Chapter 3 Igneous Rocks:Igneous Rocks, Intrusive activity, and the origin of Igneous Rocks

Plutonic rocks

Plutonic rocks are igneous rocks that solidified from a melt at great depth -Plutonic rocks are intrusive and generally consist of mineral grains coarse enough to be readily visible in a hand sample. -Igneous rocks that formed at considerable depth—usually more than several kilometers—are called plutonic rocks (after Pluto, the Roman god of the underworld). Characteristically, these rocks are coarse-grained, reflecting the slow cooling and solidification of magma -Most intrusive rock is plutonic — that is, coarse-grained rock that solidified slowly at considerable depth. Most plutonic rock exposed at the Earth's surface is in batholiths —large plutonic bodies. A smaller body is called a stock.

Composition of igneous rocks

Refers to the iron and silica content of a rock

Viscosity of magma

a measure of a material's resistance to flow -increases with decreasing temperature and high silica content

Volcanic Breccia

a pyroclastic igneous rock that consists of fragments of volcanic debris, which either fall through the air and accumulate, or form when solidifying lava breaks up during flow -Rock formed from large pieces of volcanic rock (cinders, blocks, bombs).

chill zones

finer-grained rocks that indicate magma solidified more quickly here because of the rapid loss of heat to cooler rock -In an intrusion, the finer-grained rock adjacent to a contact with country rock.

Hydrothermal veins

fractures "healed" (filled) by minerals that precipitated from hydrothermal fluids -Hydrothermal veins are closely related to pegmatites. Veins of quartz are common in country rock near granite. Many of these are believed to be caused by water that escapes from the magma. Silica dissolved in the very hot water cakes on the walls of cracks as the water cools while traveling surfaceward. Sometimes valuable metals such as gold, silver, lead, zinc, and copper are deposited with the quartz in veins

Peridotite

Ultramafic rock composed of pyroxene and olivine

The Origin of Granite

- Partially melted lower continental crust - Magmatic underplating -To explain the great volumes of granitic plutonic rocks, many geologists think that partial melting of the lower continental crust must take place -The continental crust contains the high amount of silica needed for a silicic magma. As the silicic rocks of the continental crust have relatively low melting temperatures (especially if water is present), partial melting of the lower continental crust is likely -Currently, geologists think that the additional heat is provided by mafic magma that was generated in the asthenosphere and moved upward -The process of magmatic underplating involves mafic magma pooling at the base of the continental crust, supplying the extra heat necessary to partially melt the overlying, silica-rich crustal rocks -Mafic magma generated in the asthenosphere rises to the base of the crust. The mafic magma is denser than the overlying silica-rich crust; therefore, it collects as a liquid mass that is much hotter than the crust. The continental crust becomes heated (as if by a giant hotplate). When the temperature of the lower crust rises sufficiently, partial melting takes place, creating felsic magma. The felsic magma collects and forms diapirs, which rise to a higher level in the crust and solidify as granitic plutons (or, on occasion, reach the surface and erupt violently

Chemistry of Igneous Rocks

-A magma that is rich in silica, aluminum, potassium, and sodium will crystallize minerals that contain those elements (feldspar and quartz). -A magma that is rich in iron and magnesium and calcium will contain a lot of the dark-colored ferromagnesian minerals (pyroxene, amphibole, olivine, and biotite). -Chemical analyses of rocks are reported as weight percentages of oxides (e.g., SiO2 , MgO, Na2O, etc.) rather than as separate elements (e.g., Si, O, Mg, Na). -For virtually all igneous rocks, SiO2 (silica) is the most abundant component. The amount of SiO2 varies from about 45% to 75% of the total weight of common igneous rocks -Based on the amount of silica, igneous rocks are classified into four groups, which are, in order of decreasing silica content, felsic, intermediate, mafic, and ultramafic

xenoliths

-Fragments of rock that are distinct from the body of igneous rocks in which they are enclosed

texture of a rock

-Refers to a rock's appearance with respect to the size, shape, and arrangement of its grains or other constituents. -Igneous textures can be divided into three groups: crystalline, glassy, and fragmental

"Know Fig 3.2 page 52"-from study guide

-The rock cycle with respect to a convergent plate boundary. Magma formed within the mantle solidifies as igneous rock at the volcano. Sediment from the eroded volcano collects in the basin to the right of the diagram. Sediment converts to sedimentary rock as it is buried by more sediment. Deeply buried sedimentary rocks are metamorphosed. The most deeply buried metamorphic rocks partially melt, and the magma moves upward. -An alternate way the rock cycle works is shown on the left of the diagram. Sediment from the continent (and volcano) becomes sedimentary rock, some of which is carried down the subduction zone. It is metamorphosed as it descends. It may contribute to the magma that forms in the mantle above the subduction zone

Rock cycle

-a conceptual device that shows the interrelationship between igneous, sedimentary, and metamorphic rocks -an idealized cycle of processes undergone by rocks in the earth's crust, involving igneous intrusion, uplift, erosion, transportation, deposition as sedimentary rock, metamorphism, remelting, and further igneous intrusion -Changes take place when one or more processes force Earth's material out of equilibrium. Each rock may form at the expense of another if it is forced out of equilibrium with its physical or climatic environment by either internal or surficial forces. It is important to be aware that rock moves from deep to shallow, and from high to low temperature and pressure in response to tectonic forces and isostasy (Isostasy is the state of gravitational equilibrium between Earth's crust and mantle such that the crust "floats" at an elevation that depends on its thickness and density). -The cycle can be repeated. However, there is no reason to expect all rocks to go through each step in the cycle. For instance, sedimentary rocks might be uplifted and exposed to weathering, creating new sediment. We should emphasize that the rock cycle is a conceptual device to help students place the common rocks and how they form in perspective. As such, it is a simplification and does not encompass all geologic processes. For instance, most magma comes from partial melting of the mantle, rather than from recycled crustal rocks.

