Lesson 2: Igneous Rocks

PORPHYRITIC ANDESITE

As a rock of intermediate composition, andesite's color tends toward darker gray (although it may be on the light side). It is most easily visually distinguished from rhyolite by black crystals of hornblende, which occur commonly as phenocrysts. Locality - La Paz County, Arizona (Image in class was light bone colored with a rust colored streak going through it. It was also a lot more grainy with pores)

BASALT

Basalt is the most common igneous rock found on the surface of the Earth. This type of rock makes up the ocean floor. Its dark color comes from the abundance of ferromagnesian (iron and magnesium) minerals . This specimen represents a late stage lava flow. Notice the many pore spaces in the rock. These vesicles represent gas pockets formed as the lava was cooling. The lava cooled faster than the gases were able to escape resulting in what is called "vesicular basalt." When, subsequent to cooling, the vesicles become wholly or partially filled with minerals, it is called "amygdaloidal basalt." Locality - Skunk Creek turnoff 1-17, Maricopa County, Arizona (Image in class was a faint colored rusty color with white inclusions (probably of the prior basalt description) and very porous looking)

ENVIRONMENTAL SIGNIFICANCE:

Felsic igneous rocks are formed in volcanic eruptions which impact air quality, global climate, local drainage, human safety, and structures. Most igneous minerals weather quickly in a humid environment, forming clays that are susceptible to compaction, slippage, and shrink/swell. Granitic rocks are often a source of radon, an air contaminant. Igneous rocks are often the source rocks for economic mineral deposits. Various igneous rocks are used as building materials and landscaping stone.

Igneous rocks are further classified by their mineral composition

Felsic, Intermediate, Mafic, Ultramafic

GABBRO

Gabbro is a coarse-grained rock, typically equivalent in composition to basalt, i.e., it is high is iron and magnesium-bearing minerals (pyroxenes, amphiboles, plagioclase feldspar, and olivine). These rocks will be darker in color, somewhat heavier than granitic rocks, and devoid of quartz. Locality - Gore Mt., New York

Igneous rocks - two types

Generally, igneous rocks are divided into two major divisions, based on the size of the crystals and thus, on their rate of cooling.

Each rock has a story to tell, and geologists must figure out that story.

Geologists use rock type and other specific characteristics of rocks to interpret the history of the Earth.

INTRUSIVE or PLUTONIC (Class definition)

Large crystals produced by long cooling periods deep within the Earth. Large bodies (batholiths) of these rocks tell a tale of mountain building, generally resulting from plate collisions.

EXTRUSIVE or VOLCANIC (Class Definition)

No or small crystals produced by very short cooling periods at or near the Earth's surface. These rocks document volcanic episodes and by their composition help to pinpoint areas of subduction (andesites and rhyolites), or rifting centers and hotspots (basalt).

Obsidian

Rock

Perlite

Rock

PERIDOTITE (Plate 2.12)

This rock is a good transition specimen to the Plutonic or Intrusive igneous rocks. Notice that the dark gray rock is the extrusive rock basalt. The green rock imbedded in it is composed of crystals of the mineral olivine. In this case, they are the gem grade called Peridot (pair-ah-dot). Peridotite (per-idd-o-tite) is an ultramafic rock composed almost entirely of olivine. It forms deep beneath the crust of the Earth and represents what many geologists feel is characteristic of the upper mantle of the Earth. Its presence at the surface indicates the depth to which this volcano drew upon magma. Locality - San Carlos Indian Reservation, Arizona (near the town of Peridot)

DIORITE

This rock is intermediate in composition between granite and gabbro. Silica is deficient enough that there is no visible quartz. Otherwise, it closely resembles granite in appearance. It is generally described as having a "salt and pepper" look, being composed of roughly half feldspar and half ferromagnesians.

PORPHYRITIC

Two sizes of crystals denote two stages of cooling. Both intrusive and extrusive igneous rocks can exhibit this texture.

