Igneous rocks

Igneous rocks

1. Granular

   • Rocks composed of similar sized mineral grains

2. Amygdales

   • Secondary minerals (i.e minerals that formed after the crystallization of the igneous rock infilling vesicles. Rocks, which display this are called amygdaoidal

3. Pahoehoe

   • Is a ropey structure seen on the surface of lava flow it is caused by the compression of surface lava in the ridges because it is flowing more slowly that the lava below

4. Pillow lava

   • Ball shaped masses of volcanic rock, which are formed by the eruption of lava in water

5. Columnar jointing

   • Six sided columns of igneous rock, created by the contracting of magma or lava during cooling

6. Aa

   • A blocky or clinker structure seen on the surface of lava

7. Euhedral

   • Mineral grains that show well developed crystal forms

8. Intersitial

   • Mosaic of small crystals that form in the spaces left by early-formed cyrstals

9. Anhedral

   • Mineral grains that show imperfect crystal forms

10. Glassy

    • Rocks that are composed of natural glass, such as obsidian. This texture developed when lava cools very quickly and crystals are unable to form

11. Holocrystaline

    • rocks that are wholly crystalline i.e entirely formed of crystals

12. Phorphyritic

    • the rock contains large crystals called phenocrysts set in a matrix of fine grained interstitial minerals. This texture develops due to the crystallization of minerals at different magma temperatures. The large phenocryst minerals crystalized first at the higher temperatures, and were able to grow unrestricted in the melt. The smaller matrix minerals formed later when the magma had cooloed to a lower temperature.

13. Vesicular

    • Rocks with holes created by escaping gas bubbles