Basalt

-extrusive= rocks if they form at the Earth's surface (e.g., basalt) -If you go to the island of Hawaii, you might observe red-hot lava flowing over the land and, as it cools, solidifying into the fine-grained (the grains are too small to be seen with the naked eye), black rock we call basalt. Basalt is an igneous rock, rock that has solidified from magma

volcanic neck

1)A deposit of hardened magma in a volcano's pipe 2)solid igneous core of a volcano left behind after the softer cone has been eroded -A volcanic neck is an intrusive structure apparently formed from magma that solidified within the throat of a volcano -Ex: A volcano formed above what is now Ship Rock. The magma for the volcano moved upward through a more or less cylindrical conduit. Eruptions ceased and the magma underground solidified into what is now Ship Rock. In time, the volcano and its underlying rock—the country rock around Ship Rock—eroded away. The more resistant igneous body eroded more slowly into its present shape. Weathering and erosion are continuing.

magma evolution

A change in the composition of a magma body is known as magma evolution -Magma evolution can occur by differentiation, partial melting, assimilation, or magma mixing

laccolith

A concordant intrusive structure, similar to a sill, with the central portion thicker and domed upward. Laccoliths are not common. -Relatively small, mushroom-shaped pluton that forms when magma intrudes into parallel rock layers close to Earth's surface. -When viscous magma intrudes between two layers of sedimentary rock, it may generate enough pressure to lift the overlying layer into an arch. The magma then cools to form a concordant, mushroom-shaped or domed pluton known as a laccolith.

Granite

A felsic, coarse-grained, intrusive igneous rock containing quartz and composed mostly of potassium- and sodium-rich feldspars. -Granite predominates in the continental crust. Younger granite batholiths occur mostly within younger mountain belts

rhyolite

A fine-grained, felsic, igneous rock made up mostly of feldspar and quartz.

Basalt

A fine-grained, mafic, igneous rock composed predominantly of ferromagnesian minerals and with lesser amounts of calcium-rich plagioclase feldspar -Basalt and gabbro are predominant in the oceanic crust

batholiths

A large discordant pluton with an areal extent at Earth's surface greater than 100 square kilometers -A mass of rock formed when a large body of magma cools inside the crust -The largest of all igneous intrusions

Gabbro

A mafic, coarse-grained igneous rock composed predominantly of ferromagnesian minerals and with lesser amounts of calcium-rich plagioclase feldspar -Basalt and gabbro are predominant in the oceanic crust

Magma

A molten mixture of rock-forming substances, gases, and water from the mantle -Molten rock beneath the earth's surface -usually rich in silica and containing dissolved gases -Solidifies slowly within surface -Big crystals -Felsic -Light in color -ex: granite

Tuff

A pyroclastic igneous rock composed of volcanic ash and fragmented pumice, formed when accumulations of the debris cement together. -When pyroclastic material (ash, pumice, or crystalline rock fragments) accumulates and is cemented or otherwise consolidated, the new rock is called tuff , or volcanic breccia , depending on the size of the fragments. A tuff is a rock composed of fine-grained pyroclastic particles (dust and ash). A volcanic breccia is a rock that includes larger pieces of volcanic rock

Rock

A rock is naturally formed, consolidated material usually composed of grains of one or more minerals -A naturally occurring solid mixture of one or more minerals or organic matter

stock

A smaller pluton that has a surface exposure of less than 100 square kilometers is called a stock -stock may be the exposed portion of a larger batholith.

glassy texture

A term used to describe the texture of certain igneous rocks, such as obsidian, that contain no crystals. -are composed primarily of glass and contain few, if any, crystals

pegmatite

An extremely coarse-grained (crystals ranging in size from a few centimeters to several meters in length) igneous rock -The extremely coarse texture of pegmatites is attributed to both slow cooling and the low viscosity (resistance to flow) of the fluid from which they form -the vast majority of pegmatites are silica-rich, with very large crystals of potassium feldspar, sodium-rich plagioclase feldspars, and quartz. Hence, the term pegmatite generally refers to a rock of granitic composition -Most pegmatites contain only quartz, feldspar, and perhaps mica -Pegmatites probably crystallize from a fluid composed largely of water under high pressure. Water molecules and ions from the parent, granitic magma make up a residual magma

pluton

An igneous body that crystallized deep underground

komatiite

An ultramafic extrusive igneous rock

The theory of plate tectonics incorporates the preceding concepts

Basalt is generated where hot mantle rock partially melts, most notably along divergent boundaries. -Divergent boundaries are associated with the creation of basalt and gabbro of the oceanic crust -The fluid magma rises easily through fissures, if present. The ferromagnesian portion that stays solid remains in the mantle as ultramafic rock. -Granite and andesite are associated with subduction at convergent boundaries Differentiation, assimilation, and partial melting may each play a part in creating the observed variety of rocks

