Igneous Rocks

Igneous Rocks Overview

  • Igneous rocks are formed when magma cools and solidifies, either inside the earth as intrusive rocks or on the surface as extrusive rocks.
  • They are one of the three main types of rocks, alongside sedimentary and metamorphic rocks.

Extrusive Igneous Rocks

  • Formed when magma erupts at the surface, cools and solidifies quickly.
  • Include basalt and andesite, which are common in oceanic and continental settings respectively.
  • Often have a fine-grained or glassy texture due to the fast rate of cooling which prevents the formation of large crystals.

Intrusive Igneous Rocks

  • Formed when magma cools and solidifies below the earth’s surface.
  • Include granite, diorite and gabbro, each with different mineral compositions.
  • Generally have a coarse-grained texture due to the slow rate of cooling, which allows time for large crystals to form.

Formation of Igneous Rocks

  • Igneous rocks form through the process of crystallisation where cooling magma becomes a solid mass of interlocking crystals.
  • Differs from sedimentary and metamorphic rock formation, which involve deposition and heat/pressure respectively.

Mineral Composition

  • Composed mainly of silicate minerals such as quartz, feldspar and mica.
  • Presence of specific minerals can help identify the type of igneous rock, e.g. high percentage of quartz suggests granite.

Classification of Igneous Rocks

  • Can be classified based on their texture (fine-grained, coarse-grained, glassy, etc.) and their mineral composition.
  • Other factors like the percentage of dark (mafic) or light (felsic) minerals can also be important.

Role in the Rock Cycle

  • Subsequent weathering or burial of igneous rocks can lead to their transformation into sedimentary or metamorphic rocks, completing their part in the rock cycle.
  • They can be re-melted through subduction or other processes, forming new magma and potentially new igneous rocks.

Importance of Igneous Rocks

  • Many economically valuable minerals such as copper, nickel, and precious metals are found in igneous rock formations.
  • They also provide valuable information about the earth’s interior and the history of our planet.
  • Useful for understanding plate tectonics as they often form at plate boundaries.

Remember to keep revising the various properties and types of igneous rocks, their role in the rock cycle and their overall importance in geological understanding.