Geo Exam 2
intrusion (pluton)
the structures that result from the emplacement of magma into preexisting rocks
Compare the processes of assimilation and magma mixing.
--Two processes that account for the different igneous rock compositions on the spectrum Assimilation: the heat of the rising magma body breaks off parts of the host rock and they broken parts sink AND MELT Magma mixing; occurs during the ascent of two chemically distinct magma bodies as the more buoyant mass overtakes the more slowly rising body. Once they are joined, convective flow stirs the two magmas, generating a single mass that has an intermediate composition.
Brielfly describe why partial melting results in a magma having a composition different from the rock from which it was derived.
-Rocks with a granitic composition are composed of minerals with the lowest melting temperatures (quartz and potassium feldspar) -When a rock undergoes partial melting, it will form a melt that is enriched in ions from minerals with the lowest melting temperatures, while the unmelted portion is composed of minerals with higher melting temperatures. -Separation of the two groups yields a melt with a chemical composition that is richer in silica then the rock from which it formed
What are the three factors that influence the textures of igneous rocks?
-The rate at which molten rock cools -The amount of silica present in the magma -The amount of dissolved gases in the magma
What are the three ways magma can be generated?
1. Decrease in pressure (without an increase in temperature) can cause decompression melting 2. The introduction of water can lower the melting temperature of hot mantle rock sufficiently to generate magma 3. Heating of crustal rocks above their melting temperature produces magma.
Name two plate tectonic settings in which you would expect magma to be generated.
1. Divergent plate boundaries, in association with seafloor spreading 2. Subduction zones where oceanic lithosphere descends into the mantle
List the six major igneous rock textures.
1. Glassy 2. Porphyritic 3. Phaneritic (coarse-grained) 4. Vesicular 5. Pyroclastic (fragmental) 6. Aphanitic (fine-grained)
List the four basic compositional groups of igneous rocks, in order from the group with the highest silica content to the group with the lowest silica content.
1. Granitic/felsic 70% silica 2. Andesitic/intermediate 3. Basaltic/mafic 4. Ultramafic 40% silica
List and describe the three components of magma.
1. Liquid: called melt. Mainly composed of mobile ions of the 8 most common elements in the crust 2. Solid (if any): crystals of silicate materials. As the magma body cools, the size and number of crystals increase. Last stage of cooling, magma body is like a crystalline mush/oatmeal with very little melt 3. Gaseous: called volatiles. Materials that vaporize at surface pressures. Most common are water, carbon dioxide, and sulfur dioxide
Classify each of the following rocks by their mineral composition (felsic, intermediate, or mafic): 1. Gabbro 2. Obsidian 3. Granite 4. Andesite
1. Mafic 2. Felsic 3. Felsic 4. Intermediate
Describe each of the following in terms of composition and texture: 1. Diorite 2. Rhyolite 3. Basalt porphyry
1. Phaneritic (coarse-grained) and Intermediate 2. Aphanitic (fine-grained) and Felsic 3. Porphyritic and Mafic
How does the rate of cooling influence crystal size?
1. Slow cooling promotes growth of fewer, larger crystals. 2. Rapid cooling results in a solid mass of tiny intergrown crystals.
List the two criteria by which igneous rocks are classified.
1. Their mineral composition 2. Their texture
Define viscosity and list three factors that influence the viscosity of magma.
A magma's mobility. More viscous = more resistance to flow 1. Temperature (cooler temp, more viscous) 2. Composition (more silica, more viscosity) 3. Dissolved gasses (less gases, more viscous)
What is the most common extrusive igneous rock?
Basalt
What does a porphyritic texture indicate about the cooling history of an igneous rock?
Composed of two distinctly different size crystals -Begins crystallizing in one environment, then moves to another environment with different crystallization rate
Gabbro makes up significant portion of what?