Ultramafic Rocks

An ultramafic rock is one that contains less than 45% silica and is rich in iron, magnesium, and calcium. -Ultramafic rocks are typically composed almost entirely of the ferromagnesian minerals olivine and pyroxene. No feldspars are present and, of course, no quartz. -Komatiite , the volcanic ultramafic rock, is very rare. Ultramafic extrusive rocks are mostly restricted to the very early history of the Earth -Peridotite , the coarse-grained intrusive rock, is composed of olivine and pyroxene and is the most abundant ultramafic rock. -Most ultramafic rocks come from the mantle rather than from the Earth's crust. Where we find large bodies of ultramafic rocks, the usual interpretation is that a part of the mantle has traveled upward as solid rock -In some cases, ultramafic rocks can form when ferromagnesian minerals crystallize in a mafic magma and settle down to the base of the magma chamber where they accumulate.

Potassium feldspar Bowen's Reaction series

Any magma left at the point where the two branches meet is richer in silicon than the original magma; it also contains abundant potassium and aluminum. -The potassium and aluminum combine with silicon to form potassium feldspar. (If the water pressure is high, muscovite may also form at this stage.) Excess SiO2 crystallizes as quartz -A complication is that early-formed crystals react with the remaining melt and recrystallize as cooling proceeds -For instance, early-formed olivine crystals react with the melt and recrystallize to pyroxene when pyroxene's temperature of crystallization is reached. Upon further cooling, pyroxene continues to crystallize until all of the melt is used up or the melting temperature of amphibole is reached

country rock

Any rock that was older than and intruded by an igneous body.

Geologists believe the following sequence of events accounts for most pegmatites:

As a granite pluton cools, increasing amounts of the magma solidify into the minerals of a granite. By the time the pluton is well over 90% solid, the residual magma contains a very high amount of silica and ions of elements that will crystallize into potassium and sodium feldspars. -Also present are elements that could not be accommodated into the crystal structures of the common minerals that formed during the normal solidification phase of the pluton. -Fluids, notably water, that were in the original magma are left over as well. If no fracture above the pluton permits the fluids to escape, they are sealed in, as in a pressure cooker. - The watery residual magma has a low viscosity, which allows appropriate atoms to migrate easily toward growing crystals. The crystals add more and more atoms and grow very large

Assimilation

Assimilation occurs when a hot magma melts and incorporates more felsic surrounding country rock -In assimilation, a hot, original magma is contaminated by picking up and absorbing rock of a different composition. Rocks of intermediate composition can form when magma mixing occurs between a felsic magma and a mafic magma. -A very hot magma may melt some of the country rock and assimilate the newly molten material into the magma -This is like putting a few ice cubes into a cup of hot coffee. The ice melts and the coffee cools as it becomes diluted -If a hot basaltic magma, perhaps generated from the mantle, melts portions of the continental crust, the magma simultaneously becomes richer in silica and cooler. Possibly intermediate magmas, such as those associated with circum-Pacific andesite volcanoes, may derive from assimilation of some crustal rocks by a basaltic magma.

intrusive contact

Boundary surface between two different rock types or ages of rocks -the boundary between country rock and an intrusive igneous rock

Diorite

Coarse-grained igneous rock of intermediate composition. Up to half of the rock is plagioclase feldspar and the rest is ferromagnesian minerals.

Crystalline Textures: Two critical factors determine grain size during the solidification of igneous rocks: rate of cooling and viscosity

Crystals form and grow as magma cools, and the result is a crystalline texture. The most significant aspect of texture in crystalline igneous rocks is grain (or crystal) size -If magma cools rapidly, the atoms have time to move only a short distance; they bond with nearby atoms, forming only small crystals. -Grain size is controlled to a lesser extent by the viscosity of the magma. Atoms in highly viscous lava cannot move as freely as those in a more fluid lava -Hence, a rock formed from viscous lava is more likely to be finer grained than one formed from more fluid lava

Intrusives that Crystallize at Depth: pluton

Deep intrusions: Plutons • Form at considerable depth beneath Earth's surface when rising blobs of magma (diapirs) get trapped within the crust • Crystallize slowly in warm country rock • Generally composed of coarse-grained rocks -A pluton is a body of magma or igneous rock that crystallized at considerable depth within the crust -Where plutons are exposed at the Earth's surface, they are arbitrarily distinguished by size.

Felsic igneous rocks are commonly formed adjacent to...?

Felsic igneous rocks are commonly formed adjacent to convergent boundaries - Hot rising magma causes partial melting of the granitic continental crust

sill

Fine-grained dikes and sills may also have formed in cracks during local extrusive activity -A tabular intrusive structure concordant with the country rock -When magma squeezes between horizontal layers of rock -A sill is also a tabular intrusive structure, but it is concordant. That is, sills, unlike dikes, are parallel to any planes or layering in the country rock -As magma squeezes into a crack between two layers, it solidifies into a sill. -Coarser grains in either a dike or a sill indicate that it probably formed at considerable depth.

