Chapter 5: Igneous Environments
it rises into the overriding plate--oceanic plate or a continental plate. Magma modify the crust. Magma generates above the subducting slab--mafic composition--forms by partial melting or ultramafic mantle. partial melting may generate magma of intermediate composition. 2. overriding plate--continental plate, rising magma--thick continental crust that slows its upward journey. Heat surrounds the rocks--causing localized partial melting--felsic or intermediate magma. 3. some magma may never reach the surface, erupts, forming clusters or belts of volcanoes. Overlying crust is continental, volcanoes--mountain belt. Overlying crust is oceanic--individual volcanoes along an island arc. both settings--intermediate composition (andesite). island arcs--magma can be felsic, thicken the crust
What happens when subduction-derived magma encounters overlying crust?
As minerals crystallize from cooling magma--chemical composition of the magma changes as minerals crystallize. Mafic minerals crystallize from magma--extract mafic components; magnesium, iron and calcium, remaining magma contains less of the elements. magma becomes less mafic (more intermediate or felsic). crystallization of mafic minerals is making the remaining magma less mafic.
How does crystallization of minerals change the composition of remaining magma?
the slower the magma cools the more time it has to form larger crystals. If magma cools quickly---small crystals
How does the rate of magma cooling affect the size of crystals?
two plates move away from one another (diverge) along mid-ocean ridges. mantle rocks; solid and crystalline not molten. mantle's high pressure and temperature rocks to flow as weak solid--crystalline structure, Parts of the athensophere are close to melting temperatures. 2. plates separate; solid asthenosphere rises to fill the area between the plates. rises, pressure decreases and the rock partially melts (decompression melting). 3. mafic magma rises away from the unmelted residue in the mantle accumulate in crust and upper mantle. 4. Magma rises upward through magma-filled fractures that form as plates pull apart. 5. older oceanic crust moves away from the ridge in conveyor belt manner as new oceanic crust forms along the axis of the ridge
Why does melting occur along mid-ocean ridges and why the resulting magmas are basaltic? (mafic)
Granite--(felsic) coarsely crystalline light colored igneous rock, contain some biotite and some white mica muscovite and garnet. Rhyolite--fine grained crystalline rock, contain glass, volcanic ash, pieces of pumice visible crystals (phenocrysts) quartz, feldspar or biotite. Intermediate; Diorite--between felsic/mafic compositions, plagioclase and amphibole. Andesite--fine grain--phenocrysts of cream color feldspar or dark amphibole. Mafic--coarsely crystalline, mafic rock, dark consist of pyroxene and mafic minerals along with calcium and plagioclase feldspar. Basalt--mafic lava rock, vesicles, phenocrysts of dark pyroxene, olivine. Ultramafic-- Peridotite--coarsely crystalline, contains more magnesium and iron-rich minerals especially green olivine, Upper mantle--peridotite.
List some common igneous rocks and a few characteristics of each
lithospheric plate--moving above the plume anchored into the deep mantle. active volcano overlies the hot spot--volcanic activity will cease--volcano overlies the hot spot. hot spot--succession of volcanoes along linear chain of islands & sea-mounts. rising mantle plume encounters continental lithosphere, high temps cause melting. melting occurs in the lower part of the lithosphere (in the mantle) mafic magma. melting in the upper part of the lithosphere (in the crust)--felsic magma. felsic magma--eruptions to be explosive form large volcanic depression--caldera.
Describe how a hot spot can form a sequence of volcanic islands on a moving oceanic plate
colors and size of crystals, geologists observe coarse-grained rocks by cutting a slab. Fine grained rocks--microscope
Describe how igneous rocks are classified
main difference:felsic; light colored abundant in quartz and feldspar. Mafic-- dark minerals rich in magnesium and iron
Describe the main differences between felsic and mafic rocks
many hot spots--crustal expression of rising plume of hot mantle material, may begin at the core-mantle boundary & ascend through the lower mantle--asthenosphere. mantle plume encounters the lithosphere, causes melting of the overlying lithosphere and additional melting occurs by decompression. from the lithosphere the plume reach the surface--large volcanoes on the seafloor. Big Island of Hawaii & seafloor far to the southeast.
