Download #8 - Igneous Rocks Notes

#8 ­ Igneous Rocks Notes
Igneous Rocks
(A Hot Topic)
Igneous Rocks
• Formed when melted rock (magma OR lava) cools into a solid.
• Composed of one or more kinds of minerals.
INTRUSIVE: magma never reaches the surface
Solidifies under ground & cools VERY SLOWLY
EXTRUSIVE: magma reaches the surface and becomes lava­­ Cools very quickly.
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#8 ­ Igneous Rocks Notes
Solidification
Hot, liquid rock cools back into a solid.
Crystallization: magma cools slowly. Mineral crystals form an inter­grown structure.
Glasses: magma cools quickly. No time for mineral crystals to form. Minerals stay mixed together.
Characteristics
COMPOSITION: What minerals make up the rock? Can be identified using the eyes, density & color FELSIC: high in Aluminum (Al) and Silicon (Si). Tend to be less dense and lighter color.
MAFIC: high in iron (Fe) and magneium (Mg). Tend to be more dense and darker in color.
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#8 ­ Igneous Rocks Notes
Textures of Igneous Rocks
• Size of mineral crystals depend on how and where the rock formed­­mostly by the speed of cooling.
bigger crystals = slower cooling
smaller crystals = faster cooling
• Type of mineral crystals give information about the chemical composition of the rock.
Names of Textures
• Glassy: (usually shiny)
No crystals. Rocks cooled very quickly at Earth's surface.
Usually extrusive.
• Fine (less than 1 mm­­usually not shiny)
Crystals are very small (you need a microscope to see them)
May have cooled at or near Earth's surface.
Intrusive OR extrusive
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#8 ­ Igneous Rocks Notes
Names of Textures, Continued
• Coarse (1 mm ­ 10 mm)
Crystals are large and inter­grown (stuck together like legos)
Almost always intrusive and slowly cooled.
• Very Coarse (10 mm or larger)
Crystals are extremely large­­up to a meter in diameter!
Intrusive and VERY slowly cooled.
Two Other Texture Words
Vesicular
• rock has round openings caused by frozen gas bubbles.
•
usually extrusive or formed very close to Earth's surface.
NON­Vesicular
• rock does not have frozen gas bubbles in it.
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#8 ­ Igneous Rocks Notes
Steps to Identify Igneous Rocks
Use Scheme for Igneous Rock Identification
Read from the upper right
Step 1: Identify Texture
A. Vesicular or Non­Vesicular?
Gas bubbles or none?
B. What is the texture? Glassy/Fine/Coarse/Very Coarse
Grain sizes shown in mm.
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#8 ­ Igneous Rocks Notes
Step 2: Identify Composition
Felsic (light color, low density)
OR
Mafic
(dark color,
high density)
Step 3: Where do your lines meet?
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#8 ­ Igneous Rocks Notes
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