Geology Chapter 4 Notes
Crystalline Classification
Based on composition and texture. Fine grain=very small grains Coarse grained=larger visible grains Felsic, intermediate, mafic, or ultramafic for composition
Hot Spots Products
The hot rock of a plume will reach the the base of the lithosphere and the change in pressure (decompression) causes it to go through partial melting, generating a mafic magma and rising through the lithosphere. Then pools into a magma chamber in the crust, erupting and forming a volcano. Oceanic hot spots: Mostly mafic magma like basalt Continental hot spots: Mafic and felsic, basalt or rhyolite. Generates a hot spot track.
Continental Rift Products
The rifting thins the lithosphere, causing decompression and the melting of mafic rock. The heat transfer from this then melts the crust and creates felsic rock. East African River Falley
Characteristics of Igneous Rocks
The ways to describe/classify an igneous rocks are: Crystalline Texture Fragmental Texture Glassy Texture Color(light/dark) Texture directly reflects the cooling rate of the magma.
Igneous Rocks Freeze Temp
1,100° C to 650°C, depending on composition.
Magma Composition
3 Components are solids, liquid, and gas. Solidified mineral crystals are carried in magma. The melt itself is composed of mobile ions, dominantly Si & O; with Al, Ca, Fe, Mg, Na, and K with other ions to a lesser extent. A different mixture of these elements yield to different magmas. Variable amounts of dissolved gas occur in magma. Dry magma contains scarce volatiles, while we magma may be up to 15% volatiles such as H2O, CO2, SO2, N2, and H2.
Stoping
A process in which the magma assimilates wall rock, and blocks of wall rock break off and sink into the magma.
Assimilation
Since magma sits in a magma chamber before completely solidifying, it may gather chemicals dissolved from the wall rocks of the chamber or from blocks that detached from the wall and sink into the magma. Also known as contamination.
Types of Intrusive Contact
Tabular intrusions/sheet intrusions are planar and are of roughly uniform thickness. Commonly they are in the range of centimeters to tens of meters thick, and tens of meters to tens of kilometers long. The different types of intrusions are distinguished by the basis of their shape. The 6 types of intrusions are: Dike Sill Laccolith Pluton Batholith
Diapir
A light-bulb-shaped blob of magma that pierced overlying rock and pushed it aside as it rose.
Xenolith
When magma undergoes the process of stoping, but does not melt entirely and is instead surrounded by new igneous rock, then it is a xenolith. Named after the Greek word xeno, meaning foreign.
Fractional Crystallization
While mafic magma begins to freeze, mafic minerals (iron- and magnesium rich) minerals such as olivine and pyroxene crystallize first. These first formed crystals are denser than the rest of the magma and begin to sink. Some may react chemically with the rest of the magma while sinking, but some will reach the floor of the magma chamber and become isolated from the magma. This process of crystallization in different orders and then settling is know as fractional crystallization. Progressively extracting iron and magnesium from the magma, making it more felsic.
Heat-transfer Melting
While magma from the mantle rises up into the crust, it brings heat with it. This heat raises the temperature of the surrounding crustal rock and, in some cases, the rise in temperature may be enough of an increase for the crustal rock to begin melting. Like injecting hot fudge into ice cream, the warmth from inside rises the temperature of the entire ice cream.
Volcano
A vent where molten rock comes out of the Earth. EX: Kilauea Volcano in Hawaii. It is very hot (1,200°C) lava pools around the vent. Hot, syrupy lava runs downhill as it flows. The lava flows slow, loses heat, and crusts over. Finally it stops and cools, forms an igneous rock.
Extrusive Igneous Rocks
Cools quickly at surface. Lava flows as streams or mounds of cooled melt. Pyroclastic debris consists of cooled fragments of volcanic ash (fine particles of volcanic glass) and volcanic rock that is fragmented by eruption.
Shape and Size of a Magma Body
Heat will escape from magma at an intrusion's surface, so the greater the surface area for a given volume of intrusion cools faster. So a magma roughly the shape of a pancake cools faster than one with the shape of a melon. Ratio of surface are to volume increases while size decreases, meaning a body of magma the size of a car cools faster than the one the size of a ship.
The Depth of Intrusion
Magma intruded deep in the crust where it is surrounded by warm wall rock, cools much slower than magma does intruded into cold wall rock near the ground surface. So closer to the surface wall rock = faster cooled magma. And wall rock in the deep crust = slower cooled magma.