Explain a mantle plume and its magmatic expression in both oceanic and continental plates
begins with magma forming by melting at depth, followed by movement of the magma toward the surface and then solidification of the magma into solid rock. 1. melting beneath the surface in the deeper parts of the crust or in the mantle. source area--melting occurs, most magma result from partial melting leaving most source area unmelted. 2. separate pockets of magma may accumulate large volume of magma. 3. magma chamber--injection of smaller magma in sheetlike bodies, may solidify and never reach the surface. igneous rock--solidified at depth--plutonic rock and body of the rock forms a pluton. 4. magma rises through the crust, may stop in or pass through a series of magma chambers, intrusion--body of molten rock in the subsurface--magma intrudes into the rocks, solidifies below surface--intrusive rock. 5. magma chambers below the surface beneath a volcano, magma may solidify before the next batch arrives, crystallize while some rise to the surface, rising magma carry some early formed crystals all the way to the surface--porphyritic volcanic rock. 6. magma reaches the surface erupts as lava or volcanic ash--forms when dissolved gases in the magma expand and below the magma apart into small fragments of volcanic glass. forms on the surface--extrusive rock--extruded onto the surface
Describe the processes involved in forming igneous rocks
the rocks are all mafic. The mafic magma forms by partial melting of the ultramafic mantle. Upper part of the oceanic crust---basaltic lava flows, form series of overlapping mounds---pillows. rocks called---pillow basalts, uplifted above sea level. Thin, vertical intrusions of finely crystalline basalt cut across pillow basalts. Thin intrusions---dikes--closely spaced--called sheeted dikes. thin tab conduit through magma passed. Orientated parallel to oceanic rift. Sheeted dikes merge down into gabbro--magma chambers beneath the rift--light colored and dark crystals. Base of gabbro--base of the oceanic crust---ultramafic rocks of the mantle. evidence of having partially melted to form overlying mafic rocks in the crust (pillow basalts, sheeted dikes and gabbro)
Describe the types of igneous rocks that form along mid-ocean ridges
texture of a rock--refers to the sizes, shapes and arrangements of different components. The texture of an igneous rock depends on overall crystal size, variation within that rock and presence of other features; holes and rock fragments, most obvious texture; crystals that are visible or not. Visible crystals--phaneritic. microscopic crystals--fine grained volcanic ash, volcanic glass without any crystals--Aphanitic--magma solidifies too rapidly to grow, large crystals--coarse igneous rock--pegmatite. Coarse grained (coarsely crystalline) Medium grained rocks--crystals easily visible. Fine grained--crystals too small to see. Volcanic glass or mostly glassy with some crystals. Large crystals finer grained matrix--porphyritic
Describe the various textures displayed by igneous rocks
the magma inflates a lump or buldge-shaped magma body, magma chamber grows layers tilt outward and dome shaped feature. Henry Mountains of southern Utah, uncovered by erosion, medium grained and porphyritic, intermediate composition.
Discuss the geometry of a laccolith
they form when a hot but solid igneous rock contracts as it cools. the fractures carve out columns--5 or 6 sides, common in basaltic lava flow, felsic ash flows, sills, dikes and some laccoliths.
How do columnar joints form?
two continents; Asia and India converge--collision because continental crust--buoyant and difficult to subduct. plate may slide beneath the other--descending plates--continental rather than oceanic. 3. Magma by continental collision--don't reach the surface partly because they have pass through thick continental crust. relatively high water content compared to mantle-derived magma, pass through the wet solidus as rises. Continental collisions---don't have many volcanoes. 2. metamorphism--water bearing minerals and if the descending continental crust gets hot enough, partial melting--felsic magma. 1. continental collision-- one continental plate may slide beneath another continental plate. descending continental crust gets hotter and increases pressure
How does magma form during continental collisions?
magma created at convergent boundaries at subduction zones. about 1/5 of Earth's magma forms where an oceanic plate subducts into the mantle at an ocean-ocean or ocean-continent convergent boundary. Oceanic pate--oceanic crust and lithospheric mantle convgerges with another oceanic plate or with a continental plate, subduction occurs--descends both pressure & temperature increase. 2. existing minerals in the subducting plate convert into new ones--metamorphism. Water-bearing minerals; mica break down, forcing water out of the crystalline structure. water lowers the melting temperature of the mantle above subducting plate. If temps are high enough--melting occurs and mantle- derived magma rise into the overriding plate.