Continental crust
Aphanitic (Fine-grained) texture
Cooling is relatively rapid, like at the surface or as small intrusive masses within the upper crust -Individual minerals can be distinguished only with the aid of a polarizing microscope -ex. light, intermediate, or dark in color "a phaner" = not visible
Phaneritic (Coarse-grained) texture
Cooling is relatively slow, like for large masses of magma at great depths -A mass of intergrown crystals that are roughly equal in size and large enough to distinguish the individual minerals without the aid of a microscope "phaner" = visible
Describe the process of crystallization.
Cooling reverses the process of melting -As temp of the liquid magma drops, ions pack more closely together as their movement slows -When they are cool enough, their chemical bonds will confine again to form a crystalline arrangement.
Porphyritic texture
Crystals start forming, then move to a different environment (up to the surface, for example) where cooling happens at a different rate -large crystals (phenocrysts) embedded within a matrix of smaller crystals (groundmass)
Crystal settling
During crystallization, the separation of various components -Occurs when earlier-formed minerals are denser than the liquid portion and sink toward the bottom of the magma chamber -When the remaining liquid melt solidifies either in the magma body or elsewhere, it will form a rock with a different mineral composition than the parent magma
Describe dikes and sills using the appropriate terms from the following list: massive, discordant, tabular, and concordant.
Dikes are discordant bodies that form when magma is forcibly injected into fractures and cut across bedding surfaces and other structures in the host rock Sills are nearly horizontal, concordant bodies that form when magma exploits weaknesses between sedimentary beds or other rock structures. **In general, dikes serve as tabular conduits that transport magma upward, whereas sills tend to accumulate magma and increase in thickness.
Explain how the crystals in pegmatites are able to grow so large.
Form late in the crystallization of a magma, when water and other materials such as co2, cl, and fl make up an unusually high percentage of the melt -Ion migration is enhanced in the fluid-rich environment, so the crystals formed are abnormally large -Most compositions are similar to granite -Some contain significant quantities of rare elements
Pyroclastic (fragmental) texture
Formed from the consolidation of individual rock fragments ejected from the walls of a vent during explosive volcanic eruptions -Because they are made of individual fragments rather than interlocking crystals, they often look like sedimentary rocks -ex. welded tuff
Glassy texture
Formed when molten rock is ejected into the atmosphere where it is quenched (very quickly cooled) to become a solid -Glass results when unordered ions are "frozen in place" before they are able to unite into an orderly crystalline structure -ex. obsidian
How are most basaltic magmas thought to have formed?
High pressure conditions in the upper mantle and partial melting of the ultramafic rock peridodie
Name two minerals typically found in rocks with high silica content an two minerals found in rocks with relatively low silica content.
High: quartz and potassium feldspar Low: Olivine, pyroxene
Igneous rocks
Ignis=fire Form as molten rock cools and solidifies a solid mass of interlocking silicate minerals (silicon-oxygen tetrahedrals)
Compare and contrast extrusive and intrusive igneous rocks.
Intrusive (plutonic): When magma crystallizes at depth. Mt. Rushmore, Yosemite Extrusive (volcanic): When magma crystallizes on Earth's surface. Hawaiian and Aleutian Island Chains
How do light-colored igneous rocks differ in composition from dark-colored igneous rocks?
Light: richer in silica than dark Dark: less silica than light, rich in iron/magnesium
What is magma? How does magma differ from lava?
Magma: the parent material for igneous rocks, formed by partial melting in the crust and upper mantle (to about 150mi) Lava: magma that has reached Earth's surface (explodes or oozes)
Explain the process of decompression melting.
Melting occurs at progressively higher temperatures with increased depth due to the increase in pressure exerted by the weight of overlying rocks. -In reverse, reducing confining pressure lowers a rock's melting temperature -Occurs wherever hot, solid mantle rock ascends, thereby moving into regions of lower pressure.
What is the process that is thought to generate most granitic magmas?
