Igneous Rocks By. Group 1 Jackie, Andrew, Brittany, and Jessica.

What is an Igneous Rock? An Igneous Rock are one of the three main rock types. Rocks that form when molten cools and crystallizes.

Igneous Rock Facts Igneous Rocks make up approximately 90% of the upper part of the earths crust, but their great abundance is hidden on the earth’s surface by a relatively thin but widespread layer of sedimentary and metamorphic rocks.

Classifying Igneous Rocks Igneous rocks are based on mineral composition (Color), and texture (Crystal Size). Differences in magma composition can be observed in the rocks that form when the magma cools and crystallizes. Texture is used to subdivide igneous rocks into four groups Glassy, Fine-grained, Coarse-grained, and Very coarse-grained.

Mineral Composition Rock type can be determined by estimating the relative percentages of minerals in the rocks.

Intrusive Rocks An intrusive igneous rock is when magma rocks is underground, cools and hardens. Intrusive igneous rocks form in magma chambers that are deep underground. Many kilometers below the Earth surface, molten rock called magma flows into cracks or underground chambers. There, the magma sits, cooling very slowly over thousands to millions of years. As it cools, element combine to form common silicate minerals, the building blocks of igneous rocks. The mineral crystals within them are large enough to see without a microscope. There are many different types of intrusive igneous rocks but granite is the most common type.

Extrusive Rocks Extrusive igneous rock forms when molten rock reaches the earth's surface and cools. Air and moisture cool the lava rapidly. We can see holes left in the rock where the bubbles of gas were located. These rocks are also known as vesicular rocks.

Vesicular Rocks Magma contains dissolved gases that escape when pressure on the magma lessens. The holes in the rocks are called vesicles. Example Pumice-