Dike

Fine-grained dikes and sills may also have formed in cracks during local extrusive activity -A tabular, discordant intrusive structure -A slab of volcanic rock formed when magma forces itself across rock layers -A protective wall that controls or holds back water -Discordant means that the body is not parallel to any layering in the country rock. -Dikes may form at shallow depths and be fine-grained, such as those at Ship Rock, or form at greater depths and be coarser-grained.

andesite

Fine-grained igneous rock of intermediate composition. Up to half of the rock is plagioclase feldspar with the rest being ferromagnesian minerals. -Andesite is largely restricted to narrow zones along convergent plate boundaries.

Pyroclasts

Fragments of erupted rock -When trapped gases are released during an eruption, it can lead to lava being blasted out of the vent as fragments of rock known as pyroclasts

Granite

Granite , a coarse-grained (the crystals are large enough to be seen with the naked eye) rock composed predominantly of feldspar and quartz, is an intrusive rock. In fact, granite is the most abundant intrusive rock found in the continents.

ABUNDANCE AND DISTRIBUTION OF PLUTONIC ROCKS

Granite is the most abundant igneous rock in mountain ranges. It is also the most commonly found igneous rock in the interior lowlands of continents -Granite, then, is the predominant igneous rock of the continents - basalt and gabbro are the predominant rocks underlying the oceans -Andesite (usually along continental margins) is the building material of most young volcanic mountains. Underneath the crust, ultramafic rocks make up the upper mantle.

Phaneritic rocks

Granite, Diorite, Gabbro, Peridotite --in order of: felsic, intermediate, mafic, ultramafic Increasing K and Na <-------------- Increasing Ca, Fe, and Mg --------> Increasing silica <------------------- -Helpful mnemonic GDGP ---> GrandDaddyGrandPapi This was the only thing I could think of sorry haha

Intraplate Igneous Activity

Igneous activity within a plate, a long distance from a plate boundary, is unusual. These hot spots have been hypothesized to be due to hot mantle plumes , which are narrow upwellings of hot material within the mantle. -Examples include the long-lasting volcanic activity that built the Hawaiian Islands and the eruptions at Yellowstone National Park in Wyoming

Classification of Igneous Rocks

Igneous rocks are named based on their texture and chemical composition. The texture of an igneous rock (its appearance in terms of grain size and presence of gas bubbles, for example) gives you information on where that rock formed, whether beneath the surface as an intrusive igneous rock or on the surface as an extrusive igneous rock. The chemistry of an igneous rock tells you about the origin of the magma and how it evolved before finally solidifying

Igneous Processes at Convergent Boundaries

Intermediate and felsic magmas are clearly related to the convergence of two plates and subduction. -Compared to divergent boundaries, there is less agreement about how magmas are generated at convergent boundaries

Intermediate igneous rocks are commonly formed at.. ?

Intermediate igneous rocks are commonly formed at convergent boundaries - Partial melting of basaltic oceanic crust produces intermediate magmas

intrusive bodies

Intrusive rocks exist in bodies or structures that penetrate or cut through pre-existing country rock -A body of intrusive rock classified on the basis of size, shape, and relationship to surrounding rock -Intrusions , or intrusive structures , are bodies of intrusive rock whose names are based on their size and shape as well as their relationship to surrounding rocks -They are important aspects of the architecture, or structure, of the Earth's crust

Felsic Rocks

Light colored rocks that are rich in elements such as aluminum, potassium, silicon, and sodium -Rocks with a silica content of 65% or more (by weight) are considered to be silica-rich -silica-rich igneous rocks with a relatively high content of potassium and sodium (the fel part of the name comes from feldspar, which crystallizes from the potassium, sodium, aluminum, and silicon oxides; si in felsic is for silica). Rhyolite is a fine-grained, extrusive felsic rock. Granite is the coarse-grained intrusive equivalent of rhyolite -felsic rocks are commonly light in color. A notable exception to this rule is the black glassy volcanic rock obsidian -Mineral Content: Quartz, feldspars (white, light gray, or pink). Minor ferromagnesian minerals

Lava

Liquid magma that reaches the surface; also the rock formed when liquid lava hardens. -Lava is magma on the Earth's surface -Solidifies quickly at surface -extrusive -Small crystals -mafic -dark in color -ex: basalt

Magma mixing

Magma mixing involves the mixing of more and less mafic magmas to produce one of intermediate composition -The process whereby magmas of different composition mix together to yield a modified version of the parent magmas. -If two magmas meet and merge within the crust, the combined magma should be compositionally intermediate -Rocks of intermediate composition can form when magma mixing occurs between a felsic magma and a mafic magma -you are unlikely to get a homogeneous magma or rock. This is because of the profound differences in the properties of felsic and mafic magmas, most notably their respective temperature differences -The mafic magma likely has a temperature of over 1,100 deg. C, whereas a felsic magma would likely be several hundred degrees cooler. The mafic magma would be quickly cooled, and most of it would solidify when the two magmas meet. Some of the mafic minerals would react with the felsic magma and be absorbed in it, but most of the mafic magma would become blobs of basalt or gabbro included in the more felsic magma. Overall, the pluton would have an average chemical composition that is intermediate, but the rock that forms would not be a homogeneous intermediate rock.

Addition of Water (flux melting)

Melting of the mantle occurs through decompression melting and flux melting -Melting aided by the addition of water or other fluid -the addition of water to the already hot mantle rocks lowers their melting temperature resulting in partial melting of ultramafic mantle rocks to yield mafic magma. -A rock's melting temperature is significantly lowered by water under high pressure. This process is known as flux melting . -Water sealed in under high pressure helps break the silicon-oxygen bonds in minerals, causing the crystals to liquify -flux melting is an important process at convergent plate boundaries, where subduction carries water down into the mantle.

