Technical Announcements. Employees in the News. Emergency Management. Survey Manual. Igneous rocks from the Latin word for fire form when hot, molten rock crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface.
Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies. Intrusive Igneous Rocks: Intrusive, or plutonic, igneous rock forms when magma is trapped deep inside the Earth. Great globs of molten rock rise toward the surface. Some of the magma may feed volcanoes on the Earth's surface, but most remains trapped below, where it cools very slowly over many thousands or millions of years until it solidifies.
Slow cooling means the individual mineral grains have a very long time to grow, so they grow to a relatively large size. Intrusive rocks have a coarse grained texture.
Extrusive Igneous Rocks: Extrusive, or volcanic, igneous rock is produced when magma exits and cools above or very near the Earth's surface. These are the rocks that form at erupting volcanoes and oozing fissures. The magma, called lava when molten rock erupts on the surface, cools and solidifies almost instantly when it is exposed to the relatively cool temperature of the atmosphere.
Quick cooling means that mineral crystals don't have much time to grow, so these rocks have a very fine-grained or even glassy texture. Hot gas bubbles are often trapped in the quenched lava, forming a bubbly, vesicular texture.
Ever wondered what the difference between a rock and a mineral was? This EarthWord should cover it As fall foliage begins to blanket New Hampshire, pleasantly diverting the attention of residents and visitors, scientists are preparing to unveil some of the geologic secrets of the famous yet not-well-known rocks that lie beneath the fiery cover. The photo was taken by Erin Todd during helicopter-supported geologic fieldwork in Lake Clark National Park this past summer.
The point on the left was made by an anthropology major at Stanford University. The piece on the right has not been worked. E-mail C. Jones with comments or corrections. Igneous rocks are called intrusive when they cool and solidify beneath the surface. Intrusive rocks form plutons and so are also called plutonic.
A pluton is an igneous intrusive rock body that has cooled in the crust. When magma cools within the Earth, the cooling proceeds slowly. Slow cooling allows time for large crystals to form, so intrusive igneous rocks have visible crystals.
Granite is the most common intrusive igneous rock see figure 2 for an example. Igneous rocks make up most of the rocks on Earth. Most igneous rocks are buried below the surface and covered with sedimentary rock, or are buried beneath the ocean water.
In some places, geological processes have brought igneous rocks to the surface. Figure 3. Igneous rocks are called extrusive when they cool and solidify above the surface. These rocks usually form from a volcano, so they are also called volcanic rocks figure 4.
Figure 4. 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. Text on this page is printable and can be used according to our Terms of Service. 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.
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