What Can Happen to an Igneous Rock After It Is Formed

Identify igneous rocks and the steps of the rock bicycle related to their formation.

We'll showtime by learning well-nigh igneous rocks and their formation.

What You'll Learn to Practice

• Define the characteristics of an igneous rock.
• Discuss the role of melting and subsequent cooling in the rock cycle.

Characteristics of Igneous Rocks

Magma is molten rock inside the globe. It is the source of all igneous stone. Considering the earth was largely molten at its origin, magma may be considered the offset of the stone cycle. Igneous rocks contain data about how they originate. By carefully analyzing igneous rocks and interpreting the information they comprise, nosotros can deduce processes that have identify within the earth and we can understand volcanic processes that accept place on the earth's surface.

The study of igneous rocks enables us to understand the igneous role of geologic history. For example, at the finish of the Triassic period, 245 one thousand thousand years ago, the greatest mass extinction ever known took place, wiping out more life forms on earth than the mass extinction that led to the demise of dinosaurs 65 1000000 years ago at the end of the Cretaceous. At the stop of the Triassic, a huge corporeality of basalt erupted onto the earth. Many geologists think that the gases and particles released into the atmosphere by those eruptions may accept been a major cistron in the end of Triassic mass extinction. Those scientists are studying the data contained in the basalts of that age to further test their hypotheses.

Igneous rocks contain 3 essential sources of data: their minerals, their overall chemical composition, and their igneous texture. Igneous stone names are based on specific combinations of these features. Igneous rocks also incorporate isotopic information that is used in determining accented ages and in further characterizing the origin of the magma. Special equipment and expertise is required to conduct isotopic and precise chemic analyses. Fortunately, with some bones grooming and practice anyone tin acquire to place the minerals, composition and texture of an igneous rock; name the rock; and translate key information almost its origins.

All igneous rocks, other than pure volcanic glass, contain minerals. The minerals provide details on the chemic limerick of the rock, and on the conditions in which the magma originated, cooled, and solidified. Geologists conduct chemic analyses of minerals to determine the temperatures and pressures at which they formed and to identify the dissolved gases and chemical elements that were nowadays in the magma.

Most magmas are predominantly silicate liquids, composed largely of silica tetrahedra that have not yet bonded together to become silicate minerals. The chemical composition of an igneous rock tells us about the origin of the magma, beginning with which blazon of rock melted inside the earth to grade the magma in the kickoff place, and how deep in the world the melting occurred. Once magma has formed inside the earth, its limerick may be modified. Minerals can abound from the magma and separate from it, changing the chemistry of the remaining liquid. Or, 1 torso of magma can mix with another that has a different composition.

Magmas come in a range of compositions, from rich in silica and poor and iron and magnesium (felsic) to moderate in silica and high in iron and magnesium (mafic). Felsic igneous rocks, equally a whole rock, tend to have light colors or shades: white, pinkish, light dark-brown, light gray. Mafic igneous rocks, on the whole, tend to be night colored, commonly black or dark gray. Most mafic magma originates past melting of rocks in the mantle that are extremely rich in iron and magnesium. Felsic magma usually originates in the chaff or by the shedding of mafic minerals as magma rises through the crust.

The igneous texture tells the states how the magma cooled and solidified. Magma tin solidify into igneous stone in several different means, each way resulting in a different igneous texture. Magma may stay within the earth, far below basis level, and crystallize into plutonic igneous rock (also known as intrusive igneous rock). Or, magma may catamenia out onto surface of the globe every bit a lava flow. Another way that igneous rock forms is by magma erupting explosively into the air and falling to earth in pieces known every bit pyroclastic material, also called tephra. Lava flows and pyroclastic cloth are volcanic igneous rock (as well known as extrusive igneous rock).

The igneous texture of a stone is not how it feels in your hand, not whether it is rough or smooth. The igneous texture describes whether the rock has mineral crystals or is glassy, the size of the mineral grains, and the rock'south porosity (empty spaces).

How Are Igneous Rocks Classified?

Figure 1. Granite is made of four minerals, all visible to the naked centre: feldspar (white), quartz (translucent), hornblende (black), and biotite (black, platy).

This page focuses on igneous rocks and gives yous the background needed to empathise the terms used in the igneous rock nomenclature table (at the bottom of this page).

Intrusive and Extrusive Igneous Rocks

Igneous rocks are called intrusive when they cool and solidify beneath the surface. Intrusive rocks form plutons and and so are also called plutonic. A pluton is an igneous intrusive stone trunk that has cooled in the chaff. When magma cools within the Earth, the cooling proceeds slowly. Slow cooling allows fourth dimension for large crystals to course, and then intrusive igneous rocks take visible crystals. Granite is the most common intrusive igneous rock (encounter Figure 1 for an example).