Igneous Rock Chart

an example

EXTRUSIVE IGNEOUS ROCKS: List

(Rocks that have cooled at Earth's surface)

INTRUSIVE IGNEOUS ROCKS: List

(Rocks that have cooled below Earth's surface)

PEGMATITE

A pegmatite is a coarse-grained igneous rock usually of granitic composition. In pegmatites, the crystals are much larger than in normal granite. In this type of rock, the large crystals are not due to slow cooling, but instead are the result of hydrothermal solutions. This sample is included with the igneous rocks because of its occurrence. As granitic magmas form and solidify, they give rise to water that was part of their chemistry, but was excluded as the magma cooled and crystals began to form. This water, in the form of super-heated steam, contains many dissolved minerals. As this aqueous fluid migrates away from the solidifying magma, (via fractures in the rocks), it begins to deposit minerals as the temperatures and pressures decrease. In this way, many valuable mineral deposits are formed. Pegmatites are particular types of hydrothermal deposits that are rich in silicates: usually quartz, feldspars and mica. The aqueous solution allows a great deal of freedom for the ions to move to the sites of growing crystals (much more so than for magmas), which means large crystals are possible over a much shorter period of time.Locality - Bradshaw Mountains, Arizona (Image in class was more mustardy yellow with black)

PORPHYRITIC BASALT

A porphyry is an igneous rock that contains at least two distinct grain sizes. Remember that the grain size represents the length of time the crystals had to form from the molten state. Porphyries represent different cooling stages of the molten rock. In this case, the light gray feldspar grains were allowed to form slowly beneath the surface. After they formed, much of the crystalline and residual molten material was extruded onto the surface where it solidified rapidly, producing the fine grained matrix rock. Other types of porphyries are possible for igneous rocks. This specimen is also a scoria; it is vesicular, with lots of gas bubble holes, but not as frothy or lightweight as pumice. Locality - Gila County, Arizona (Image in class had many small porous inclusions)

GRANITE (Plate 2.15 a, b, c)

Although they differ somewhat in appearance, these three rocks are all granites. They share a coarse-grained texture with individual crystals of various minerals and they share a basic composition of quartz, feldspar and mica (either black biotite or clear muscovite). Localities - various unknown

Bowen's Reaction Series

Bowen's Reaction Series gives you some idea of the general temperature conditions under which various rock-forming minerals are stable. When rocks and their component minerals are found at the Earth's surface, weathering processes will be most rapidly effective on those minerals farthest out of their "comfort zone."

PORPHYRITIC GRANITE

Coarse-grained igneous rocks may also display two grain sizes. This granite is an excellent example. Very large feldspar crystals are surrounded by smaller (yet still large) crystals of quartz and feldspar. Initially, the magma cooled very slowly and the larger feldspar crystals formed; then a period of faster, but still slow, cooling took place, during which the smaller crystals formed. Locality - Bagdad, Arizona

Types of Granite

Colors: Bown w/ White, Red w/ Brown, White w/ Moss Green... etc.

VOLCANIC AGGLOMERATE (TUFF)

During a violent volcanic eruption, debris (lava, cooled volcanic rocks and country rock) can be pulverized and blown out in a vast, hot cloud of ash and angular rock fragments. When this material comes to rest, it is often still hot enough to weld together to form a rock. The texture of this rock is very clastic or fragmental, and out of context, can be confused with sedimentary rocks. Locality - Wikieup, Arizona (Image in class depicts a rust red colored rock with slightly smaller white flecks in it)

the Rock Cycle

Earth materials undergo constant changes as their finite supply is caught in an ongoing cycle of events.

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.

Igneous rocks - Intrusive or extrusive

Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies.

IGNEOUS ROCKS

Igneous rocks are those that have formed directly from a melt, either magma (molten rock that does not reach Earth's surface) or lava (molten rock that flows at the Earth's surface). The group is characterized by interlocking grains (crystals) of different minerals

Igneous rocks are good markers for mountain-building events, volcanic events, and other tectonic events

In addition, dates based on the decay rates of radioactive elements can be determined for igneous rocks. These dates can provide absolute dates (an age in number of years) for sedimentary rocks bracketed by igneous events.

