Notes on Types of Rocks

Types of Rocks

What is a Rock?

  • Definition: A rock is a naturally occurring solid mixture of one or more minerals, or organic matter.
  • Classification: Rocks are classified based on:
  • How they are formed
  • Composition
  • Texture
  • Change: Rocks change over time through the rock cycle.

The 3 Types of Rocks

  1. Igneous Rocks
  2. Sedimentary Rocks
  3. Metamorphic Rocks

Igneous Rocks

  • Formation: Igneous rock begins as magma, which can form from:
  • Heating of rock
  • Release of pressure
  • Changes in rock composition
  • Temperature of Magma: Freezes between 700 °C to 1,250 °C.
  • Composition: A mixture of many minerals.
Types of Igneous Rocks
  • Felsic: Light-colored rocks (e.g., Granite, Rhyolite).
  • Mafic: Dark-colored rocks (e.g., Gabbro, Basalt).
  • Grain Size:
  • Coarse-Grained: Large crystals (e.g., Granite).
  • Fine-Grained: Small crystals (e.g., Rhyolite).
  • Intrusive vs. Extrusive:
  • Intrusive: Formed from magma that cools slowly beneath the Earth’s surface.
  • Extrusive: Formed from lava that erupts onto the Earth's surface and cools rapidly.
    • Example: Obsidian - a dark volcanic glass that cools too quickly for crystals to form.
    • Is it fine-grained or coarse-grained? Fine-grained
    • Is it Intrusive or Extrusive? Extrusive

Sedimentary Rocks

  • Formation Process: Formed by erosion and deposition of sediments.
  • Sediments are moved and deposited in layers, with older layers at the bottom.
  • Layers (strata) compact and cement together over time.
  • Characteristics: Formed at or near Earth's surface, no heat or pressure involved.
  • Types of Sedimentary Rocks:
  • Clastic: Made of fragments of other rocks, cemented with calcite or quartz.
  • Chemical: Minerals crystallize from solution (e.g., Limestone - primarily calcium carbonate).
  • Organic: Composed of plant/animal remains (e.g., Coal - formed from plant materials, found in swamp environments).

Metamorphic Rocks

  • Definition: Metamorphic means to change shape.
  • Formation: Changes occur with temperature and pressure, but rocks remain solid.
  • Usually occurs deep within the Earth.
Types of Metamorphism
  1. Contact Metamorphism: Rock changes due to heat from nearby magma. Minerals are transformed into new minerals.
  2. Regional Metamorphism: Results from pressure buildup in rocks deep within the Earth, where large pieces of crust collide and deform the rock through heat and pressure.
Characteristics of Metamorphic Rocks
  • Foliated: Contains aligned grains of flat minerals (e.g., Gneiss - banded appearance, rich in quartz or feldspar).
  • Non-Foliated: Mineral grains are not aligned (e.g., Marble - formed from the metamorphism of limestone, primarily calcium carbonate).
Classification of Metamorphic Rocks
  • Determine if rock samples are foliated or non-foliated:
  • Amphibolite: Typically non-foliated.
  • Quartzite: Non-foliated, composed of quartz grains.
  • Phyllite: Foliated.

Conclusion

  • A comprehensive overview of rocks includes understanding their definitions, formation processes, classifications, and characteristics. This knowledge is essential in geology and earth sciences.