Igneous rocks make up most of the rocks on Earth. Nigh igneous rocks are cached beneath the surface and covered with sedimentary rock, or are buried below the body of water water. In some places, geological processes accept brought igneous rocks to the surface. Effigy 2 below shows a landscape in California's Sierra Nevada made of granite that has been raised to create mountains.

Figure 2. California's Sierra Nevada is intrusive igneous rock exposed at Earth'south surface.

Igneous rocks are called extrusive when they absurd and solidify above the surface. These rocks unremarkably course from a volcano, so they are also called volcanic rocks(Effigy 3).

Figure 3. Extrusive igneous rocks class after lava cools above the surface.

Figure iv. Cooled lava forms basalt with no visible crystals. Why are there no visible crystals?

Extrusive igneous rocks absurd much more speedily than intrusive rocks. There is little time for crystals to course, so extrusive igneous rocks have tiny crystals (Figure 4).

Classifying Igneous Rocks

As we just learned, there are 2 chief types of igneous rocks: intrusive rocks (also known as plutonic rocks) and extrusive rocks (also known equally volcanic rocks). Volcanic rocks break down into two more categories: (a) lava flows and (b) tephra (pyroclastic material).

Igneous rocks are classified on the ground of their composition and their texture. Magma, and the igneous rock it becomes, has a range of chemic compositions. For example, basalt is a mafic lava flow stone which originates from melting of the upper drape. The mode that magma turns into a solid stone gives information technology a distinctive igneous texture. For example, magma that becomes a pluton by slowly crystallizing (growing minerals) within the crust volition develop a very unlike texture from magma that becomes an ash flow tuff as a result of semi-molten volcanic ash spewing across a mural so settling downwardly and welding itself together into solid rock.

Igneous Rock Textures

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

Aphanitic (a = non, phaner = visible) rocks in contrast to phaneritic rocks, typically form from lava which crystallize rapidly on or near Earth' surface. Because extrusive rocks make contact with the temper they cool quickly, so the minerals do not take time to class large crystals. The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye. Examples of aphanitic igneous stone include basalt, andesite andrhyolite.

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 baggy glass with few or no crystals. Examples includeobsidian and pumice.

Pegmatitic texture occurs during magma cooling when some minerals may abound so large that they become massive (the size ranges from a few centimetres to several metres). This is typical of pegmatites.

Phaneritic (phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below Earth's surface. As magma cools slowly the minerals have fourth dimension to grow and class 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 apace. The before formed minerals volition have formed slowly and remain as large crystals, whereas, sudden cooling causes the rapid crystallization of the remainder of the melt into a fine grained (aphanitic) matrix. The result is an aphanitic rock with some larger crystals (phenocrysts) imbedded within its matrix. Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted earlier completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals.

Pyroclastic (pyro = igneous, clastic = fragment) textures occur when explosive eruptions nail the lava into the air resulting in fragmental, typically glassy material which fall equally volcanic ash, lapilli and volcanic bombs.

Effigy v. Different cooling rate and gas content resulted in these different textures.

Volcanic Rocks

Let us start with textures associated with rocks formed by lava flows. Magmas that erupt equally lava onto the earth's surface cool and solidify rapidly. Rapid cooling results in an aphanitic igneous texture, in which few or none of the individual minerals are big enough to run into with the naked eye. This is sometimes referred to as a fine-grained igneous texture.

Some lava flows, still, are not purely fine-grained. If some mineral crystals start growing while the magma is still secret and cooling slowly, those crystals abound to a large enough size to be easily seen, and the magma then erupts as a lava flow, the resulting texture will consist of coarse-grained crystals embedded in a fine-grained matrix. This texture is called porphyritic.

If lava has bubbles of gas escaping from it as information technology solidifies, it will cease up with "frozen bubble holes" in it. These "frozen bubble holes" are called vesicles, and the texture of a rock containing them is said to be vesicular.

If and 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 information technology, it may freeze into glass, with no minerals (drinking glass past 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 mutual rock that has a burnished texture, and is substantially volcanic glass. Obsidian is unremarkably black.

At present permit the states briefly consider textures of tephra or pyroclastic rocks. Like lava flow rocks, these are also extrusive igneous rocks. However, instead of originating from lava that flowed on the world's surface, tephra is volcanic material that was hurled through the air during a volcanic eruption.

