a rock is called a porphyry
Two stages of cooling can form a porphyry
In the first stage the magma cools slowly at great depth, and large crystals can form, the rest of the magma remains liquid
Then the magma moves up, and the rest of the magma cools into fine-grained rock around the first-stage crystals

9. Families of Igneous Rocks: Granite
Forms from felsic magmas
Mainly orthoclase feldspar and quartz
Likely minerals are plagioclase feldspar, mica, and some horneblende
Usually light coloured
Granite: coarse grained
Rhyolite: fine grained
Obsidian & pumice: glassy
Granite
Rhyolite
Obsidian

10. Families of Igneous Rocks: Gabbro
Forms from mafic magmas
Mainly dark plagioclase feldspar and augite
Other likely minerals are olivine, hornblende, and biotite
Generally dark in colour and dense
Gabbro is coarse grained
Basalt is fine grained
Gabbro
Basalt

11. Families of Igneous Rocks: Diorite
Composition and colour between those of granite and gabbro
Diorite has less quartz than granite, and less plagioclase than gabbro
Andesite is the fine-grained member
Diorite
Andesite

12. Some Igneous Rocks do not have a Family…
Granodiorite is a coarse-grained rock, with a composition between granite and diorite
Pyroxenite is nearly all pyroxene
Dunite is almost all olivine
Peridotite is a mixture of pyroxene and olivine.
Granodiorite
Dunite
Pyroxenite

13. Common Igneous Rocks
Specific rocks can be identified by their minerals and texture
Granite is made of quartz, orthoclase feldspar, and at least on other mineral, such as mica or hornblende, and is coarse-grained
Quartz: little chips of cloudy or grayish glass
Feldspar: smooth cleavage surfaces, is gray, pink or orange
Mica: black biotite, is shiny little flakes
Horneblende: tiny dull black chunks or sticks

14. Granite is the most common continental igneous rock.
Obsidian
Pumice
Granite is the most common continental igneous rock.
Felsite is the general name for any light-coloured, fine-grained rock in the granite family.
Rhyolite is an example
Obsidian is a volcanic glass of the granite family. It is usually dark brown or black, hard and brittle, with conchoidal fracture.
Pumice is formed from felsic lava that hardened while steam and other gases were still bubbling out of it.

15. The most common rock of the gabbro family is basalt
Scoria
The most common rock of the gabbro family is basalt
Basalt ranges from dark green to black
Gabbro has about the same composition as basalt, but is coarse grained
Basalt glass is like obsidian but has a mafic composition
Scoria is full of holes like pumice