Intrusive Igneous Settings
Magma rises and intrudes into preexisting wall rock by slowly filtering up through minute holes in the surface between grains and/or forcing open cracks. The magma that does not reach the surface freezes solid underground while still in contact with the preexisting rock and becomes an intrusive rock. The preexisting rock in which magma intrudes is know as wall rock. The boundary between wall rock/intrusive rock is know as an intrusive contact.
Magma Rising
Magma rises due to two reasons. Magma is relatively buoyant so the buoyancy drives the magma upward, similar to buoyancy causing a wooden block to be forced up through the water. Secondly, magma rises because of the pressure created by the overlying rock that literally squeezes magma upward. Like when your barefoot forces down on mud and squeezes the mud up between your toes.
Why Do Magmas Vary Chemically?
Many factors dictate magma composition, these factors are as follows: Source rock composition Partial melting Assimilation Magma Mixing
Chill Margin
Rim of quenched magma at contact.
Glassy Texture
Rocks made of a solid mass of glass, or tiny crystals surrounded by glass, fracture conchoidally. Cooled so quickly that the atoms didn't even have time to arrange into crystalline structure. More common in felsic igneous rock.
Extrusive Igneous Setting
Some volcanoes erupt streams of low-viscosity lava that flood down the flanks of the volcano. When this lava freezes, it forms a thin lava flow, cooling within days to months. (Mafic/low-viscosity lavas) While some erupt viscous masses of lava that pile into rubbly domes. Finally, some erupt explosively, that sends clouds of volcanic ash and debris skyward, and/or avalanches of ash. These clouds of volcanic ash and avalanche of debris combine to create a deadly pyroclastic flow. (Felsic/high-viscosity lavas) The type of eruption that occurs is majorly dependent on the magma's composition and volatile content. Volatile-rich felsic lavas tend to erupt explosively and forms thick ash and debris deposits. While mafic lavas tend to have low viscosity and spread in broad/thin flows.
Speed of Flow on Magma (Viscosity)
Viscosity is the resistance to flow, and it affects the speed at which magmas or lavas move. Magmas with low viscosity flow easier than those with high viscosity, just as water flows more easily than molasses.
The Presence of Circulating Groundwater
Water that passes through magma absorbs and carries heat away, like a coolant that flows around an automobile engine to reduce heat.
Dikes
A tabular intrusion that cuts across preexisting layering(bedding/foliation), it is a nearly vertical/wall-like intrusion. Dikes form in regions where the crust is being stretched horizontally, like in a rift. While the magma that makes a dike forces its way up into a crack, the crust opens up sideways.
Sills
A tabular intrusion that injects parallel to layering, it is a nearly horizontal/tabletop-shaped intrusion. Sills occur near the surface of the Earth, so the pressure of the magma effectively pushes up the rock above the sill, leading to the uplift of the Earth's surface.
Volcanic Arcs Products
A volcanic arc is a chain of volcanoes. These form on the overriding plate, adjacent to the deep-ocean trenches that mark convergent plate boundaries. While the oceanic crust rocks contain volatile compounds like water, the subduction in these arcs carries the crust down into the hot asthenosphere and wet crustal rocks warm up. The crust becomes so hot the volatiles separate from the crust and adds the volatiles to the hot ultramafic rock, causing partial melting and yielding mafic magma. It will then either rise directly to erupt as basaltic lava, or undergoes fractional crystallization before erupting as an intermediate or felsic lava.
Major Types of Magma
All magma contains silica, a compound of silicon and oxygen (SiO2), but carry more elements such as Al, Ca, Na, K, Fe, and Mg. The two main categories are either silicate or mafic. Silicate is heavily consisting of silica in the rock, while mafic contains higher proportion of iron oxides (FeO/Fe2O3) and magnesium oxides (MgO). The "ma-" in mafic stands for magnesium, while "-fic" originates from the Latin word for iron. Ultramafic contains an even high proportion of Fe and Mg oxides relative to silica. While felsic magmas have a relatively high proportion of silica in relation to the Mg and Fe oxides. The 4 types are broken up based off of the percentage of silica that the magma is made of: Felsic (or silicic) magma consists of 66%-76% silica. Intermediate magma consists of 52%-66% silica. Mafic magma consists of 45%-52% silica. Ultramafic magma consists of 38%-45% silica.
Pluton
Blob-shaped intrusions that range in size from tens of meters across to tens of kilometers across that are formed when volumes of magma cool slowly at depth. Molten rock that reaches the surface erupts as lava. Development of a pluton is not fully known yet, but is believed to be either a frozen "diapir", result of stoping, or that pluton is formed by the injection of numerous superimposed dikes or sills, which form together as one mass and recrystallize and becomes a singe, massive body.