A pyroclastic rock made of fine-grained volcanic ash may exist said to have a fine-grained, fragmental texture. Volcanic ash consists mainly of fine shards of volcanic glass. Information technology may exist white, greyness, pink, brownish, beige, or blackness in color, and information technology may accept some other fine crystals and stone debris mixed in. The term "fine-grained, fragmental" is easy to misfile with the term fine-grained (aphanitic). 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 fibroid-grained, fragmental texture. However, a better discussion that will avoid confusion is to say information technology has a brecciated texture, and the stone is commonly called a volcanic breccia. The bigger chunks of cloth in a volcanic breccia are more than than 1 cm (5/8 inch) beyond, and sometimes are much bigger.

Plutonic Rocks

When magma cools slowly underground and solidifies there, information technology usually grows crystals big enough to exist seen easily with the naked eye. These visible crystals comprise the whole rock, not only part of it every bit in a porphyritic, fine-grained igneous stone. The texture of an igneous rock made up entirely of crystals big enough to be hands 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 stone, 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. This hydrothermal fluid gets into cracks and voids in the earth'due south crust, and equally it cools information technology may grow very big minerals from the dissolved chemical elements. A rock consisting of such large minerals is said to have a pegmatitic texture, which means the average mineral size is greater than ane cm in diameter (and sometimes is much larger). The name of an igneous rock with a pegmatitic texture is pegmatite. Pegmatites are normally found in or near the margins of bodies of granite.

Igneous Stone Compositions

The nearly mutual igneous compositions can exist summarized in iii words: mafic (basaltic), intermediate (andesitic), and felsic (granitic).

Felsic composition is higher in silica (SiO₂) and low in iron (Fe) and magnesium (Mg). Mafic limerick is higher in fe and magnesium and lower in silica. Intermediate compositions contain silica, fe, and magnesium in amounts that are intermediate to felsic and mafic compositions.

Limerick and Colour

Composition influences the color of igneous rocks. Felsic rocks tend to be light in color (white, pink, tan, light brown, lite gray). Mafic rocks tend to exist night in colour (blackness, very dark brownish, very dark grayness, nighttime green mixed with black). The color distinction comes from the differences in iron and magnesium content. Fe and, to a lessor extent, magnesium give minerals a darker color. Intermediate igneous rocks tend to take intermediate shades or colors (greenish, grey, brown).

The association between color and composition is useful because before yous can name and translate an igneous rock yous demand to decide both its texture AND its composition. If you take an aphanitic igneous stone, which has no crystals large enough to come across without a microscope, you lot can estimate its composition based on its color: pinkish or about white, felsic; medium grayness, intermediate; very dark or black, mafic.

This color rule works most of the time but at that place are 2 problems that you demand to continue in mind. First, the rule does not piece of 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 corporeality of iron, besides little to color minerals very darkly, can color drinking glass darkly.

The second problem is that when igneous rocks have been exposed to air and h2o for a long time, they start to atmospheric condition, which changes their color. Geologists working in the field carry a rock hammer, so they can suspension off the weathered, outer parts of rocks to see the "fresh," unweathered rock within.

If you can meet and identify the minerals in an igneous rock, you tin gain farther information about the igneous limerick. Igneous rocks with quartz in them are usually felsic. Igneous rocks with olivine in them are commonly mafic. Igneous rocks with neither quartz nor olivine in them are most ordinarily intermediate.

Origins of Igneous Rocks

Once you have determined the texture and composition of an igneous rock, you tin name it and you can as well say something of import about how it formed. For instance, a coarse-grained, felsic igneous stone is not only a granite, it is an intrusive igneous rock that formed from tedious cooling and crystallization of a trunk of magma inside the globe's crust. The intrusion of large bodies of granite—batholiths—is usually part of the origin of a mountain range. Similarly, a fine-grained, mafic igneous stone is not only a basalt, it is an extrusive igneous rock that formed from rapid cooling and crystallization of a lava flow at earth's surface.

