Igneous Rocks I

Introduction to Rocks and Minerals

  • Good morning and welcome back to class.

  • Previous topic: Minerals.

  • Current topic: Rocks.

  • Most rocks are primarily composed of minerals.

Definition and Composition of Rocks

  • Rocks: Coherent, naturally occurring solids.

    • Mostly made of minerals, but some are not.

    • Example of a non-mineral rock: Coal.

      • Reason: Composed of compressed organic matter, thus classified as organic and not a mineral.

      • Coal is still considered a rock despite being non-mineral.

  • Some rocks are monomineralic.

    • Mono: one.

    • Example: Limestone, composed entirely of Calcite (a carbonate mineral).

    • Students must distinguish monomineralic from polymineralic rocks in exams.

Polymineralic Rocks

  • Most rocks are polymineralic (aggregates of more than one mineral).

    • Example: Granite.

      • Important in construction (e.g., countertops).

      • Geologic definition: Indicates presence and specific ratios of minerals.

      • Must contain:

        • Quartz (light gray with a whitish appearance)

        • Two types of Feldspars: white and fleshy pink colored.

        • Variety of dark-colored minerals, potential presence of Biotite (a black mica).

      • Construction vs. geologic naming: Construction uses the term granite loosely.

      • Example: A black countertop may not qualify as granite under geological definitions due to lacking quartz and feldspar.

Types of Rocks

  • Three main types of rocks:

    • Igneous Rocks

      • Formed from the melting of preexisting rocks.

      • Melting occurs deep within the crust and upper mantle, leading to magma that cools and solidifies resulting in igneous rock.

      • Types of crystallization locations:

        • Intrusive: Magma trapped in subsurface.

        • Extrusive: Magma erupts on the surface.

    • Sedimentary Rocks:

      • Formed from weathering and erosion of rocks at the earth's surface.

      • Fragments are transported and deposited, then buried by additional sediment, undergoing Lithification to become sedimentary rock.

      • Example: Sandstone, formed from sand-sized sediments.

    • Metamorphic Rocks:

      • Form under high temperatures and pressures, typically in the crust or upper mantle.

      • Formation involves solid-state recrystallization and is often challenging to understand.

      • Example: Gneiss (with a silent G).

The Rock Cycle

  • Diagram from the textbook showing rock transformations.

  • Igneous rocks transform over time; the earth’s geological processes cause changes.

    • Example of cycle: Igneous rock -> erosion -> sediment -> lithification -> sedimentary rock -> increased pressure/temperature -> metamorphic rock -> melting -> new igneous rock.

    • Note: Change occurs over millions of years.

Introduction to Igneous Rocks

  • Igneous rocks: Solidified molten rock.

    • Transition from liquid to solid.

    • Construction: Most of the Earth’s crust consists of igneous rock.

  • Magma: Melt in the subsurface.

  • Lava: Melt at the surface.

    • Must distinguish between the two based on location.

Types of Igneous Rocks by Crystallization Location

  • Extrusive Rocks:

    • Cooled at or near the surface.

    • Characterized by rapid cooling, leading to small crystals.

  • Intrusive Rocks:

    • Cooled at depth within the earth.

    • Characterized by slower cooling, allowing larger crystals to form.

Methods of Creating Melt

  • Three primary methods of generating melt within the crust and mantle:

    • Pressure Release:

      • Drop in pressure can cause melting.

      • Example: Thinning the lithosphere via rifting resulting in decompression melting.

    • Adding Water:

      • Water lowers melting temperatures leading to melting processes.

      • Water introduced through subduction of water-rich tectonic plates.

      • Important note: The subducting plate itself typically does not melt but allows the overlying plate to melt due to added water.

    • Adding Heat:

      • Heat transfer from an already molten rock body raises temperatures of surrounding materials, causing melting.

Composition of Magma

  • Magma contains three states of matter:

    • Solids: Minerals carried within the melt.

    • Liquids: Mobile ions (predominantly silicon and oxygen).

    • Gases: Includes H2O, CO2, SO2 (usually a few percent of total composition).

Classification of Magmas

  • Four main types based on the relative proportion of silicate minerals:

    • Felsic: Rich in feldspar and silica (e.g., Quartz).

    • Mafic: Rich in magnesium and iron.

    • Intermediate: Mixture of mafic and felsic characteristics.

    • Ultramafic: Very high in magnesium.

Processes of Rock Melting

  • Partial Melting:

    • Rocks do not melt completely but only some minerals do.

    • Silica-rich minerals melt first at lower temperatures (e.g., Quartz and Feldspar).

    • Remaining rock consists of higher melting point minerals (e.g., Olivine and Pyroxene).

Varying Magma Composition

  • Magma composition can vary based on:

    • Extent of melting (e.g., 15% vs. 30% melting may yield different magma compositions).

    • Assimilation: Incorporation of surrounding rock materials into rising magma, changing its composition.

    • Fractional Crystallization: As magma cools, early forming minerals preferentially remove specific elements, changing the magma composition over time.

  • Example of elements: Early minerals (