Most probably form when hot basaltic magma becomes trapped because of its greater density (ponds) below continental crust -The heat from the hot basaltic magma partially melts the overlying crustal rocks which are silica rich and have much lower melting temperatures -ex. past volcanic activity in Yellowstone National Park
Vesicular texture
Often form in the upper zone of a lava flow, where cooling occurs rapidly enough to preserve the openings produced by the expanding gas bubbles An extrusive rock that has many voids with nearly spherical openings left by escaping gas bubbles -ex. pumice
List these three magmas in order, from the MOST silica-rich to the least silica-rich, based on their composition: basaltic magma, rhyolitic magma, and andesitic magma.
Rhyolitic, Andesitic, Basaltic
Explain why magmas with a higher silica content have a higher viscosity.
Silica impedes the flow of magma because silicate structures begin to link together into long chains early in the crystallization process
Igneous rocks are composed mainly of which groups of minerals?
Silicate minerals (silicon and oxygen tetrahedrals)
Define Bowen's reaction series.
Start with basaltic magma. As it cools, certain minerals will form and their elements will drop out of the magma mix. Then, with the elements that are left in the magma mix, different minerals will crystallize and form, and their elements will drop out of the magma mix. -Each rock group consists of minerals that crystallize in the same temperature range.
How does the crystallization and settling of the earliest formed minerals affect the composition of the remaining magma?
The earliest formed minerals at the higher temperatures suck up all the elements they need (ex. iron, magnesium, and calcium) until the magma is left with more of other elements (ex. sodium and potassium) that form other minerals farther down in the cooling process
Magmatic differentiation
The formation of a magma body having a mineralogy or chemical composition that is different than the parent magma -Palisades sill in NY
What is meant by the term country rock?
The host rock in the crust that the magma moves through
What is the geothermal gradient? Describe how the geothermal gradient compares with the melting temperatures of the mantle rock peridodite at various depths.
The increase in temperature with depth. -The melting point curve for peridodite is higher than the geothermal gradient. Thus, under normal conditions, the mantle is solid. However, tectonic processes trigger melting through various means, including reducing the melting point of mantle rock.
Intrusive igneous bodie are generally classified according to what?
Their shape--either tabular or massive AND by their orientation with respect to the host rock
How are granite and rhyolite different? In what way are they similar?
They are both made of quartz and feldspar. Granite is coarse-grained. Rhyolite is fine-grained. Granite is widely distributed as large intrusive masses. Rhyolite deposits are less common and less voluminous
In what ways does tuff and volcanic breccia differ from other igneous rocks such as granite and basalt?
They are pyroclastic rocks formed from volcanic ash and other small particles. Their names do not imply mineral composition like granite and basalt. Instead, they are frequently identified with a modifier. -ex. Rhyolite tuff indicates a rock composed of ash size particles (tuff) having a felsic composition (rhyolite)
What role do water and other volatiles play in the formation of magma?
Water causes rock to melt at lower temperatures -Occurs mostly at convergent plate boundaries -Oceanic crust with water in it goes under the continental crust. The water moves up out of the oceanic crust into the asthenosphere under the continental crust. This water heats up the hot mantle above it and the water induces some melting. -Partial melting of the mantle rock peridodite generates hot basaltic magma
Explain the link between dissolved gases and magma viscosity.
Water vapor dissolved in magma tends to increase fluidity because it reduces formation of long silicate chains by breaking silicon-oxygen bonds. -the loss of gases renders magma more viscous -gases give magma their explosive character
laccolith
igneous rock forcibly injected between sedimentary strata, so as to arch the beds above while leaving those below relatively flat
columnar jointing
occurs when igneous rocks cool and develop shrinkage fractures that produce elongated, pillar-like columns that most often have six sides
xenolith
suspended blocks of country rock -proof of stoping (blocks of the roof overlying a hot, rising mass become dislodged and sink through the magma without melting)
batholiths
the largest intrusive igneous bodies -occur as mammoth linear structures several hundred km long and up to 100km wide -produced by hundreds of discrete injection of magma that form smaller intrusive bodies that ultimately crowd against or penetrate one another -must have a surface exposure greater than 100km sq to be considered a batholith
Texture
the overall appearance of a rock based on the size, shape, and arrangement of its mineral grains, NOT HOW IT FEELS TO TOUCH