Decompression Melting

Melting of the mantle occurs through decompression melting and flux melting -Melting that occurs as rock ascends due to a drop in confining pressure -Decompression melting takes place when a body of hot mantle rock moves upward and the pressure is reduced -The melting point of a mineral generally increases with increasing pressure. Pressure increases with depth in the Earth's crust, just as temperature does. So a rock that melts at a given temperature at the surface of the Earth requires a higher temperature to melt deep underground. -decompression melting is an important process at divergent plate boundaries, where mantle material is rising and melting beneath mid-oceanic ridges.

Heat for Melting Rock comes from...?

Most of the heat that contributes to the generation of magma comes from the very hot Earth's core (where temperatures are estimated to be greater than 5,000 celsius). -Heat is conducted toward the Earth's surface through the mantle and crust.

Differentiation

No single process can satisfactorily account for all igneous rocks. In the process of differentiation, based on Bowen's reaction series, a residual magma more silicic than the original mafic magma is created when the early-forming minerals separate out of the magma -To produce magmas of different composition from an initially mafic magma -The process by which different ingredients separate from an originally homogeneous mixture is differentiation -Differentiation in magmas takes place through a number of processes -If crystals are no longer in contact with the melt, they will not react with it, and the elements they contain are effectively removed from the melt -For example, if olivine pyroxene, and calcium-rich plagioclase crystallize from a mafic magma, they take up magnesium, iron, and calcium from the melt. If these early-formed crystals are removed from the melt, the magma becomes depleted in those elements and enriched in silicon, aluminum, sodium, and potassium. In other words, it becomes more felsic in composition. This differentiation process, often called fractional crystallization, can occur in a number of ways

The Rock Cycle and Plate Tectonics

One way of relating the rock cycle to plate tectonics is illustrated by an example from what happens at a convergent plate boundary. -Magma is created in the zone of melting above the subduction zone. The magma, being less dense than adjacent rock, migrates upward toward the surface. A volcanic eruption takes place if magma reaches the surface. The magma solidifies into igneous rock. -The igneous rock is exposed to the atmosphere and subjected to weathering and erosion. The resulting sediment is transported and then deposited in low-lying areas. In time, the buried layers of sediment lithify into sedimentary rock. The sedimentary rock becomes increasingly deeply buried as more sediment accumulates and tectonic forces push it deeper. - After the sedimentary rock is buried to depths exceeding several kilometers, the heat and pressure become too great, and the rock recrystallizes into a metamorphic rock. -As the depth of burial becomes even greater (several tens of kilometers), the metamorphic rock may find itself in a zone of melting. Temperatures are now high enough so that the metamorphic rock partially melts. Magma is created, thus completing the cycle

Relationships between Rock Types and Their Usual Plate-Tectonic Setting: Granite and rhyolite

Original Magma: Felsic Final Magma: Felsic Processes: Partial melting of lower crust Plate-Tectonic Setting: 1. Convergent boundary 2. Intraplate • over mantle plume

Relationships between Rock Types and Their Usual Plate-Tectonic Setting: Basalt and gabbro

Original Magma: Mafic Final Magma: Mafic Processes: Partial melting of mantle (asthenosphere) Plate-Tectonic Setting: 1. Divergent boundary—oceanic crust created 2. Intraplate • plateau basalt • volcanic island chains (e.g., Hawaii)

Relationships between Rock Types and Their Usual Plate-Tectonic Setting: Andesite and diorite

Original Magma: Mafic (usually) Final Magma: Intermediate Processes: Partial melting of mantle (asthenosphere) followed by: • differentiation or • assimilation or • magma mixing Plate-Tectonic Setting: Convergent Boundary

Partial melting

Partial melting produces magmas less mafic than their source rocks, because lower melting point minerals are more felsic in composition -When the temperature of the lower crust rises sufficiently, partial melting takes place, creating felsic magma. The felsic magma collects and forms diapirs, which rise to a higher level in the crust and solidify as granitic plutons (or, on occasion, reach the surface and erupt violently) -Partial melting of the mantle usually produces basaltic magma, whereas granitic magma is most likely produced by partial melting of the lower continental crust. -As might be expected, the first portion of a rock to melt as temperatures rise forms a liquid with the chemical composition of quartz and potassium feldspar -Partial melting of the lower continental crust likely produces felsic magma -The magma rises and eventually solidifies at a higher level in the crust into granite, or rhyolite if it reaches Earth's surface -Geologists generally regard basaltic magma (Hawaiian lava, for example) as the product of partial melting of ultramafic rock in the mantle, at temperatures hotter than those in the crust. The solid residue left behind in the mantle when the basaltic magma is removed is an even more silica-deficient ultramafic rock

vesicular texture

Rock that has a spongy appearance due to trapped gas bubbles in the lava. -A magma deep underground is under high pressure, generally high enough to keep all its gases in a dissolved state. On eruption, the pressure is suddenly released and the gases come out of solution. When a lava solidifies while gas is bubbling through it, holes are trapped in the rock, creating a distinctive vesicular texture -Vesicles are cavities in extrusive rock resulting from gas bubbles that were in lava, and the texture is called vesicular. A vesicular rock has the appearance of Swiss cheese (whose texture is caused by trapped carbon dioxide gas). Vesicular basalt is quite common

