IGNEOUS ROCKS - SLG 8.2.1

• The word "igneous" came from the Latin word "ignis" which means fire.

  Igneous rocks are formed from a hot and molten mass of melted rock materials called magma or lava.
• Magma - is a molten rock material mostly found in the Earth's mantle.

  Lava - when magma reaches the surface.
• There are two classifications of Igneous rocks, extrusive and intrusive igneous rocks.
• Extrusive - came from the Latin word "extrudere" which means to thrust out. It is magma that has cooled and solidified on the surface of the Earth. Is often volcanic.
• Intrusive - came from the Latin word "intrudere" which means to thrust out. It is a type of igneous rock that forms when magma cools and solidifies within the Earth's crust. Is often plutonic.
• There are 4 types of Igneous rocks based on its composition.

  1. Granitic (Felsic)
  2. Basaltic (Mafic)
  3. Andesitic
  4. Ultramafic
• Granitic rocks are made up almost entirely of light-colored silicates which are quartz and potassium feldspar. Most granitic rocks contain 10% of dark silicates.
• Basaltic rocks are composed mainly of dark colored silicate materials and calcium-rich plagioclase but no feldspar such as pyroxene and olivine. They also mafic rocks. They are usually denser than granitic rocks and are mostly found on the ocean floor.
• Andesitic rocks are intermediate between granitic and basaltic. They have an intermediate composition. They contain at least 25% of dark silicate minerals (amphibole, pyroxene, and biotite mica) together with the other minerals - plagioclase and feldspar.
• Ultramafic rocks or peridotite are the most common rocks found in the upper mantle. They are mostly composed of olivine and pyroxene. They are almost entirely made up of ferromagnesian (iron and magnesium) minerals.
• Texture - refers to the overall appearance of the rocks based on size, shape, and arrangement of the mineral grains.

  1. Fine-grained
  2. Coarse-grained
  3. Poryphyritic
  4. Vesicular
  5. Glassy
  6. Pyroclastic
• Fine-grained - crystals are too small to be seen by the naked eye and need to be seen through a polarizing microscope. Light, intermediate, or dark in color.
• Coarse-Grained - consists of a mass of intergrown crystals that are almost of equal size. The crystals are large enough which makes it possible to identify the minerals without the use of a microscope.
• Poryphyritic - large crystals are embedded in a bed of smaller crystals. They are composed of 2 distinct sizes of minerals
• Vesicular - has almost spherical-shaped voids or holes called vesicles. These holes are formed due to bubbles that escape as the lava is soldified.
• Glassy - its texture is that of a glass, shiny and smooth. It is formed when a volcanic material is ejected into the atmosphere where it cools quickly.
• Pyroclastic - formed from the merging of individual rock fragments of pyroclastic materials ejected during an explosive eruption. It is also known as fragmental texture.