Igneous Rocks

Charlene Estrada

Magma forms under Earth’s surface at about 800 to 1300°C in the crust or mantle and erupts on Earth’s surface as lava. When magma or lava cools, it solidifies by crystallization in which minerals grow within the magma or lava. The rock that results from this is an igneous rock from the Latin word ignis, meaning “fire.” [2] Igneous rocks are traditionally defined as the solid products from the cooling and hardening of molten magma in many different environments. We identify these rocks by their composition and texture.

Igneous Rock Composition

Composition refers to a rock’s chemical and mineralogical make-up. For igneous rock, the composition is generally divided into four groups: ultramafic, mafic, intermediate, and felsic. These groups refer to differing amounts of silica (SiO2), iron (Fe), and magnesium (Mg) found in the minerals that make up the rocks.

Classification of igneous rocks by composition. Darker rocks are ultramafic or mafic whereas lighter rocks are intermediate/felsic.
Figure. Classification of igneous rocks by composition. Darker rocks are ultramafic or mafic, whereas lighter rocks are intermediate/felsic. Y-axis refers to the abundance of Fe and Mg-containing minerals in each rock type.

Ultramafic refers to rocks composed of mostly olivine and some pyroxene. These rocks have even more magnesium and iron and even less silica than ‘ordinary’ mafic rocks. Ultramafic rocks are rare on the surface, but they make up the primary components of the upper mantle. Ultramafic rocks are very poor in silica, in the 40% or less range (this means that the rock would be less than 40 weight percent silica).

Mafic refers to igneous rocks with abundant ferromagnesian minerals (those with the elements Mg and Fe in their chemical formulae) plus plagioclase feldspar. Such minerals are dark-colored and include pyroxene and olivine. Mafic rocks are low in silica (in the 45-50% range), but they make up most of the oceanic crust and lithosphere.

Intermediate describes the igneous rock composition between mafic and felsic. It contains roughly equal amounts of light and dark minerals, including light grains of plagioclase feldspar and dark grains of amphibole It is intermediate in silica (in the 55-60% range).

Felsic refers to a predominance of light-colored minerals, including Feldspar and silica quartz. These minerals have more silica as a proportion of their overall chemical formulae. Minor amounts of dark-colored minerals, such as biotite mica, may sometimes be present. Felsic igneous rocks are rich in silica (in the 65-75% range) and tend to represent the continental crust or lithosphere composition.

Igneous Rock Texture

Phaneritic/Coarse-grained texture, where individual crystals and minerals are easily spotted by the naked eye within the rock.
Figure. Phaneritic/Coarse-grained texture, where the naked eye easily spots individual crystals and minerals within the rock.

If magma cools slowly, deep within the crust, the resulting rock is called intrusive. The slow-cooling process beneath the surface allows crystals to grow large, giving the rock a coarse-grained or “phaneritic” texture. The individual crystals in a coarse-grained texture are visible to the unaided eye.

Aphanitic/Fine-grained texture, where individual crystals and minerals cannot be spotted by the naked eye within the rock.
Figure. Aphanitic/Fine-grained texture, where the naked eye within the rock cannot spot individual crystals and minerals.

When lava erupts onto the surface or rises into shallow in a mountain and cools, the rock that will cool from it is called an extrusive igneous rock. Extrusive igneous rocks have a fine-grained or “aphanitic” texture, in which the grains are too small to see with the unaided eye. This fine-grained texture tells us that the quickly-cooling lava did not have time to grow large crystals.

Porphyritic texture illustrated by large, visible crystals found within a fine-grained matrix in igneous rock.
Figure Porphyritic texture illustrated by large, visible crystals found within a fine-grained matrix in igneous rock.

Some igneous rocks have a mixture of large crystals within a fine-grained matrix. Such a texture is called porphyritic. A porphyritic texture tells us that the magma underwent multiple cooling stages; it first cooled slowly when it was deep under the surface, then rose to a shallow depth where it cooled quickly.

Vesicular texture, in which the rock contains many pores where gases escaped during formation.
Figure. Vesicular texture, in which the rock contains many pores where gases escaped during formation.

All magmas contain dissolved gases called volatiles. When magma quickly rises to the surface as lava, these volatiles sometimes become trapped in the cooling molten rock and form a bubbling texture that appears sponge-like. Such a texture is called vesicular because the holes in the rock are called vesicles by scientists.

Glassy texture, which is very smooth without the presence of crystals or minerals
Figure. Glassy texture, which is very smooth without the presence of crystals or minerals

Lava will sometimes cool so quickly that not even microscopic crystals will form in it. As a result, volcanic glass will form with a shiny, smooth appearance that reflects light. This texture is called glassy. Just like the mineral quartz, a glassy rock will have a conchoidal fracture with distinctive, rounded fracture edges. This is because, like quartz, most glassy rocks are made of the compound SiO2 in the form of the mineraloid amorphous silica.