Intermediate Rocks

Rocks with a silica content between 55% and 65% are classified as intermediate rocks . Intermediate rocks contain significant amounts (30-50%) of dark ferromagnesian minerals like pyroxene and amphibole as well as light-colored plagioclase feldspar and small amounts of quartz. These can easily be discerned in diorite , the coarse-grained intermediate rock. -Examine a picture of diorite. You can see that about half the mineral grains are dark in color and the other half are white. -Andesite, the fine-grained intermediate rock, is typically medium-gray or greenish-gray in color. -Mineral Content: Feldspars (white or gray) and about 35-50% ferromagnesian minerals. No quartz. -Color of Rock (most commonly): Medium-gray or medium green

Mafic Rocks

Rocks with a silica content of between 45% and 55% (by weight) are considered silica-poor - The remainder is composed mostly of the oxides of aluminum, calcium, magnesium, and iron. Rocks in this group are called mafic — silica-deficient igneous rocks with a relatively high content of magnesium, iron, and calcium. (The term mafic comes from magnesium and ferrum, the Latin word for iron.) -Mafic rocks are made up predominantly of gray plagioclase feldspar and the ferromagnesian minerals pyroxene and olivine and tend to be dark in color. -Mafic magma that cools slowly beneath the surface forms the coarse-grained, intrusive rock gabbro. If mafic magma erupts on the surface, it forms the dark, fine-grained, extrusive rock basalt

Granite is considerably more common than rhyolite, its volcanic counterpart. Why is this?

Silicic magma is much more viscous (that is, more resistant to flow) than mafic magma. Therefore, a silicic magma body will travel upward through the crust more slowly and with more difficulty than mafic or intermediate magma. Unless it is exceptionally hot, a silicic magma will not be able to work its way through the relatively cool and rigid rocks of the upper few kilometers of crust. Instead, it is much more likely to solidify slowly into a pluton

Metamorphic rock (continued from rock cycle)

Tectonic forces are required to transport sedimentary (and volcanic) rock to lower levels in the crust. Heat and pressure increase with increasing depth of burial. If the temperature and pressure become high enough, as occurs in the middle and lower levels of continental crust, the original sedimentary rock is no longer in equilibrium and recrystallizes. The new rock that forms is called a metamorphic rock. If the temperature gets very high, the rock partially melts, producing magma and completing the cycle.

The continuous branch of Bowen's reaction series

The continuous branch on the right-hand side of the diagram contains only plagioclase feldspar. Plagioclase is a solid solution mineral in which either sodium or calcium atoms can be accommodated in its crystal structure, along with aluminum, silicon, and oxygen. The continuous branch describes the evolution of plagioclase from more calcium-rich compositions to more sodium-rich compositions with decreasing temperature -Plagioclase feldspar forms with a chemical composition that evolves - (from Ca-rich to Na-rich) with decreasing temperature

Discontinuous Branch of Bowen's Reaction Series

The discontinuous branch on the left-hand side of the diagram describes the formation of the ferromagnesian minerals olivine, pyroxene, amphibole, and biotite. This series is called the discontinuous series because these minerals have very different crystalline structures -As one mineral becomes chemically unstable in the remaining magma, another begins to form

sedimentary rock (continued from rock cycle)

The igneous rock, being out of equilibrium, may then undergo weathering and erosion, and the debris produced is transported and ultimately deposited (usually on a sea floor) as sediment. If the unconsolidated sediment becomes lithified (cemented or otherwise consolidated into a rock), it becomes a sedimentary rock. The rock is buried by additional layers of sediment and sedimentary rock. This process can only bury layered rock in the uppermost crust to a depth of several kilometers

geologists have not been able to make coarsely crystalline granite artificially because...

The temperature and pressure at which granite apparently forms can be duplicated in the laboratory—but not the time element. According to calculations, a large body of magma requires over a million years to solidify completely. This very gradual cooling causes the coarse-grained texture of most intrusive rocks. Only very fine-grained rocks containing the minerals of granite have been made from artificial magmas, or "melts." -another problem in trying to apply experimental procedures to real rocks is determining the role of water and other gases in the crystallization of rocks such as granite. Only a small amount of gas is retained in rock crystallized underground from a magma, but large amounts of gas (especially water vapor) are released during volcanic eruptions

crystal settling

This differentiation process, often called fractional crystallization, can occur in a number of ways. One example is crystal settling , the downward movement of minerals that are denser (heavier) than the magma from which they crystallized -A process that occurs during the crystallization of magma, in which the earlier-formed minerals are denser than the liquid portion and settle to the bottom of the magma chamber. -If crystal settling takes place in a mafic magma chamber, olivine and, perhaps, pyroxene crystallize and settle to the bottom of the magma chamber -This makes the remaining magma more felsic -The remaining magma is, therefore, depleted of calcium, iron, and magnesium. Because these minerals were economical in using the relatively abundant silica, the remaining magma becomes richer in silica as well as in sodium and potassium

Glassy textures

With extremely rapid or almost instantaneous cooling, individual atoms in the lava are "frozen" in place, forming glass rather than crystals. -Ex: Obsidian, a dark volcanic glass, is one of the few rocks that is not composed of minerals. Because obsidian breaks with a conchoidal fracture