Igneous Rock Classification

Pegmatitic Texture (Extremely Coarse-Grained) Originates from water-rich intrusions, which cool and crystallize hugger-mugger
Composition	Virtually Common Minerals	Rock Name
felsic	Na-plagioclase, orthoclase, quartz, biotite, amphibole, muscovite	pegmatite
Phanertitic Texture (Coarse-Grained) Originates in deep intrusions, which absurd and crystallize slowly underground
Composition	Most Common Minerals	Rock Proper noun
felsic	Na-plagioclase, orthoclase, quartz, biotite, amphibole, muscovite	granite
intermediate	Na-plagioclase, quartz, orthoclase, amphibole, biotite	granodiorite
	Na-plagioclase, amphibole, pyroxene, biotite	diorite
mafic	Ca-plagioclase, pyroxene, olivine, amphibole	gabbro
Aphanitic Texture (Fine-Grained) Originates in lava flows (or very shallow intrusions), which cool rapidly
Composition	Almost Mutual Minerals	Rock Name
felsic	Na-plagioclase, orthoclase, quartz, biotite, amphibole, muscovite	rhyolite
intermediate	Na-plagioclase, quartz, orthoclase, amphibole, biotite	dacite
	Na-plagioclase, amphibole, pyroxene, biotite	andesite
mafic	Ca-plagioclase, pyroxene, olivine, amphibole	basalt
Frothy Texture (Porous, Pumiceous) Originates in gas-charged volcanic eruptions, commonly pyroclastic
Composition	Nigh Common Minerals	Rock Name
felsic	glass (may contain a few minerals typical of felsic rocks)	pumice
mafic	glass (may incorporate a few mineral typical of mafic rocks)	scoria
Note: Basalt with fewer holes, known as vesicles, is called vesicular basalt. Scoria has more holes and may be blackness or ruddy in color.
Glassy Texture Originates from cooling also rapid to allow crystal lattices to grade
Limerick	Nearly Common Minerals	Stone Name
felsic to mafic	glass (no minerals)	obsidian
Note: Obsidian that is transparent at thin edges and has good conchoidal fracture is probably felsic.
Fragmental Texture—Fibroid (Contains Big Rock Fragments) Originates from pyroclastic (explosive) eruptions
Composition	Near Common Minerals	Stone Name
felsic to mafic	variable (depending on stone fragments and ash content)	volcanic breccia
Fragmental Texture—Fine (Mainly Volcanic Ash) Originates from pyroclastic (explosive) eruptions
Limerick	Most Mutual Minerals	Stone Name
felsic	may contain a few minerals typical of felsic rocks	rhyolitic tuff
medium	may contain a few minerals typical of intermediate rocks	andesitic tuff
mafic	may contain a few minerals typical of mafic rocks

Identifying Igneous Rocks

Igneous rocks tin can be distinguished from sedimentary rocks past the lack of beds, lack of fossils, and lack of rounded grains in igneous rocks, and the presence of igneous textures. A granite, for example, can be distinguished from a sandstone considering rather than being a mixture of weathered, rounded grains compressed and cemented together, granite consists of a small number of minerals in shiny black, white, or pink colors, with excellent crystal forms, grown together into a completely interlocking pattern. Sandstones, past dissimilarity, accept sedimentary bedding (layers) and consist of rounded grains with some spaces between the grains, which you can see with a mitt lens or magnifying glass.

Igneous rocks can be distinguished from most regional metamorphic rocks past the lack of foliation (layering) in igneous rocks. Unfoliated metamorphic rocks lack igneous textures and ordinarily contain minerals not establish in igneous rocks.

Granite may look like gneiss at beginning glance, merely granite has no layering, no preferred orientation of the minerals. The minerals in a granite grow randomly in all directions, rather than disposed to grow parallel to each other.

Igneous rocks are classified on the basis of their texture and their limerick. Come across the previous sections for descriptions of the dissimilar igneous textures and compositions.

The igneous stone classification tables that accompany this section are bundled on the basis of igneous textures first, and farther broken down on the basis of igneous composition. Think that igneous composition is estimated on the ground of color: light = felsic composition, medium = intermediate limerick, and dark = mafic composition.

Watch this video for an instance of identifying igneous rocks:

Uses of Igneous Rocks

Figure 6. This diorite statue was made in approximately 2090 BC.

Igneous rocks have a wide variety of uses. I important use is every bit rock for buildings and statues. Diorite was used extensively by aboriginal civilizations for vases and other decorative artwork and is still used for art today (Figure 6).

Granite (Figure 7) is used both in building construction and for statues. It is too a pop choice for kitchen countertops. Peridotite is sometimes mined for peridot, a blazon of olivine that is used in jewelry.

Pumice is commonly used as an abrasive. Pumice is used to smooth peel or scrape upwardly grime around the house. When pumice is placed into giant washing machines with newly manufactured jeans and tumbled, the result is "stone-done" jeans. Ground upwardly pumice rock is sometimes added to toothpaste to act equally an abrasive textile to scrub teeth.

Figure seven. Granite is an igneous rock used unremarkably in statues and building materials.

Check Your Understanding

Respond the question(south) below to encounter how well you sympathize the topics covered in the previous section. This short quiz doesnot count toward your grade in the class, and y'all tin can retake it an unlimited number of times.

Utilise this quiz to check your agreement and decide whether to (1) study the previous section farther or (2) motility on to the next section.

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Source: https://courses.lumenlearning.com/wmopen-geology/chapter/outcome-igneous-rocks/

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