BASALT crystals

In basalt, crystals have had a chance to form; however, they are much too small to be seen by the unaided eye. The rock will generally be black to very dark gray in color, although a rusty red color is also common, as the abundant iron in the rock may become oxidized during steamy phases in the eruptive cycle. Locality - Coconino County, Arizona (Image in class wasn't this color, it was a creamy white/khaki color, but the texture was similar to this image)

Intermediate

Intermediate silica content, results in rocks with no visible quartz. The mineralogy changes to include plagioclase feldspar and increasing percentages of amphibole, pyroxene and biotite (all dark-colored minerals). Coarse grained intermediate rocks are generally salt and pepper colored. Intermediate magmas are generated at convergent plate boundaries.

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.

Consider the following example of the rock cycle:

Magma deep beneath the Earth crystallizes to form a mass of igneous rock called granite. Over millions of years, mountain building and erosion remove everything above the granite, until it is exposed at the Earth's surface. There it is attacked by rain, snow, and heat/cold. The feldspars and micas break down chemically, and debris composed of quartz grains, clays, and rock fragments forms. This fragmental debris, or sediment, is picked up by water and moved downstream. The finest particles are carried the longest distance and are deposited in the delta of a river. This deposit is buried by more and more sediment. Eventually, it is so deep that the pressure of the overlying material causes the once loose sediment to become hardened or lithified into a sedimentary rock called shale. Further burial, with increasing pressure and heat, causes re-crystallization and alignment of crystals to produce a crystalline, foliated metamorphic rock, called schist. Should the temperature continue to rise, the schist may melt to follow another path within the Rock Cycle.

Igneous rock formation

Rapid cooling promotes the swift growth of many crystals, but they are small in size (in fact, they are generally too small to be seen); or precludes the formation of any crystals, as in volcanic glasses. Magmas that take many thousands of years to solidify are distinguished by their large crystal size.

Composition of magma - Igneous rock

The composition of magma depends on the chemistry of the parent rock from which it forms. If the parent is deficient in silica, then when it melts, the resulting lava's composition will reflect that deficiency. In addition, magmas are generally the result of partial melting; i.e., when a parent rock begins to melt, the components that formed last (when the parent's magma was coolest) are the first to melt - in reverse of Bowen's Reaction Series

Components of magma - I. Rock

The earliest components to melt become buoyant and move away from the unmelted rock, thus differentiating the composition of the resultant magma from that of the parent rock.

Igneous rock grains

The size of the grains depends on the rate of cooling of the molten mass.

Ultramafic

These contain less than 45% silica and are composed nearly entirely of ferromagnesian minerals (such as olivine or hornblende), and consequently are dark-colored. Ultramafic magmas are generated within the Earth's mantle.

Mafic

These generally dark-colored rocks are rich in magnesium and iron, and low in silica (50%). Common minerals include plagioclase feldspar, pyroxene, amphibole, and olivine. Mafic magmas are generated at rift zones, hot spots, and back-arc spreading centers, as a result of partial melting of upper mantle rocks.

Felsic

These generally light-colored rocks are rich in potassium, sodium, aluminum and silica. Coarse-grained felsic rocks have visible quartz, due to the high percentage (65%+) of silica. Other dominant minerals include orthoclase feldspar, muscovite or biotite mica, and sometimes amphibole. Felsic magmas are generated at subduction zones and other convergent boundaries, as a result of partial melting of subducting oceanic plates (often with contributions of oceanic sediments).

PUMICE

This classic piece of pumice displays a glassy luster when viewed under magnification. Pumice forms as abundant volatiles (gases) escape from a glass flow, usually of rhyolitic composition. This forms a sort of volcanic froth that congeals to pumice. Pumice often contains so many gas pore spaces that it may float for months. Occasionally, pieces of pumice that originated in Japan are found on the western shores of North America. This particular sample will float for a couple of hours before the pores fill with water and the rock sinks. Locality - Northern Arizona

RHYOLITE

This example of a fine-grained igneous rock with a different composition than basalt. Rhyolite is deficient in ferromagnesians and abundant in silica, aluminum and sodium. This combination makes it lighter in weight and also lighter in color. This characteristic is often used to distinguishes between the two in the field. The composition difference composition is also responsible for the violent manner in which rhyolite comes into existence. Mount St. Helens is an example of how rhyolitic material can be generated. Rhyolite tuff is produced from volcanic ash that is lithified. Upon closer inspection, it can be seen that this rock is composed of angular fragments of minerals - a testimony to the violent explosions that distributed ash across the countryside. The garnet(s) nestled in the gas pocket grew by pneumatolitic processes; i.e., directly from a gas to a solid. Locality - Wikieup, Arizona (Image in class was more of a rich beige color, with a big hole going through the center of the rock - don't know if that's significant)

