Igneous Rocks

Intrusive and extrusive rocks formed from the cooling and crystallization of magma.

Magma vs. Lava

• Magma is molten rock beneath Earth’s surface

• Lava is magma that has flowed out onto Earth’s surface

Two types of Igneous Rocks

Extrusive Igneous Rocks• Fine grained igneous rocks that cool

quickly on the Earth’s surface(forms from Lava). Ex. Rhyolite

Intrusive Igneous Rock• Course grained Igneous rock that cools

slowly beneath the Earth’s surface(forms from magma).Ex. granite

Rhyolite- extrusive rapid cooling

Granite- Intrusive slow cooling

Composition of Magma

• A slushy mix of : Molten rock Gases Mineral crystals

Elements in Magma

• Oxygen (O)• Silicon (Si)• Aluminum (Al)• Iron (Fe)

• Potassium (K)• Sodium (Na) • Magnesium

(Mg)• Calcium (Ca)

• Magma is classified based on the amount of silica it contains.

SiO2

Types of magma Group• Rhyolitic • Andesitic • Basaltic

SiO2 Content

70%60%50%

Origins of Magma

• In labs, most rocks melt when they hit 800°C to 1200°C

• These temps are found in the upper mantle & lower crust

• Scientists think this heat comes from the decay of radioactive elements & the remaining energy from Earth’s molten formation

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Check Point

1. What is the difference between magma and lava?

2. What are the two types of igneous rocks?

3. Which type of magma has the most silica content?

Factors That Affect Magma Formation

1.Temperature– Temp ↑ with depth in Earth’s crust

2. Pressure– Pressure ↑ with depth because of the weight

of overlying rock– As pressure ↑, so does the melting point

3.Water content– Rocks & minerals contain little water– The higher the water content , the lower the

melting point 4.Mineral’s composition

– Different minerals have different compositions and therefore have different melting points

How Rocks Melt

• Partial Melting – process in which different minerals melt into magma at different temperatures, changing its composition.– Some minerals melt, while others remain solid– As minerals melt, different elements are

added to the magma “stew,” thereby changing its composition

• Fractional Crystallization – process in which different minerals crystallize from magma at different temperatures, removing elements from magma.– Changes composition of magma by removing

elements

Bowen’s Reaction Series• Discovered by a geologist named Bowen• Sequential, predictable, dual-branched pattern in

which minerals crystallize from cooling magmaTwo main patterns or branches of crystallization

1. Continuous, gradual change of mineral composition

• Ex. feldspars (change from calcium rich to sodium rich)

• Magma is made up of different minerals that crystallize at different temperatures.

• So during the cooling and crystallization process some minerals become solid while others remain molten.

Bowen’s Reaction continued…

2. An abrupt change of mineral type Iron-rich minerals change into other

minerals.  Ex. At 18000C Olive forms once that substance reaches 1557 degrees a new mineral is formed (pyroxene).

Magmatic Differentiation

• As minerals form in the Bowen series the solid more dense ones fall to the bottom and are not available for the reaction anymore so the material that is available is now different.

Bowens reaction

https://www.youtube.com/watch?v=K6aQlYA4BHU

• www.youtube.com/watch?v=R4X8ukSpbQo

• https://www.youtube.com/watch?v=ApLp7GMDOE0

Crystal separations

• Under certain conditions newly formed crystals can be separated from rock that explains why olivine is found in the rock that is why olivine did not convert to pyroxene during cooling.

Layered Intrusions• Sometimes during

cooling layered intrusions form. Scientist are uncertain why. Maybe temperature differences. Valuable rare metals maybe found here. Ex. Platinum, chromium, nickel, and gold.

Check Point

1. What happens to temperature with depth?

2. As depth increases does pressure decrease or increase?

3. What is partial melting?4. Summarize Bowen’s reaction?

5.2 Classifying Igneous Rocks• Igneous rocks are broadly classified as

intrusive or extrusive. • However, they are further classified by

–mineral composition• what minerals are found in the rock

–color–grain size

• if lava cools quickly then small crystals form Example: Obsidian has no visible mineral grains

• if magma cools slowly then large crystals form Example: Gabbro has large mineral grains

–texture•early forming minerals may have well shaped crystals

• later forming minerals may have irregular shaped crystals

•porphyritic texture – contains both large & small crystals

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Classification of Igneous Rocks

Four Main Groups of Igneous Rocks

1. Felsic– Light colored– High silica content– Contain quartz,

feldspar,orthoclase, and plasioclase–Ex, granite

2. Mafic

–Dark colored–Low Silica–Plagioclase, biotite, amphibole,

pyroxene, and olivine–Rich in Iron and Magnesium–Example, Gabbro

3. Intermediate page107

• Contain moderate amounts of biotite, amphibole, pyroxene, and olivine.

• Ex. diorite

4. Ultramafic

–Low Silica–High Iron and Magnesium–Ex, Peridotite and dunite

Check Point

• What are the four mineral composition types and two facts about each? (you have 5 minutes)

• What is a pegmatite?• What is a kimberlite?• Answer #6 on p. 106

Igneous Rocks as Resources

Igneous rocks are good resources as building materials because of their strength, durability, ability to resist weathering, and beauty.

 

Ore DepositsOre deposits can be found within igneous rocks or

in the cracks and voids surrounding the rocks, called veins– Dissolved silica & common metals (Ex: gold,

silver, lead, copper) are released at the end of magma crystallization in a hot, mineral rich fluid that fills the veins.

– Fluid solidifies to form metal-rich quartz veins– Ex, gold-bearing quartz in Sierra Nevada

Mountains in California

• Pegmatite- vein deposits of extremely large-grained minerals that can contain rare ores such as lithium and beryllium

 • Kimberlites – rare, ultramafic rock that

contain diamonds and other minerals formed only under very high pressures

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