How is magma generated in a subduction zone?
Obsidian (mostly felsic)-- shiny volcanic glass, forms when lava flow cools too rapidly to form crystals. Pumice (felsic)--volcanic rock, vesicles, floats on water, begins as volcanic glass, convert into microscopic crystals. Scoria (mafic)--volcanic rock many vesicles, mass of rock fragments. Pegmatite (any composition but mostly felsic)-- magma crystallizes deep within the crust--dissolved water that it grows, large crystals. Tuff (felsic to intermediate) can be also mafic- volcanic rock mix of volcanic ash, pumice crystals and rock fragments, thick pyroclastic flow, weight of overlying materials compacts ash and pumice into lenses forming welded tuff. Volcanic breccia (almost any composition)-- volcanic rocks with fragments break apart of the lava that solidifies during flow, fragmental rocks form from mixture of volcanic rock, ash and mud.
List the main characteristics of obsidian, pumice, scoria, tuff, breccia, and pegmatite, and indicate where each of these rock types fits into an igneous classification system based on composition
a solidified magma chamber--pluton--cylindrical, sheetlike or very irregular in shape. Irregular plutons--vertical cylinders, less than 100 km2 is a stock.--magma solidified at depth and later was uplifted. Sheetlike plutons--tabular shape, horizontal, vertical or inclined, parallel to or cutting across layers. Batholith--more than 100km2, form from multiple magma emplaced into the same part of the crust. Huge gray granite -- Sierra Nevada batholith of California
Summarize the different geometries of large magma chamber and how they are expressed in the landscape
Magma chamber- underground body of molten rock, very dynamic with magma evolving, crystallizing, being replenished by additions of new magma. 1. Large magma chambers can be consist of a single magma type, involve more than one influx of magma. 2. crystallization--early formed minerals that remove chemical components from the magma--sink or rise. 3. new pulse of magma rises into the chamber, mix with existing magma or remain distinct, adds a new pulse of thermal energy. 4. Partially crystallized magma--heated by new hotter pulse of magma. 5. melt the wall rocks. 6. partial melting of wall rocks--assimilated into the existing magma. 7. two magmas mix depends--relative densities, crystal content, viscosities and the temperature which they crystallize.
What is a magma chamber? and what are the processes that occur in one
a dike-- sheetlike intrusion cuts across any layers present in the host rocks, a sill-- intrusion that is parallel to layers in the host rocks. Dikes are steep because magma pushes through the rocks in horizontal direction rises vertically. Sills form by pushing adjacent rocks upward rather than sideways.
What is the difference between a dike and a sill and why does each of them have the orientation that it does?
complete melting-- composition identical to that of the source. most rocks melt by partial melting--felsic minerals melt at lower temperatures than mafic minerals, partial melting produces magma more felsic than the source. overall composition of the mantle is ultramafic----partial melting--mantle--mostly mafic. felsic source area; continental crust is melted, magma will be felsic---intermediate source is almost melted--intermediate composition.
describe how melting can influence magma composition
magma cools and solidifies from the outside--magma cool--mafic minerals crystallize first, composition of remaining magma less mafic (more felsic) partial crystallization of mafic magma--produces magma of more intermediate composition. Heavy mafic minerals may sink through the magma and collect in layers at the bottom--crystal settling--lower parts of the magma chamber more mafic, remaining felsic. Felsic crystals may be less dense than magma--float upward--top of magma chamber more felsic. Magma mixing--two different magmas come into contact and mix, producing magma has composition intermediate between the two magmas. Mafic magma -- hotter than melting temperature of felsic rocks so mafic can melt felsic wall rocks..---- wall rocks around a magma melt--incorporated into the magma by assimilation.