"RAINBOW" OBSIDIAN

This piece of obsidian shows the well-developed conchoidal fracture typical of amorphous (no structure) materials. This is the property that allowed primitive people to fashion projectile points from massive chunks of obsidian or similar fine-grained, homogenous rocks. The cutting edge on obsidian has recently been found to be finer than that of the best surgical scalpels, and scalpels of obsidian have been used for delicate eye surgery. (Image in class is a smooth blackish 'rainbow' oil spill looking rock with the oil spill looking section depicted as a swirly inside the obsidian. Has to be cracked open to see full rainbow effect: "This particular specimen shows iridescence when viewed in reflected light. This color is due to the internal interference of light as it encounters many minute layers within the rock. This rainbow of interference is also found in broken glass, oil slicks on water, and certain types of insect wings. Locality - Northern California")

RHYOLITE (Plate 2.9)

This rhyolite is similar in formation to the previous specimen but shows flow banding, swirls, and additional banding due to the stretching out of the very viscous unmixed lava. (Image in class was a much richer rust color)

rocks are all bound in the dynamic web of Earth's systems

weathering, erosion, and plate tectonics

GRANITE (Plate: 2.14)

This rock is characteristic of the continents. It forms the core of the major continental mountain ranges. Composed of feldspars, quartz, and micas, it contrasts with the mafic and ultramafic rocks rich in ferromagnesians (iron and magnesium minerals) and poor in silica, and that make up the ocean floors. Granite is thought to originate from partial melting of basaltic rocks and further contamination by sedimentary rocks during the subduction process of plate tectonics. In particular, the partial melting of basalt (along with oceanic sediments and adjacent crustal rocks) allows the lighter weight minerals to migrate toward the surface of the Earth where they form granitic rocks. Thus, the origin of the continents can be explained by this process in action over many hundreds of millions of years - a process that is still going on today. (Image in class was grainy, porous, and bone colored. "This particular specimen of granite contains an abundance of pinkish orthoclase or potassium feldspar. The light gray streaks are crystals of quartz. Muscovite mica is present but in only minor amounts. The specimen also goes by the name of "graphic" granite due to the unusual pattern of gray quartz crystals resembling Egyptian hieroglyphics. Locality - Buckeye Hills, Arizona")

OBSIDIAN IN PERLITE

This specimen is an example of an older glass (obsidian) flow that is beginning to devitrify, i.e., the glass is crystallizing into minerals, probably due to the action of hot water that infiltrated the rock. The still glassy black globule is known as an Apache Tear. Obsidian represents the volcanic rock that has the fastest cooling rate. Cooling was so rapid that an organized mineral structure did not have the time needed to form. Eventually all glasses will crystallize (in millions of years). Obsidians most commonly form from magmas that have lost their dissolved gases. This is similar to a bottle of pop that has gone "flat" by losing its dissolved carbon dioxide. Locality - Superior, Arizona (Image: Small piece of obsidian inclusion in a big chunk of perlite)

THUNDEREGG

While this is not specifically a wholly igneous rock, the image shows a secondary feature formed by hydrothermal fluids associated with volcanism and volcanic igneous rocks. Viscous lavas, such as rhyolite, make it difficult for gases to escape, and large gas bubbles often become trapped. When the lava cools, large voids appear within the rock. Penetrating hot water, laden with silica, later leaves deposits of quartz within these voids to be discovered by a hammer-wielding adventurer. Locality Pima County, Arizona

Three types of rocks

igneous, sedimentary, and metamorphic

The Rock Cycle diagram

the rock cycle is a continuous process of cycling and recycling materials available for the formation of rocks. Each rock forms under a specific set of conditions and is subject to alteration when those conditions change. (It is this re-processing that has erased much of the rock record of Earth's history.)