Magma hardens to form what kind of rock

The most common type of extrusive rock is basalt. Intrusive, or plutonic, igneous rocks form underneath the Earth's surface when magma flows into underground chambers or tunnels. The rock is not exposed to the atmosphere above surface, so the magma cools slowly which allows large mineral crystals to form within the rock. It takes thousands of years for Intrusive rocks to form.

Hypabassal, or subvolcanic, rock derives from magma that has solidified at a shallow depth of the volcano, mainly in dykes and sills. This type of rock is formed in between extrusive and intrusive rock, and similarly has a texture in between that of intrusive and extrusive rock. This type of rock is rarer than extrusive and intrusive varieties, and often occurs at continental boundaries and oceanic crusts.

Andesite is the most common type of hypabassal rock. Over different types of igneous rocks have been discovered to date. These vary in terms of appearance, grain size and amount of time that it takes for the lava to cool. A common igneous rock rule is that if lava cools at a faster rate, the rock formed will have finer grains and have a glassy appearance; if rock cools at a slower rate, the grains will be larger and more coarse.

Porphyritic rock is a type that has a combination of large and small grains; this occurs when a rock has a mixed cooling history. You may have done an experiment at school with a substance called salol.

If molten salol cools slowly, you get big crystals. If it cools quickly, you get small crystals. Igneous rocks Igneous rocks are formed from molten rock that has cooled and solidified. What are igneous rocks like?

The size of the crystals depends on how quickly the molten magma solidified: magma that cools slowly will form an igneous rock with large crystals lava that cools quickly will form an igneous rock with small crystals This means that we get two main types of igneous rock, extrusive and intrusive, as shown in the table: Extrusive Intrusive Where the magma cooled On the surface Underground How fast the magma cooled Quickly Slowly Size of crystals Small Large Examples Obsidian and basalt Granite and gabbro Extrusive igneous rocks form from magma that erupted onto the surface as lava , where it cooled quickly.

Obsidian has tiny crystals. The diagram above shows you where magma is produced at a subduction zone. Magma is less dense than the surrounding rock which causes it to rise.

When magma reaches the surface it is then called lava and the eruptions of lava and ash produce volcanoes. The lava that reaches the Earth's surface will harden and become igneous rock. When the magma does not reach the surface it produces a variety of geologic structures.

When lava reaches the surface of the Earth through volcanoes or through great fissures the rocks that are formed from the lava cooling and hardening are called extrusive igneous rocks. Some of the more common types of extrusive igneous rocks are lava rocks, cinders, pumice, obsidian, and volcanic ash and dust. This is the volcano Paricutin that is located in Mexico. It is erupting cinders and pumice which are examples of extrusive igneous rocks.

Millions and even billions of years ago molten rock was cooling and thus hardening into igneous rocks deep under the surface of the Earth. These rocks are now visible because mountain building has thrust them upward and erosion has removed the softer rocks exposing the much harder igneous rocks.

These are called intrusive igneous rocks because the magma has intruded into pre-exiting rock layers. Types of intrusive igneous rocks are granite and basalt. The diagram above shows you a large intrusive igneous body called a batholith. A batholith is the largest of the intrusive bodies. They are larger than square kilometers and usually form granite cores. As you can see in the diagram above a batholith is a very large intrusive igneous body.

There are two types of intrusive bodies that we are going to discuss 1 Discordant and 2 Concordant. A discordant igneous rock body cuts across the pre-exiting rock bed. Batholiths and dikes are examples of discordant rock bodies. A dike is a vertical or near vertical intrusive igneous rock body that cuts across rock beds.

They frequently form from explosive eruptions that crack the area around a volcano with the magma filling the cracks forming a dike. A concordant igneous rock body runs parallel to the pre-existing bedrock. Laccoliths and sills are examples of concordant igneous rock bodies. A laccolith is a dome shaped intrusive body that has intruded between layers of sedimentary rock. The rising magma forces the overlying layers to rise up into a dome. A sill is similar to a dike with the exception that sills run parallel to the existing rock bed instead of cutting through it.

The composition of igneous rocks falls into four main categories.