Why igneous rocks are crystalline

The size of the crystals depends on how quickly the molten magma solidified:. This means that we get two main types of igneous rock, extrusive and intrusive, as shown in the table:. Extrusive igneous rocks form from magma that erupted onto the surface as lava , where it cooled quickly. On the other hand, intrusive igneous rocks form from magma that cooled slowly, deep underground. Igneous rocks do not contain any fossils.

This is because any fossils in the original rock will have melted when the rock melted to form magma. You may have done an experiment at school with a substance called salol.

The composition of the rocks melted, and thus the composition of the magma produced, depends on the tectonic environment. Figs 4. The melts eventually pool in large cavities to form magma chambers. An earlier formed mineral may react with the magma and be converted to the next mineral in the series e.

Early Ca-rich plagioclase transform to more Na-rich plagioclase through reaction with an increasingly silicic melt as the temperature drops. As early-formed crystals remove certain elements from the melt e. Mg, Fe by olivine and pyroxene , the remaining melt becomes enriched in elements left behind e. Si, Na, K, Al. As a result, minerals crystallizing later from the compositionally modified melt will be composed of those elements left behind in the liquid.

This process by which an initial parent magma of one composition can produce rocks of a variety of compositions via crystallization is called magmatic differentiation. Finally at late stages of differentiation, felsic rocks are primarily formed from elements left within the remaining liquid, Si, K, Na, and Al. The above scenario occurs during equilibrium crystallization.

During equilibrium crystallization, the crystals are in continuous contact with the evolving magma. These crystals can therefore change through continual reaction with the evolving liquid. Ca-rich plagioclase can become progressively more Na rich and olivines can be converted into pyroxenes through continued reaction with the evolving magma as temperatures drop. Some igneous rocks show compositionally zoned crystals where the rims have a more evolved composition relative to the cores.

Plagioclase can have Ca-rich cores and Na-rich rims. Olivine may be surrounded by a rim of pyroxene. In both cases, only partial reaction with the melt has taken place. Somehow, these early formed crystals became separated or isolated from the remaining melt before any reaction could occur.

Fractional crystallization is the term used for this separation and removal of successive fractions of crystals formed from a cooling magma. Extrusive rocks are formed on the surface of the Earth from lava , which is magma that has emerged from underground. Intrusive rocks are formed from magma that cools and solidifies within the crust of the planet. When lava comes out of a volcano and solidifies into extrusive igneous rock, also called volcanic, the rock cools very quickly.

Crystals inside solid volcanic rocks are small because they do not have much time to form until the rock cools all the way, which stops the crystal growth. If lava cools almost instantly, the rocks that form are glassy with no individual crystals, like obsidian.

There are many other kinds of extrusive igneous rocks. Intrusive rocks, also called plutonic rocks, cool slowly without ever reaching the surface. They have large crystals that are usually visible without a microscope. This surface is known as a phaneritic texture. Perhaps the best-known phaneritic rock is granite. One extreme type of phaneritic rock is called pegmatite , found often in the U. Pegmatite can have a huge variety of crystal shapes and sizes, including some larger than a human hand.

Rock texture with crystals that are invisible without magnification. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited. Tyson Brown, National Geographic Society.