Igneous Rocks An igneous rock is one that is formed from the cooling and solidification of molten (liquid) rock.

Extrusive vs. Intrusive Igneous Rocks Intrusive (plutonic) rock is one that forms from the cooling of magma (liquid rock) within the lithosphere of the Earth. Intrusive rocks are igneous rocks that have cooled slowly over time. Batholiths, laccoliths, dikes and sills are examples of igneous intrusions. Extrusive (volcanic) rock is one that forms from the cooling of lava (liquid rock) found at Earth’s surface. Extrusive rocks are igneous rocks that have cooled quickly over time. Lava flows and volcanoes are examples of igneous extrusions. Intrusions Extrusions

Formation of Igneous Rocks Igneous rocks form as a result of the solidification of liquid rock into solid rock. Most igneous rocks are made through a process of solidification known as crystallization. As an igneous rock cools into a solid form, different minerals form crystals that are inter-grown with other mineral crystals. This is known as crystallization. Inter-grown crystals of several minerals formed by crystallization.

Formation of Igneous Rocks – Crystal Size and Texture The texture of an igneous rocks and the type of crystals it exhibits depends on the environment in which it had formed. GRANITEGABBROGranite and Gabbro are both examples of coarse-textured igneous rocks. Both rocks have very large crystals that are easy to see. The development of LARGE CRYSTALS in an igneous rock indicate a LONG and SLOW PERIOD OF COOLING. Coarse-grained igneous rocks cooled deep within the lithospshere (intrusions) where the temperature and pressure are higher than that of Earth’s surface.

Formation of Igneous Rocks – Crystal Size and Texture The development of SMALL CRYSTALS in an igneous rock indicate a QUICK PERIOD OF COOLING. Fine-grained, igneous rocks cooled on top of the Earth’s surface (extrusions) where the temperature and pressure are not as extreme as deeper within the Earth. Some fine-grained, igneous rocks have vesicles – large air pockets that formed within the rock when it is cooled quickly. Sometimes igneous rocks cool so quickly that mineral crystals do not have time to form. Instead igneous rocks form a non-crystalline solid known as glass. RHYOLITEBASALT OBSIDIAN Rhyolite and Basalt are both examples of fine-textured igneous rocks. Both rocks have very small crystals that can only be seen through magnification. vesicles glassy texture

Identification of Igneous Rocks RT = Pg. 6

Identification of Igneous Rocks Which igneous rock is intrusive and has crystal sizes greater than 10 mm? PEGMATITE Describe RHYOLITE. -- Extrusive (volcanic) -- Crystal Size is less than 1 mm -- Fine texture -- Non-vesicular -- Light in color -- Low Density -- Felsic composition (rich in Si and Al – silicon and aluminum) -- Mineral Composition consists of potassium feldspar, quartz, plagioclase feldspar, biotite and amphibole.