Why do igneous rocks have large crystals

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. Granite has large crystals. On the surface. Any interactives on this page can only be played while you are visiting our website.

You cannot download interactives. Metamorphic rocks start as one type of rock and—with pressure, heat, and time—gradually change into a new type of rock. Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. Pele's hair. Media Credits 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.

Media If a media asset is downloadable, a download button appears in the corner of the media viewer. Text Text on this page is printable and can be used according to our Terms of Service. Interactives Any interactives on this page can only be played while you are visiting our website. Similarly, a rock with small crystals probably formed at or near the surface and cooled quickly.

Sometimes, a rock will contain both aphanitic and phaneritic crystals in it. This means that something truly odd happened to the magma before it was erupted. Since we know that large crystals need time to grow, the magma must have spent some time deep underground. But the smaller crystals mean that the rest of the cooling happened very quickly. If a rock has both crystal types, it means that the mamga spent some time in a magma chamber, where the large crystals grew, then was violently erupted onto the surface, where the small crystals were formed.

We can simulate the growth of minerals using some common materials; just about everyone has grown salt and sugar crystals from a supersaturated solution. However, those experiments take more time than is usually available in the classroom, so we have developed a demonstration based on one given in Jackson, J. Melting the mothballs and crayon mixture. Cooling the mothballs and crayon mixture. Extrusive igneous rocks form after lava cools above the surface.

Figure 5. Cooled lava forms basalt with no visible crystals. Why are there no visible crystals? Extrusive igneous rocks cool much more rapidly than intrusive rocks.

There is little time for crystals to form, so extrusive igneous rocks have tiny crystals figure 5. As we just learned, there are two main types of igneous rocks: intrusive rocks also known as plutonic rocks and extrusive rocks also known as volcanic rocks.

Volcanic rocks break down into two more categories: a lava flows and b tephra pyroclastic material. Igneous rocks are classified on the basis of their composition and their texture. Magma, and the igneous rock it becomes, has a range of chemical compositions. For example, basalt is a mafic lava flow rock which originates from melting of the upper mantle.

The way that magma turns into a solid rock gives it a distinctive igneous texture. For example, magma that becomes a pluton by slowly crystallizing growing minerals within the crust will develop a very different texture from magma that becomes an ash flow tuff as a result of semi-molten volcanic ash spewing across a landscape and then settling down and welding itself together into solid rock.

Igneous textures include the rock textures occurring in igneous rocks. Igneous textures are used by geologists in determining the mode of origin of igneous rocks and are used in rock classification. There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic.

Because extrusive rocks make contact with the atmosphere they cool quickly, so the minerals do not have time to form large crystals. The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye.

Examples of aphanitic igneous rock include basalt, andesite and rhyolite. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur.

The result is a natural amorphous glass with few or no crystals. Examples include obsidian and pumice. Pegmatitic texture occurs during magma cooling when some minerals may grow so large that they become massive the size ranges from a few centimetres to several metres.

This is typical of pegmatites. As magma cools slowly the minerals have time to grow and form large crystals. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye. Examples of phaneritic igneous rocks are gabbro, diorite and granite. Porphyritic textures develop when conditions during cooling of a magma change relatively quickly.