Scoria

a highly vesicular basalt, actually contains more gas space than rock

Pumice

a very light and porous volcanic rock formed when a gas-rich froth of glassy lava solidifies rapidly. -In more viscous lavas, where the gas cannot escape as easily, the lava is churned into a froth (like the head in a glass of beer). When cooled quickly, it forms pumice, a frothy glass with so much void space that it floats in water

Igneous rocks

any rock formed when magma cools and solidifies -formed when magma cools beneath Earth's surface or when lava cools at Earth's surface; forms in or around volcanoes; requires heat -Magma may contain suspended solid crystals and gas, Igneous rocks form when magma solidifies

Fragmental texture

composed of fragments of igneous material -The characteristic appearance of a rock composed of broken fragments of various materials.

Pegmatite dikes

def: A tabular, discordant intrusive structure. -Most pegmatites are found in sheets of rock (dikes and veins) near large masses of igneous rocks called batholiths -Pegmatite dikes are fairly common, especially within granite plutons, where they apparently filled cracks that developed in the already solid granite. Some pegmatites form small dikes along contacts between granite and country rock, filling cracks that developed as the cooling granite pluton contracted

crystalline texture

held together by interlocking crystals -for instance, the minerals quartz and feldspar

phenocrysts

large crystals in porphyritic rocks -Some rocks contain phenocrysts , larger crystals that are enclosed in a groundmass of finer-grained crystals or glass

Phaneritic

large crystals, slow cooling -For our purposes, coarse-grained or phaneritic (from phaner, meaning visible) rocks are defined as those in which the crystals are large enough to be seen easily with the naked eye.

batholith

large mass of intrusive igneous rock believed to have solidified deep within the earth -A large discordant pluton with an areal extent at Earth's surface greater than 100 square kilometers -Although batholiths may contain mafic and intermediate rocks, they almost always are predominantly composed of granite

aphanitic rocks

rhyolite, andesite, basalt, Komatiite (very rare) -in order of: felsic, intermediate, mafic, ultramafic Helpful mnemonic RABK-->ResidentAssistantBurgerKing

diapirs

rising blobs of magma -Bodies of rock (e.g., rock salt) or magma that ascend within Earth's interior because they are less dense than the surrounding rock -Detailed studies of batholiths indicate that they are formed of numerous, coalesced (come together to form one mass or whole) plutons - Apparently, large blobs of magma worked their way upward through the lower crust and collected 5 to 30 kilometers below the surface, where they solidified -These blobs of magma, known as diapirs , are less dense than the surrounding rock that is pliable and shouldered aside as the magma rises

instrusive igneous rock

rock formed from the cooling and solidification of magma beneath Earth's surface -Magma solidifies very slowly beneath the surface. The resulting intrusive igneous rock may be exposed later after uplift and erosion remove the overlying rock -Two textures: pegmatitic, phaneritic

porphyritic texture

rock texture characterized by large, well-formed crystals surrounded by finer-grained crystals of the same or different mineral

extrusive igneous rock

rock that forms from the cooling and solidification of lava at Earth's surface -If magma is brought to the surface (where it is called lava) by a volcanic eruption, it will solidify into an extrusive igneous rock -Six textures: porphyritic, aphanitic, glassy, vesicular, frothy, pyroclastic

aphanitic

small crystals, fast cooling -Extrusive rocks are typically fine-grained or aphanitic - their crystals are too small to see easily with the naked eye. The grains, if they are crystals, are small because magma cools rapidly at the Earth's surface, so they have less time to form. -Some intrusive rocks are also fine-grained; - these occur as smaller bodies that apparently solidified near the surface upon intrusion into relatively cold country rock

The Origin of Andesite

subduction zone generates ultramafic/mafic magmas through dewatering, which evolve to andesite through differentiation, assimilation and magma-mixing. -Magma for most of our andesitic composite volcanoes (such as those found along the west coast of the Americas) seems to originate from a depth of about 100 kilometers. This coincides with the depth at which the subducted oceanic plate is sliding under the asthenosphere -Partial melting of the asthenosphere takes place, resulting in a mafic magma. In most cases, melting occurs because the subducted oceanic crust releases water into the asthenosphere. The water collected in the oceanic crust when it was beneath the ocean and is driven out as the descending plate is heated. The water lowers the melting temperature of the ultramafic rocks in this part of the mantle. In other words flux melting takes place. Partial melting produces a mafic magma -Hot asthenospheric rock continues to flow into the zone of partial melting -asthenospheric ultramafic rock is dragged downward by the descending lithospheric slab. More ultramafic rock flows laterally to replace the descending material. A continuous flow of hot, fertile (containing the constituents of basalt) ultramafic rock is brought into the zone where water, moving upward from the descending slab, lowers the melting temperature. After being depleted of basaltic magma, the solid, residual, ultramafic rock continues to sink deeper into the mantle. -On its slow journey through the crust, the mafic magma evolves into an intermediate magma by differentiation and by assimilation of silicic crustal rocks -geologists believe that partial melting of the subducted crust produces the magma for andesitic volcanoes in South America. Here, the oceanic crust is much younger and considerably hotter than normal -Most subduction zones are a long distance from the divergent boundaries of their plates, so steep subduction and magma production from the asthenosphere are the norm.

