Igneous Rocks

• Igneous rocks are the types of rocks formed from the intense pressure and heat that occur within the earth. The rocks liquify due to the very high temperatures within the mantle of the earth. This liquified substance -- magma, or molten rock -- carries an amalgam of many different minerals. When magma reaches the surface of the earth, this liquid rock becomes lava. Many different kinds of igneous rocks exist, categorized based on their mineral composition and whether they cooled within or outside the earth.
  
  

Intrusive Rocks

• One form of igneous rocks, intrusive rocks, derive directly from magma and solidifies within the earth. Since intrusive rocks are within the earth, they cool very slowly -- taking anywhere from thousands to millions of years to cool enough to completely solidify. The cooling rate of intrusive rocks enables the crystals to form that are visible to the naked eye, which gives them a coarse grain compared to extrusive rocks. This visible grain is called a phaneritic texture. Granite is one example of an intrusive igneous rock.
  
  

Extrusive Rocks

• Extrusive rocks are formed from lava and form outside of the earth. When lava is exposed to the atmosphere or water outside of the earth, this causes the lava to cool very rapidly in comparison to intrusive rocks. This rapid cooling does not allow the rock time to form large crystals in the way that intrusive rocks do. Extrusive rocks have a fine-grained texture known to geologists as aphanitic, because the mineral crystals present within the rock are very small. Examples of extrusive igneous rocks are basalt and obsidian.
  
  

Mineral Composition

• Igneous rocks are further subdivided by the presence of certain minerals in their composition. Three subclassifications for intrusive and extrusive rocks are basic, intermediate or acidic. These classifications refer to how much silica is present in the rock's composition. Silica is a mineral predominantly found in sand. Rocks with a high concentration of silica are known as acidic rocks. Granite is an example of an intrusive rock that contains a high concentration of silica in the form of its quartz crystals. Rhyolite is the extrusive counterpart of granite due to its high silica concentration, but fine-grained texture.