Igneous Rocks and Processes

What is a Rock?

• A rock is an aggregate of one or more minerals.
• Exceptions:
  • Coal: Considered a rock but not made of minerals.
  • Obsidian: An igneous rock, but not made of minerals.

Categorizing Rocks

• Rocks are categorized based on the origin of their minerals.
• Types:
  • Igneous
  • Sedimentary
  • Metamorphic

Igneous Rocks

• Made from molten rock (magma or lava).
• Liquid rock cools and minerals crystallize.
• Magma vs. Lava:
  • Magma: Molten rock underground.
  • Lava: Molten rock erupted to the surface.

Sedimentary Rocks

• Formed from preexisting rocks at the surface.
• Weathering and erosion break down rocks.
• Material is collected and undergoes lithification (loose material turns into rock).
  • Example: Sand turns into sandstone.

Metamorphic Rocks

• Formed from preexisting rocks subjected to higher temperatures and pressures.
• Minerals recrystallize under extreme conditions.
• Recrystallization happens while the rock is solid.
• If melting occurs before recrystallization, it falls under the igneous category.

The Rock Cycle

• Relationship between rock types.
• Classic cycle:
  • Magma/Lava cools to form Igneous Rock.
  • Igneous Rock weathers and erodes to create sediment.
  • Sediment compacts into Sedimentary Rock.
  • Sedimentary Rock is subjected to high temperatures and pressures to become Metamorphic Rock.
  • Metamorphic Rock melts to form Magma/Lava, restarting the cycle.
• Any rock type can be metamorphosed or weathered.
• Igneous rocks can be metamorphosed.
• Metamorphic rocks can undergo further metamorphism.
• Any rock exposed at the surface can break down and erode into sediment.
• Igneous rocks can be remelted.
• If a rock is heated rapidly, it might skip the metamorphic process and melt directly.
• The rock cycle is more of a web, showing connections between rock types.

Creating Igneous Rocks

• Earth is mostly solid (core, mantle, crust).
• Crust and upper mantle are where most rocks are found.
• To create igneous rocks, existing rock must be melted.

Factors Influencing Magma Formation

• Composition: Different minerals have different melting temperatures.
• Partial Melting: Only some minerals melt, changing the magma composition.
  • Example: Granite
    • Minerals: Orthoclase feldspar (pink), Quartz (grayish, glassy), Biotite mica (black).
    • Melting order: Biotite melts first, then feldspar, then quartz.
    • The resulting magma composition will reflect the minerals that melted.
• Silica Content vs. Melting Temperature:
  • Low silica compositions:
    • High melting temperatures.
    • Olivine (isolated tetrahedrons).
  • High silica compositions:
    • Low melting temperatures.
    • Quartz and feldspar.

Order of Melting

• Minerals with high silica melt first.
• Minerals with low silica melt last.
• Order of melting (solid to liquid)/Order of crystallization (liquid to solid) go in opposite directions.

Melting and Pressure

• Geothermal Gradient: Temperature increases with depth in the Earth.
• Pressure increases with depth.
• Pressure prevents melting by confining materials.
  • When things melt, they expand.
• Higher pressure increases melting points.

Decompression Melting

• Melting due to a decrease in pressure.
• Occurs at spreading centers or divergent plate boundaries.
• Stretching and thinning of the Earth's crust reduces pressure on the mantle.
• Hot mantle rock rises and melts due to decreased pressure.

Role of Water

• Adding water destabilizes minerals and lowers their melting points.
• Subduction zones: Oceanic plates subduct under continents, bringing water into the mantle.
• Water is squeezed out of the subducting plate, lowering the melting point of the surrounding mantle.

Classifying Magma

• Primary classification is based on silica content.

Composition Types:

• Granitic: High silica content (e.g., granite).
• Andesitic: Intermediate silica content (e.g., andesite).
• Basaltic: Low silica content (e.g., basalt).
• Ultramafic: Very low silica content (mantle compositions).
  • Mafic = basaltic
  • Felsic = granitic

Texture of Igneous Rocks

• Texture: Size, shape, and arrangement of minerals.
• For igneous rocks, focus is on crystal size.

Phaneritic (Coarse Grained)

• Visible mineral grains.
• Cooled slowly underground (magma).
• Crystals have time to grow.

Aphanitic (Fine Grained)

• Microscopic mineral grains.
• Cooled quickly at the surface (lava).
• Crystals are tiny.

Extrusive vs Intrusive

• Extrusive (Volcanic): Rocks cooled at the surface/lava.
• Intrusive (Plutonic): Rocks cooled underground/magma.

Porphyritic (Two Grain Sizes)

• Both coarse and fine grains.
• Cooled slowly at depth, then erupted and cooled quickly.
• Large crystals (phenocrysts) in a fine-grained matrix or groundmass.
• Still classified as extrusive/volcanic.

Glassy

• No mineral crystals (amorphous).
• Very rapid cooling.
• Example: Obsidian.
• Lava cools so quickly that atoms don't have time to organize into crystals.

Naming Igneous Rocks

Igneous Features

• Volcanoes: Created by volcanic rocks.
• Intrusions/Plutons: Features created by magma cooling underground.
• Batholiths: Large underground magma chambers that have cooled.
  • Example: Sierra Nevada.
• Tabular Plutons: Dikes and Sills: Smaller injections of magma.
• Dike: Vertically oriented, cutting across layers.
• Sill: Horizontally oriented, filling in between layers.