How can igneous rocks be formed

Igneous rocks are classified according to their mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. Two important variables that are used for the classification of igneous rocks are particle size and the mineral composition of the rock. Feldspar, quartz, olivines, micas, etc.

Types of igneous rocks with other essential minerals are very rare. In simplified classification, igneous rocks are separated by the type of feldspar present, the presence or absence of quartz, and — in cases where feldspar or quartz are not present — by the type of iron or magnesium minerals present.

Rocks containing quartz are silica-oversaturated, while rocks with feldspathoids are silica-undersaturated. Igneous rocks which have crystals large enough to be seen with the unaided eye are classified as phaneritic, while those with crystals too small to be seen are aphanitic. Typically, rocks belonging to the phaneritic class are intrusive in origin, while aphanitic rocks are extrusive.

An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is classified as porphyry. Porphyritic textures develop when lava cools unevenly, causing of some of the crystals to grow before the main mass of the molten rock. So the next time you find yourself somewhere, just standing about, remember that the ground you walk on was formed under from a pretty hellish process.

It began deep in the Earth, where silicate rock, tormented by extreme heat and intense pressure, became a hot, oozing mess.

Once it was churned up to the surface. We have written many articles about igneous rocks for Universe Today. Geological Survey Website. Listen here, Episode Earth. Search Search.

Apply Filter. It is smoothly rounded on three sides and a sheer vertical face on the fourth. Half Dome, which stands nearly 8, feet 2, meters above sea level, is composed of granodiorite, and is the remains of a magma chamber that cooled slowly and crystallized Is glacier ice a type of rock?

Glacier ice, like limestone for example , is a type of rock. Glacier ice is actually a mono-mineralic rock a rock made of only one mineral, like limestone which is composed of the mineral calcite. The mineral ice is the crystalline form of water H 2 O. Most glacier ice forms through the metamorphism of tens of thousands of individual What are sedimentary rocks?

Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock. Common Sedimentary Rocks Are there geologic maps or publications for where I live?

Detailed geologic mapping has not been completed for the entire United States, but maps are available for most locations. Geologic maps at many scales and from many sources are listed in the National Geologic Map Database. Download digital geologic maps for entire states Where can I find information about the geology and natural history of National Parks?

Our National Parks are the showcases of our nation's geological heritage. The National Park Service has websites for most individual parks that include information about their geology and natural history. The website has listings for regions of the country What is the difference between a rock and a mineral? A mineral is a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties.

Common minerals include quartz, feldspar, mica, amphibole, olivine, and calcite. A rock is an aggregate of one or more minerals, or a body of undifferentiated mineral What are metamorphic rocks? This texture is called porphyritic. If so many bubbles are escaping from lava that it ends up containing more bubble holes than solid rock, the resulting texture is said to be frothy.

Pumice is the name of a type of volcanic rock with a frothy texture. If lava cools extremely quickly, and has very little water dissolved in it, it may freeze into glass, with no minerals glass by definition is not a mineral, because it does not have a crystal lattice. Such a rock is said to have a glassy texture. Obsidian is the common rock that has a glassy texture, and is essentially volcanic glass.

Obsidian is usually black. Now let us briefly consider textures of tephra or pyroclastic rocks. Like lava flow rocks, these are also extrusive igneous rocks. A pyroclastic rock made of fine-grained volcanic ash may be said to have a fine-grained, fragmental texture. Volcanic ash consists mainly of fine shards of volcanic glass. It may be white, gray, pink, brown, beige, or black in color, and it may have some other fine crystals and rock debris mixed in.

An equivalent term that is less ambiguous is tuffaceous. Rocks made of volcanic ash are called tuff. A pyroclastic rock with many big chunks of material in it that were caught up in the explosive eruption is said to have a coarse-grained, fragmental texture.

However, a better word that will avoid confusion is to say it has a brecciated texture, and the rock is usually called a volcanic breccia. When magma cools slowly underground and solidifies there, it usually grows crystals big enough to be seen easily with the naked eye. These visible crystals comprise the whole rock, not just part of it as in a porphyritic, fine-grained igneous rock.

The texture of an igneous rock made up entirely of crystals big enough to be easily seen with the naked eye is phaneritic. Phaneritic texture is sometimes referred to as coarse-grained igneous texture. Granite, the most well known example of an intrusive igneous rock, has a phaneritic texture. Sometimes an intrusion of magma that is crystallizing slowly underground releases large amounts of hot water.

The water is released from the magma as extremely hot fluid with lots of chemical elements dissolved in it. A rock consisting of such large minerals is said to have a pegmatitic texture, which means the average mineral size is greater than 1 cm in diameter and sometimes is much larger.

The name of an igneous rock with a pegmatitic texture is pegmatite. Pegmatites are commonly found in or near the margins of bodies of granite. The most common igneous compositions can be summarized in three words: mafic basaltic , intermediate andesitic , and felsic granitic. Felsic composition is higher in silica SiO 2 and low in iron Fe and magnesium Mg.

Mafic composition is higher in iron and magnesium and lower in silica. Intermediate compositions contain silica, iron, and magnesium in amounts that are intermediate to felsic and mafic compositions. Composition influences the color of igneous rocks. Felsic rocks tend to be light in color white, pink, tan, light brown, light gray. Mafic rocks tend to be dark in color black, very dark brown, very dark gray, dark green mixed with black.

The color distinction comes from the differences in iron and magnesium content. Iron and, to a lessor extent, magnesium give minerals a darker color. Intermediate igneous rocks tend to have intermediate shades or colors green, gray, brown.

The association between color and composition is useful because before you can name and interpret an igneous rock you need to determine both its texture AND its composition.

If you have an aphanitic igneous rock, which has no crystals big enough to see without a microscope, you can estimate its composition based on its color: pink or nearly white, felsic; medium gray, intermediate; very dark or black, mafic. This color rule works most of the time but there are two problems that you need to keep in mind. First, the rule does not work for glassy igneous rocks. Obsidian, which is volcanic glass, is usually black, even though it has a felsic composition.

That is because a tiny amount of iron, too little to color minerals very darkly, can color glass darkly. The second problem is that when igneous rocks have been exposed to air and water for a long time, they start to weather, which changes their color. If you can see and identify the minerals in an igneous rock, you can gain further information about the igneous composition.