Explain the Igneous rocks form?

Igneous rocks form from the crystallization of magma. There are two main kinds of igneous rocks: intrusive and extrusive

Molten rock within the Earth is called magma. When magma cools, it crystallizes into rocks and minerals. Generally, the longer the magma has to cool, sometimes thousands of years, the larger the crystals can grow inside the rock.

Intrusive rocks are rocks formed within the Earth, and can take thousands of years to cool and solidify. Extrusive rocks are produced when magma reaches the surface of our planet, at volcanic sites.

Magma originates at depths as great as two hundred kilometers. This molten rock consists mostly of the minerals which constitute the crust, including many silica-rich minerals. Magma also contains dissolved gasses, which can explode out when magma reaches the surface and becomes lava. The escaped gasses cause these rocks to have thousands of tiny holes where the gas bubbles escaped.

Magma reaches the surface of the Earth as volcanoes. Lava is simply magma which has reached the surface. There are two main types of volcanic eruptions on the Earth which produce lava and new rock material: andesitic and basaltic eruptions.

Andesitic volcanoes erupt violently, as was the case with Krakatoa and Mount Saint Helens. These andesitic volcanoes are fed by silica-rich magma sources from within the Earth. The silica adds an elastic quality to the molten rock; this flexibility allows the rocks to stretch and expand with trapped gasses and water until they "pop". This explosion releases the lava and gasses in a sudden and violent manor, capable of destroying an entire island, mountain, or area.

Basaltic eruptions, like the ones on the Hawaiian islands, are much more gentle than the andesitic variety. This is because the lava contains little or no silica and thus flows evenly, like a river, from the mouth of the volcano. These lava flows are visible in Iceland and on the big island of Hawaii. The basaltic eruption produces new rock which is black and filled with jagged edges and gas bubbles.