 

Pyroclastic texture shown by angular rock fragments mixed with fine ash and volcanic glass in igneous rock.
Figure Pyroclastic texture shown by angular rock fragments mixed with fine ash and volcanic glass in igneous rock.

The final volcanic texture is a result of explosive, violent eruptions. These eruptions produce lava and clouds of ash, rock, gases, and glass. The eruption’s solid material, called tephra, eventually falls back onto the earth and consolidates into a solid mass. The rock that will form from this process has a pyroclastic texture. This texture consists of volcanic ash, glass shards, and small rock fragments.

 
 

Igneous Rock Field Guide

Video. Classifying igneous rocks by texture and composition explained (7:37).

Komatiite

Komatiite Interactive Model. Komatiite is a dark greenish rock with bladed black/brown minerals.
Figure. Komatiite, South Africa. Click on this image to see a 3D interactive model by Sara Carena CC BY-NC.

“KO-MAT-EE-ITE”

Most commonly confused with: basalt

An ultramafic, fine-grained extrusive igneous rock. This rock will form from rapidly cooling lava, but it is very rare. Molten ultramafic rock was more prevalent on early Earth, and it can be found in the mantle. Komatiite is composed primarily of olivine and pyroxene minerals, which causes the rock to take on a dark, greenish color.

Chances are that if you are holding a fine-grained, dark igneous rock, it will be basalt since komatiites are not very common but look twice if it has a strong green tint. Olivine and pyroxene are more susceptible to weathering than minerals found in felsic rocks; therefore, this igneous rock may erode easier than a felsic counterpart.

Peridotite

Peridotite Interactive Model. Peridotite is a bright green, coarse-grained rock made mostly of olivine.
Figure. Peridotite. Click on this image to go to a 3D interactive model by Sara Carena CC BY-NC.

“PUR-ID-DOH-TITE”

Most commonly confused with: olivine (mineral), gabbro

An ultramafic, coarse-grained (intrusive) igneous rock. Peridotite will form under  Earth’s surface from slowly-cooling magma. It is less rare than Komatiite, but still not very common. Ultramafic magma composes the Earth’s upper and lower mantle; therefore, when a plume of magma rises to the lithosphere and cools as a “Xenolith,” peridotite will form.

Peridotite is composed of visible, and sometimes large, crystals of olivine and pyroxene. It is often dark and with distinctively green crystals. When distinguishing this rock from gabbro, consider the percentage of olivine in the rock; peridotite has at least more than 25%. As with komatiite, peridotite will also erode more easily than other igneous rocks because it contains minerals that are more susceptible to weathering.

Basalt

Basalt Interactive Model. Basalt is a very fine-grained, dark gray rock.
Figure. Basalt. Click on this image to go to a 3D interactive model by Sara Carena CC BY-NC.

“BAH-SALT”

Most commonly confused with: komatiite, shale (sedimentary), limestone (sedimentary)

A mafic, fine-grained (extrusive igneous rock. About 90% of all volcanic rocks that form are on the Earth’s surface are basalt as this rock represents a common magma composition of the upper mantle mixed with the crust. Basalt is the final product when this magma erupts as lava and cools.

Basalts are characterized by low (~50%) silica content and minerals with iron (Fe) and magnesium (Mg). Such minerals typically include amphibole and pyroxene, and sometimes small amounts of olivine. Additionally, basalts include a significant amount of calcium-plagioclase feldspar in their matrix. Basalt often is dark gray, and unlike komatiite, does not have a green tint. When distinguishing this rock from shale and limestone, use a hand lens to identify the notable minerals in its matrix. Additionally, basalt is more resistant to scratching than shale and limestone; try scratching it with a penny!

Gabbro

Gabbro Interactive Model. Gabbro is a coarse-grained, dark gray rock with visible black and green minerals that are rough to the touch.
Figure. Gabbro. Click on this image to see a 3D interactive model by Sara Carena CC BY-NC.

“GAB-BRO”

Most commonly confused with: peridotite

A mafic, coarse-grained (intrusive) igneous rock. Gabbro makes up the majority of oceanic lithosphere. It often forms at divergent boundaries when magma with a composition similar to the upper mantle rises and slowly cools beneath the surface. This rock has the same general composition as basalt, but its minerals are easily visible to the naked eye due to its slow cooling history.

Gabbro is dark gray or black, sometimes with noticeable flecks of white calcium plagioclase or green olivine. Gabbro can be distinguished from peridotite by its limited olivine content. If the number of olivine crystals in an unknown, coarse-grained rock is small or nonexistent, then you are looking at gabbro.

