Intro to Rocks

Major Rock Types:

There are three major rock types

1. Igneous—Rocks formed from cooling of magma or lava.

2. Sedimentary—Rocks formed from sediments worn from other rocks.

3. Metamorphic– rocks formed by changing the chemistry, mineralogy, or texture of other rocks.

Rock Cycle

• Rock types are all connected in a cycle of formation, change, and destruction which we call the Rock Cycle.

• Let us start the rock cycle with molten rock (magma), which cools and forms igneous rocks. These rocks become uplifted as mountains are formed (orogeny). There it is attacked by the weather and starts to erode.

Rock Cycle

• This weathered (eroded) material is carried away by streams, rivers, wind, glaciers and deposited elsewhere as sediments.

• The sediments are then buried and lithified (turned into solid rock) being subjected to heat, pressure, and fluids the sedimentary rocks becomes metamorphic rocks.

• Metamorphic rocks maybe uplifted and eroded or may become heated to the point that it again becomes Magma.

Rock Cycle

http://www.matt-willard.com/Artwork/Graphic_Design/Rock-Cycle.jpg

What are Igneous Rocks?• The term Igneous comes form the Latin

word ignis meaning fire. These are rocks which form from cooling magma or lava.

• Magma: Molten or partially molten rock materials and dissolved gases beneath earth’s surface.

• Lava: Is also molten or partially molten rock material and dissolved gases which erupts at the Earth’s surface.

Classifying Igneous Rocks• Two methods are used: Texture and

chemisty.– Texture: a term which involves how a rock

looks. This method involves:• 1. The size of the mineral grains (crystals)

involved.• 2. Does the rock have holes (vesicles) in it. If it has

a lot of holes it is called a vesicular texture. This is an indication that the rock was lava and at the surface as it erupted or cooled.

• 3. Is the rock a coherent mass of mineral grains or from smaller chunks of igneous rock which has been cemented or welded together (pyroclastic texture).

Texture• Using the texture characteristics Igneous

rocks can be classified as either Intrusive or Extrusive.– Intrusive: Rocks composed of large crystals.

This indicates slow cooling below the earth’s surface.

– Extrusive: Rocks composed of small, microscopic, or no crystals (obsidian) indicates rapid cooling at the earth’s surface.

Chemical Classification• Based on the Chemical Composition

igneous rocks can be broken into 4 general types.– Felsic: High in silica (65%+)

• Usually light colored• Examples Rhyolite (extrusive) and Granite

(intrusive).

– Intermediate: Lower silica content (55-65%)• Darker than felsic, lighter than mafic• Example Andesite/dacite (extrusive) and

Diorite/granodiorite (intrusive).

Chemical ClassificationContinued

– Mafic: low silica content (45-55%)• Ususally dark colored• Example basalt (extrusive) and gabbro (intrusive).

– Ultramafic: Extremly low silica content (less than 45%)

• Usually dark colored, but high olivine content tend to produce green colors. There are also other rare colors.

• Example periodotite (intrusive)

Classification of Igneous Rocks

http://geology.csupomona.edu/alert/igneous/ignrxclass.gif

How does Magma form?• Magma originates from melting rocks. But

rocks are made up of different minerals which melt at different temperatures (not uniformly). Some may not melt completely resulting in Partial melting.

• Melting temperatures may be affected by environmental conditions such as pressure, amount of water. Higher pressures increase melting temperatures, presence of water lowers the melting temperature.

Magmatic Differentiation• Partial melting also results in Partial

freezing and called magmatic differentiation. Freeze does not mean cold.– Those minerals which have a higher melting

temperature will also freeze first which will change the chemical composition of the remaining melt.

– This process continues until all the magma has frozen or solidified.

Chemical Classification

• There are three general paths that igneous rock may take as they cool depending on their chemical composition.– Continuous Reaction Series: deals with those melts

that are calcium rich or sodium rich composition (plagioclase feldspars).

• As the plagioclase feldspars crystallize, the first crystals are calcium rich which leaves the sodium rich melt behind. This continues till the melt reaches equilibrium again. Again calcium crystals come out followed by sodium. This continues till the entire melt is solid.

Bowen’s Reactionhttp://imnh.isu.edu/digitalatlas/geo/rocks/bowens/bowens.jpg

Chemical Classification

• Discontinous Series: As a mafic melt cools slowly the first crystals that form are olivine, followed by pyroxenes with the olivine being converted to pyroxenes. As the temperature farther lowers amphiboles crystallizes and all the pyroxenes convert to amphiboles. Farther lower of temperature results in Mica forming with the amphiboles all convert to Mica.

• Discontinous series can be seen as separate crystallizations and conversions, so that there is only one type of mineral present at a time.

Chemical Classification

• Fractional crystallization says that after crystals form, they are somehow separated from the remaining molten material and don’t re-enter the reaction.

• One possible way that this might occur is the crystals as they form settle out due to gravity. Or as crystals form the melt moves to another location (squeezed out so to speak).

Origin of Different types of Magmas• Mafic Magmas (basaltic) has two sources:

– 1. Mid-Ocean Ridges, where mafic magma rises up to form new ocean floor.

– 2. Mid-Plate Volcanoes are sites where mafic magma rise up from great depths (where one plate is being subducted under another plate) this results in eruptions on the surface.

• Mafic to intermediate volcanism can also occur in this location.

Origin of Different types of Magmas

• Felsic Magmas forms as oceanic plates move along picking up mud, silt, and wet sediments. As one oceanic plate is pulled under another plate the wet sediment are also pulled under. As the plate and sediments heat up due to pressure the water is driven out and into the mantle. This water lowers the melting point and rise (lower density), resulting in mixing with the material from the overlying continent (more melting occurs). Finally the melt reaches the surface as volcanoes or cools within the crust.

Summary• You need to know the following:

– 1. Different classes of rocks.– 2. How they are formed, and how they are tied together through

the rock cycle. Able to draw out the rock cycle and explain (p.119).

– 3. What are igneous rocks?– 4. Classification and Types of igneous rocks (p.124 text)

• Texture (intrusive vs. extrusive)• Chemical classification

– 5. Magmatic differentation• General idea• Bowen’s series

– 6. Fractional crystallization– 7. Where do different igneous rocks form (pp.122-123) – 8. Name and definitions of various intrusive bodies (p. 125 text)

SITES USED

Thanks to Greg Anderson for use of lecture notes.

http://pasadena.wr.usgs.gov/office/ganderson/es10/lectures

Also:

http://www.matt-willard.com/Artwork/Graphic_Design/Rock-Cycle.jpg

http://geology.csupomona.edu/alert/igneous/ignrxclass.gif

http://imnh.isu.edu/digitalatlas/geo/rocks/bowens/bowens.jpg