I. Rock cycle

Std 3c. Know how to explain the properties of rocks based on the physical and chemical conditions in which they formed, including plate tectonic processes Obj: Know the steps of the rock cycle Obj: Explain how igneous rocks form, the subclasses and give examples of each

A. Circular series of processes in which rock forms, changes from one form to another, is destroyed, and forms again by geologic processes 1. Any of the three major types of rock can be changed into another type

The rock cycle

B. Three types of rock 1. Classified by way rocks form a. Igneous – magma cools + hardens b. Sedimentary - sediments compressed or cemented together c. Metamorphic - existing rock altered by changes in temperature, pressure, or by chemical processes

C. Properties of rocks 1. Determined by how and where rock formed 2. Chemical stability of minerals in rock determines rate at which rock weathers and breaks apart a. Depends on strength of chemical bonds between atoms in mineral b. Minerals crystallize from magma in specific order dependent on melting point

1) Minerals with highest freezing points crystallize first c. Minerals with lower melting points melt first 3. Physical stability a. Natural zones of weakness determined by how + where rocks form b. Uplifting to Earth’s surface decreases pressure, allowing joints or fractures to open

c. Weaknesses now exposed to air + water begin to weather (wear away/erode) d. Whether rock melts to form magma depends on temperature, pressure, + presence of fluids in rock

II. Igneous rock A. Classified according to where magma cools + hardens B. 2 classes 1. Intrusive - cooling + solidification of magma beneath Earth’s surface 2. Extrusive - cooling + solidification of lava on Earth’s surface

C. Texture 1. Crystal size determined mainly by cooling rate of magma 2. Coarse-grained - large mineral crystals a. Intrusive cool slowly b. Granite, gabbro 3. Fine-grained - small mineral crystals a. Extrusive cool rapidly b. Basalt

Basalt – extrusive Granite - intrusive Granite formed by slowly cooling pockets of magma trapped beneath earth's surface. Used for long lasting monuments + for trim + decoration on buildings.

4. Other textures a. porphyritic - large crystals embedded in a mass of smaller crystals b. glassy - highly viscous magma cools very rapidly, few crystals form, small amount of dissolved gases 1) obsidian, extrusive c. vesicular - highly viscous magma contains large percentage of dissolved gases trapped as holes in the rock 1) pumice, extrusive

Obsidian - extrusive Obsidian forms when lava cools quickly above ground. Actually glass, not a mixture of minerals. Edges very sharp – Indians made arrowheads and spearheads with this. Scoria - extrusive Scoria forms when lava cools quickly above ground. You can see where little pockets of air had been. Actually a kind of glass, not a mixture of minerals.

Pumice - extrusive Pumice forms when lava cooled quickly above ground. You can see where little pockets of air had been. So light many pumice rocks will actually float in water. Actually a kind of glass, not a mixture of minerals. Because it is so light, it is used quite often as a decorative landscape stone. Ground to a powder, it is used as an abrasive in polish compounds and in Lava© soap.

D. Three mineral families 1. Composition determined by chemical composition of parent magma 2. Felsic - magma or igneous rock rich in feldspars + silica, generally light colored 3. Mafic - magma or igneous rock rich in magnesium + iron, generally dark colored 4. Intermediate - less silica than felsic but more silica than mafic

Extrusive and Intrusive Igneous