Like basalt, gabbro will erode at Earth’s surface faster than other felsic, intrusive rocks. The effects of weathering by the atmosphere may be more noticeable on gabbro because it contains larger crystals of iron-rich minerals.

Andesite

 

Andesite Interactive Model. Andesite is light gray and porphyritic, meaning that it is mostly fine-grained with larger dark minerals in its matrix.
Figure. Andesite. Click on this image to go to a 3D interactive model by Sara Carena CC BY-NC.

“AN-DEH-SITE”

Most commonly confused with: rhyolite

An intermediate, extrusive igneous rock. Andesite cools from lavas that are between mafic and felsic in composition. As such, andesite typically has a silica content of 55-60%. Andesite can often be found near volcanoes along the Pacific Ring of Fire or at stratovolcanoes, which are sometimes called Andesite volcanoes. Andesitic lavas are typical of subduction zones, such Andes Mountains subduction zone for which it received its name.

Andesite is commonly light gray. Unlike other extrusive igneous rocks that cool from lava, andesite is also porphyritic, which means that medium and small dark crystals can be seen in its fine-grained matrix by the naked eye. These crystals are usually amphibole and pyroxene minerals, whereas the light matrix is mostly composed of calcium plagioclase feldspar. The porphyritic texture of andesite clearly distinguishes it from rhyolite, which is sometimes a light beige-grey color.

Diorite

Diorite interactive model. Diorite is a coarse-grained rock with highly visible white and black minerals.
Figure. Diorite. Click on this image to go to a 3D interactive model by Sara Carena CC BY-NC.

“DYE-O-RITE”

Most commonly confused with: granite

An intermediate, coarse-grained (intrusive) igneous rock. Diorite cools slowly from molten rock that forms beneath the Earth’s surface along subduction zones, such as the Andes Mountains and other convergent margins at the Ring of Fire.

Diorite has a “cookies and cream” or “Dalmation”-like appearance, which is caused by black amphibole and white plagioclase crystals that are easily identifiable by the naked eye. Although this rock is much lighter in color than gabbro, it should be easily distinguished from granite due to the abundance of dark crystals in its matrix.

Rhyolite

Rhyolite Interactive Model. Rhyolite is a fine-grained rock with light-tan to pink coloring.
Figure. Rhyolite. Click on this image to go to a 3D interactive model by Dr. Parvinder Sethi CC BY

“RYE-O-LITE”

Most commonly confused with: andesite, tuff

A felsic, fine-grained (extrusive) igneous rock. Rhyolite rapidly cools from a high-silica (65-75%) lava. Rhyolite is not as common as its coarse-grained counterpart, granite, because felsic lavas cannot move very far once they erupt.

Rhyolite is typically light tan to pinkish tan in color, and individual crystals are usually difficult to see with the naked eye. Therefore, this rock can be distinguished from andesite, which often has medium and small crystals within its matrix. There is also no pyroclastic debris within rhyolite, which can be confirmed by examining the rock under light; unlike tuff, rhyolite does not have visible flecks of volcanic glass in its matrix. Rhyolite can be found at explosive volcanoes, such as at Yellowstone National Park.

Granite

Granite Interactive Model. Granite is a coarse-grained rock with multi-colored minerals in its matrix, but is mostly light-colored. It is rough to the touch.
Figure. Granite. Click on this image to go to a 3D interactive model by Sara Carena CC BY-NC.

“GRAN-IT”

Most commonly confused with: diorite

A felsic, coarse-grained (intrusive) igneous rock. Granite is often used to approximate the composition of the continental crust in both composition and density. This estimation is typical because granite usually forms within the cores of mountains and thick lithosphere when magma with high silica content slowly cools and crystallizes.

Granite can be identified by its light to pinkish color and the presence of abundant quartz. Granite commonly has large amounts of salmon-pink potassium feldspar and white sodium-feldspar (plagioclase), which can even have visible cleavage planes on the crystals. Some varieties of granite have black flecks in the matrix, which are typically biotite mica. To distinguish granite from the intermediate coarse-grained rock, diorite carefully examines the minerals in its matrix. Granite, unlike diorite, has few dark minerals and a tendency to have more pink K-Feldspar.

Obsidian

Obsidian Interactive Model. Obsidian is an extremely smooth, glassy rock with no visible minerals and conchoidal fracture at its broken edges.
Figure. Obsidian. Click on this image to go to a 3D interactive model by jonathan.davidson CC BY.

“OB-CID-DEE-AN”

Most commonly confused with: chert (sedimentary)

A felsic, glassy (extrusive) igneous rock. Obsidian is commonly found along cooled lava fields with rhyolite. Obsidian forms when lava cools so quickly that it does not have time to even develop microscopic crystals. When cooled, obsidian is very smooth, brittle, and shiny. Although obsidian is felsic and has a high silica content, is often black, brown, or red.