describe how partial crystallization, assimilation, and magma mixing can change a magma
Thermal energy; heat moves from the hotter interior to the cooler surface. Heat flow--2 adjacent masses have different temperatures. Three ways: 1) Conduction- heat transfer by direct contact. 2) Radiant Heat transfer-- heat radiates through the air. 3) Convection-- heat transfer by flow of a liquid or by solid but weak material, material flows in a circular path--convection cell. Via plate tectonics-- solid asthenosphere rises beneath mid-ocean ridge, hot rocks--convection adding material to the oceanic lithosphere. Sea-water; hot crust of mid-ocean ridge; hotter and and rises; convection cell helps cool the oceanic crust. newly created lithosphere cool by conduction of heat to cooler rocks and sea-water. underlying asthenosphere cools, hardens and part of the lithosphere, cooled oceanic lithosphere subducts back into the asthenosphere.
describe three ways that heat is transferred from a warmer mass to a cooler one and an example of conduction and convection by plate tectonics
viscosity--measure of materials resistance to flow. viscous magma--does not flow easily, fluid (less viscous) magma flows more easily. controlled by temperature, composition and crystal content. viscous magma strongly resists flowing, erupts on the surface it does not spread out, piles up forming mounds or domes of lava. Fluid (less viscous)--flows more easily and may spread out in thin layers on the surface, travel longer distances from its source and cover large areas with lava. Temperature; low temp-- temperature of magma--important control factor of viscosity, barely hot enough to be molten, flows only with difficulty, very viscous. High Temperature; very hot, low viscosity flows very easily. Mafic magma--hotter than felsic--less viscous than felsic if two are at the same temperature. Composition: Abundant Silicate Chains--- Silicon and oxygen tetrahedra long silicate chains that don't move easily out of the way. Felsic and most intermediate magmas have high silicon and oxygen content--very viscous. Few silicate chains---mafic magma contains less silicon/oxygen than intermediate or felsic, less connected--less viscous, flows more easily. Volatiles-- abundance of silicate chains--water dissolved in magma disrupts long chains decreasing the viscosity. water and other volatiles decrease viscosity. Percentage of Crystals: Abundant crystals; magma cools, crystals begin to form, flow slowly, more viscous (has more resistance to flow)O than magma with fewer crystals. Few crystals--few crystals has few internal obstructions and flows more easily (less viscous), smoothly, flow faster and farther
explain the factors that control the viscosity of a magma
mafic minerals (olivine and pyroxene) are the first to crystallize from mafic-magma, typically don't crystallize from felsic magma. Amphibole and Biotite--intermediate composition but also present in mafic rocks or felsic, crystallize at temperatures lower than olivine and pyroxene. Plagioclase feldspar--calcium rich or sodium rich, crystallizes at high temperature sometimes with olivine and pyroxene. Plagioclase with less calcium--lower temperatures. Light color felsic minerals quartz, K-feldspar and muscovite crystallize at low temperatures, only minerals form from felsic magma--lack chemical components required to grow mafic minerals.
explain the order in which minerals crystallize from a magma (Bowen's Reaction Series) and compare it to the order in which they melt
1. a small volcano--partially eroded revealing cross section, many necks form as erosion wears down a volcano exposing the harder, more resistant rocks that solidified inside the magnetic conduit of the volcano. 2. magnetic conduits forming well beneath the volcano.
explain two ways that a volcanic neck can form
light colored minerals which is felsic like quartz, K-feldspar and Na-rich plagioclase feldspar reside in silica-rich felsic and some intermediate rocks, but are uncommon in mafic rocks, which contain less silica. Biotite--many felsic rocks but it and amphibole are more abundant in intermediate rocks. Mafic minerals--pyroxene and Olivine along with Ca-Rich plagioclase are dominant in mafic and ultramafic rocks.
summarize the main minerals that are present in felsic, intermediate, mafic and ultramafic rocks