The Geothermal Gradient

the gradual increase in temperature with depth in the crust -the rate at which temperature increases with increasing depth beneath the surface -Data show the geothermal gradient, on the average, to be about 3 deg. C for each 100 meters (30 deg. C/km) of depth in the upper part of the crust, decreasing in the mantle. -rocks melt over a range of temperatures. This is because they are made up of more than one mineral, and each mineral has its own melting point -In order for magma to form, conditions must change so that the geothermal gradient can intersect the zone of partial melting. The two most common mechanisms believed to create these conditions are decompression melting and the addition of water.

Bowen's Reaction Series

the simplified pattern that illustrates the order in which minerals crystallize from cooling magma according to their chemical composition and melting point -Early in the twentieth century, N. L. Bowen conducted a series of experiments that determined the sequence in which minerals crystallize in a cooling magma. Bowen's experiments involved melting powdered rock, cooling the melt to a given temperature, and then observing the mineral present in the cooled rock. By repeating this process to different temperatures, he was able to observe that as you cool a basaltic melt, minerals tend to crystallize in a sequence determined by their melting temperatures. This sequence became known as Bowen's reaction series -Bowen's experiments showed that in a cooling magma, certain minerals are stable at higher melting temperatures and crystallize before those that are stable at lower temperatures. At higher temperatures, the sequence is broken into two branches. -Bowen's reaction series can also be used to consider the formation of magma by melting rock. If you heat a rock, the minerals will melt in reverse order. In other words, you would be going up the series. Any quartz and potassium feldspar in the rock would melt first. If the temperature were raised further, biotite and sodium-rich plagioclase would contribute to the melt. Any minerals higher in the series would remain solid unless the temperature were raised further.

Surficial Processes

weathering, erosion, deposition modify earth's landscape

Shallow Intrusive Structures

• Form <2 km beneath Earth's surface • Chill and solidify fairly quickly in cool • country rock • Generally composed of fine-grained rocks -Intrusions occurring at or near the Earth's surface -Some igneous bodies apparently solidified near the surface of the Earth (probably at depths of less than 2 kilometers) -Shallow intrusive structures tend to be relatively small compared with those that formed at considerable depth. -Because the country rock near the Earth's surface generally is cool, intruded magma tends to chill and solidify relatively rapidly. Also, smaller magma bodies will cool faster than larger bodies, regardless of depth. For both of these reasons, shallow intrusive bodies are likely to be fine-grained

Igneous Processes at Divergent Boundaries

• Mafic igneous rocks are commonly formed at divergent boundaries - Increased heat flow and decreased overburden pressure produce mafic magmas from partial melting of the asthenosphere -The crust beneath the world's oceans (over 70% of Earth's surface) is mafic volcanic and intrusive rock, covered to a varying extent by sediment and sedimentary rock. -Most of this basalt and gabbro was created at mid-oceanic ridges, which also are divergent plate boundaries. -Geologists agree that the mafic magma produced at divergent boundaries is due to partial melting of the asthenosphere. -Along divergent boundaries, the asthenosphere is relatively close (5 to 10 kilometers) to the surface -The probable reason the asthenosphere is plastic or "soft" is that temperatures there are only slightly lower than the temperatures required for partial melting of mantle rock. If extra heat is added, or pressure is reduced, partial melting should take place -As the hot asthenosphere gets close to the surface, decrease in pressure results in partial melting. In other words, decompression melting takes place. The magma that forms is mafic and will solidify as basalt or gabbro. The portion that did not melt remains behind as a silica-depleted, iron-and-magnesium-enriched ultramafic rock. -Some of the basaltic magma erupts along a submarine ridge to form pillow basalts (described in chapter 4), while some fills near-surface fissures to create dikes. Deeper down, magma solidifies more slowly into gabbro. The newly solidified rock is pulled apart by spreading plates; more magma fills the new fracture and some erupts on the sea floor. The process is repeated, resulting in a continuous production of mafic crust.

Unlike the volcanic rock in Hawaii, nobody has ever seen magma solidify into intrusive rock. So what evidence suggests that bodies of granite (and other intrusive rocks) solidified underground from magma?

• Mineralogically and chemically, intrusive rocks are essentially identical to volcanic rocks. • Volcanic rocks are fine-grained or glassy due to their rapid solidification; intrusive rocks are generally coarse-grained, which indicates that the magma crystallized slowly underground. Experiments show that the slower cooling of liquids results in larger crystals. • Experiments have confirmed that most of the minerals in these rocks can form only at high temperatures. Other experiments indicate that some of the minerals could have formed only under high pressures, implying they were deeply buried. More evidence comes from examining intrusive contacts. (A contact is a surface separating different rock types.) • Preexisting solid rock, country rock, appears to have been forcibly broken by an intruding liquid, with the magma flowing into the fractures that developed. Country rock , incidentally, is an accepted term for any older rock into which an igneous body intruded. • Close examination of the country rock immediately adjacent to the intrusive rock usually indicates that it appears "baked," or metamorphosed, close to the contact with the intrusive rock. • Rock types of the country rock often match xenoliths , fragments of rock that are distinct from the body of igneous rocks in which they are enclosed. • In the intrusive rock adjacent to contacts with country rock are chill zones, finer-grained rocks that indicate magma solidified more quickly here because of the rapid loss of heat to cooler rock.