Obsidian is usually identifiable by its glassy texture alone. Like the mineral quartz, it displays conchoidal fracture, which can appear as curved or rounded fracture edges. The sedimentary rock chert, which is mostly composed of silica, also has conchoidal fracture; however, this rock is often much duller in both luster and color.

Because obsidian is both brittle and hard, it has been traditionally used as tools and weapons by indigenous people for thousands of years. Many of these weapons hold an edge after centuries, which demonstrates this rock’s resistance to weathering.

BACKYARD GEOLOGY: APACHE TEARS
A rounded pebble of dark obsidian, known as an Apache Tear.
Figure An obsidian Apache Tear.

Volcanic deposits are common throughout the western United States and Mexico, and these rocks have played an important role Native American culture and history. Obsidian can be reshaped and carved to form surgical instruments or weapons such as knives. There are also naturally occurring varieties of rounded obsidian that are said to absorb grief and depression. Today these stones are called “Apache Tears”. There are variations in the folklore behind this stone, the tragic theme of loss unites each story.

The most popular legend behind the Apache Tears focuses upon a Pinal Apache Tribe that was vastly outnumbered by the U.S. military during the nation’s westward expansion in the 1800s. The majority of the warriors were killed in a surprise raid near Picacho Peak, AZ. The remaining Native Americans preferred suicide over imprisonment or execution.

The family of the warriors gathered not far from the peaks, where the remains of the brave warriors remained at the base. The women grieved deeply, day and night, for their lost men. The Great Father placed their tears in the obsidian. Should a person find one of these stones, they would not need to grieve since the women of the Pinal Apache Tribe have already given their tears inside the stone [3].

Scoria

Scoria interactive model
Figure. Scoria. Click on this image to go to a 3D interactive model by Sara Carena CC BY-NC.

“SCOR-EE-AH”

Most commonly confused with: pumice

A mafic, vesicular (extrusive) igneous rock. Scoria forms when lava containing volatile gases erupts. The gases trapped within the lava form large cavities, or vesicles, as it rapidly cools at the surface. Scoria cools from mafic lava, and contains microscopic crystals of silica-poor, ferromagnesian minerals such as amphibole, pyroxene, and calcium plagioclase.

Scoria is usually dark gray and is easily distinguished from other mafic rocks by its vesicular texture. Although pumice is also vesicular, scoria is much darker and denser. Despite having large cavities, scoria will never float above water.

Some varieties of scoria are rust-red, which reflects the tendency of the minerals within the mafic rock to oxidize, or rust, under Earth’s atmosphere. Furthermore, the abundant vesicles make a greater surface area available for weathering, which causes this rock to weather faster than other mafic rocks.

Pumice

Pumice Interactive Model
Figure. Pumice. Click on this image to see a 3D interactive model by Sara Carena CC BY-NC.

“PUHM-IS”

Most commonly confused with: tuff, scoria

A felsic, vesicular (extrusive) igneous rock. Pumice forms when felsic lava, which can contain significantly higher amounts of volatile gases, very rapidly cools. As with scoria, the cooling rock traps many vesicles of gas, and it takes on a “frothy” appearance as it solidifies. Pumice often cools too quickly to form minerals, and it is sometimes referred to as volcanic glass. We can find this rock along the slopes of explosive volcanoes with pyroclastic deposits.

Pumice is usually a light tan, pink, or gray. It is a very low density, and most varieties contain enough vesicles that when placed in a bowl or cup of water, the rock will float. This unique characteristic will often distinguish pumice from similar-looking rocks such as tuff or scoria.

Video. How to tell if a rock is a pumice? (0:32).

Tuff

Tuff interactive model
Figure. Rhyolite. Click on this image to go to a 3D interactive model by rocksandminerals CC BY.

“TOUGH”

Most commonly confused with: pumice, rhyolite

A felsic to intermediate, pyroclastic igneous rock. Tuff forms from the solid debris or tephra of an explosive volcanic eruption. When ash, volcanic glass, and rock fragments are ejected by the volcano or related pyroclastic flow eventually accumulate together, they form tuff. This type of rock usually indicates a violent eruption, and it is commonly found along the Ring of Fire and stratovolcanoes.

Tuff often does not have any single distinguishing mineralogical composition, although it often contains angular rock fragments and shiny flecks of glass within a fine-grained matrix of ash. Tuff is typically pale tan or gray. Its pyroclastic texture sometimes makes it less dense than other volcanic rocks; however, unlike pumice, it does not float in water.

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Introduction to Historical Geology Copyright © by Charlene Estrada is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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