How Earth s Rocks Were Formed
I. Uniformitarianism- James Hutton s 2-part theory states: A. The geologic processes now at work were also active in the past B. The present physical features of Earth were formed by these processes gradually *Earthquakes don t happen because God is mad at us!
II. 3 types of Rocks Igneous Sedimentary Metamorphic
A. Igneous- are formed by the cooling of magma or lava 1. 2 types of igneous rocks a. Plutonic (intrusive)-form underground from cooled magma Ex: granite
b. Volcanic (extrusive)- form when lava cools Ex: obsidian
2. Two different types of magma or lava a. felsic- 1. high silica 2. low Ca, Fe, Mg 3. plutonic rocks b. Mafic- 1. low silica 2. high Ca, Fe, Mg 3. volcanic rocks
3. Textures of igneous rocks are determined by size and shape of crystals. a. Hardens quickly small or no crystals Ex: obsidian b. Thick magma that hardens slowly large crystals Ex: granite
c. Porphyry- 2 textures in 1 rock. Large crystals surrounded by a fine-grained rock mass. Porphyritic texture - a mixture of coarse and fine grained textures in the same rock - indicates different cooling rates.
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B. Sedimentary Rocks- form when sediments cement together-3 types 1. Clastic Sedimentary rock-form from fragments of other rocks a. Water is the main agent for weathering, but it also contains natural cements (silicate, calcite & rust) b. Pressure from overlying sediments can cause sediments to stick together.
b. Water moves and drops sediments according to size 1. Large pebbles and gravel are in shallow regions conglomerate
2. Intermediate regions have sand sandstone a. sandstone is made of quartz and is porous & permeable Don t throw sandstone in a campfire!!!
3. Deep, calm regions have silt and clay shale a. shale is smooth, soft & impermeable
2. Chemical sedimentary rock- formed when minerals fall out of water solutions and form chemical sediments. These are cemented together. a. examples: limestone, rock salt Limestone is made from the calcite which came from the beds of evaporated seas and lakes and from sea animal shells. It is used in concrete.
3. Organic sedimentary rocks- come from the remains of animals & plants Example: Limestone from shell- producing organisms like clams & oysters
4. Stratification- sedimentary rocks are arrange in visible layers
5. Fossils are trapped in sedimentary rocks only!!! a. Why? The heat or pressure needed to form other types of rock would destroy the fossil. b. Fossil- remains, impressions, or any other evidence of plants & animals preserved in rock
C. Metamorphic Rock -formed from existing rock by the action of heat, pressure, and chemicals. They form in 2 ways:
1. Regional metamorphism when large areas are under intense heat & pressure a. Rocks formed this way are very dense b. Examples: sandstone quartzite limestone marble
c. Foliation- the flat or wavy sheet-like layering within some metamorphic rocks Quartzite is a non-foliated metamorphic rock Phyllite is a foliated metamorphic rock
d. The metamorphism of shale: Shale Slate Phyllite Gneiss schist
2. Contact Metamorphism- occurs when hot magma forces its way into overlying rock & bakes what it comes in contact with a. Changes in rock are less drastic than in regional b. No foliation
How is a metamorphic rock made?
III. The Rock Cycle A. Rocks are related!! B. Igneous rock is considered parent rock
Igneous Melting to become magma & cooling Weathering & Erosion to form sediments that cement together Metamorphic Sedimentary Heat & Pressure
C.Why is the rock cycle not as simple as the diagram would indicate? Because there are detours!
Igneous Melting to become magma & cooling Weathering & Erosion to form sediments that cement together Metamorphic Sedimentary Heat & Pressure
The End!!!!!!!! Your written test will be tomorrow & the practical will be Thursday!