Source Rock Composition
Compositions of a melt reflects the composition of the solid which it was derived from. Not all magma comes from the same source rock, so not all magma will have the same composition.
Decompression Melting
Deeper in the Earth's center contains more pressure, this higher pressure prevents atoms from breaking free of solid mineral crystals. Due to the pressure preventing a melt, a decrease in pressure allows for a melt to occur. This happens when a hot mantle rock rises to the shallower depths in the Earth. For example: In mantle plumes, beneath rifts, and beneath mid ocean ridges create cases that forces a volume of hot asthenosphere (mantle rock) to rise to the lower pressured areas, and melt. A mantle plume will propel the mantle rock upwards to a lower pressure area, allowing for a melt. Decompression beneath a rift occurs due to the two plates separating and bringing up new Earth from below, rising the mantle rocks up as well. Decompression beneath a mid ocean ridge is the result of the two plates that are beneath the ocean to separate like the rift does, causing the volume of asthenosphere to rise to the lower pressure.
Magma Mixing
Different magmas formed in different locations, with difference sources may enter a magma chamber. In some events, the originally distinct magmas mix to create a new, different magma. Thoroughly mixing a felsic magma with a mafic magma in equal proportions produces an intermediate magma.
Partial Melting
Due to the temperature and pressure conditions that occur in the Earth only about 2%-30% of an original rock can melt to produce magma at a given location. Meaning the temperature at sites of magma production just never get hot enough to fully melt the whole rock before the magma has a chance to migrate from its source. Magmas that are formed through partial melting are more felsic than the original rock that they originated from. Such as a partial melted ultramafic rock would result in a mafic magma.
Melts Transforming Into Rocks
If melts stayed forever in their point of origin and nothing around it changed then it would stay molten. But melts always eventually solidify or "freeze". This happens because in some cases, volatiles escape from the melt so that the freezing temperature of the rock rises. Meaning if a melt's temperature stays the same, but the escaped volatiles causes a rise in its freezing temperature, then it will solidify. More often, freezing simply takes place when the melt cools below its freezing temperature. Since temperature decreases upward, toward the Earth's surface, then magma enters a cooler environment automatically as it rises. If it stays trapped underground as an intrusion, it will slowly loses heat to the surround wall rock, drops below its freezing temperature, and solidifies. If melt extrudes as lava at the ground surface, it cools in contact with air or water.
Melting Due to Added Volatiles (Flux Melting)
Magma often forms at locations where chemicals called volatiles, such as H2O and CO2, mix with hot mantle rock. When these volatiles mix with hot, dry rock, the help break chemical bonds so that the rock begins to melt. Essentially, the addition of these volatiles simply decreases a rock's melting temperature.
Magma Formation Locations and Reasons
Magma only forms in special tectonic settings. It forms through either the process of decreasing pressure, resulting from the addition of volatiles, or melting as a result of heat transfer from rising magma.
Magma
Melted rock below ground, consists of more magnesium than lava.
Lava
Melted rock once it has reached the surface.
Mid-Ocean Ridge Products
Most igneous activity takes place here. Rifting spreads the plates apart, causing decompression melting. Basaltic magma wells up to fill chambers. Then moves upward to create dikes or extrude as pillow basalt.
Where Does Igneous Activity Occur?
Occurs in different plate-tectonic setting: Volcanic arcs bordering deep ocean trenches Isolated hot spots(independent tectonic plate boundaries) Continental rifts Mid-ocean ridges Established or newly formed tectonic plate boundaries
Laccolith
Point where some intrusions start to inject between layers but then domes upward and creates a blister-shaped intrusion. A blob-shaped intrusion that range in size from tens of meters across to tens of kilometers across.
Crystalline Texture/Igneous Rocks
Rocks consisting of minerals that grow when a melt solidifies interlocks like pieces of a jigsaw puzzle, known as crystalline igneous rocks. Occurs because when some grains have already developed, they interfere with the growth of minerals/grains that form later. Indicates that the melt cooled more slowly, grain size depends on cooling. A faster cooled melt contains smaller grains(aphanitic), while a slower cool is much larger grains(phaneritic). In porphyritic(combination of aphanitic and phaneritic), the larger grains are known as phenocrysts, while the smaller grains are groundmass.
Fragmental Texture/Igneous Rocks
Rocks that consist of igneous chunks and/or shards that are packed together, welded together, or cemented together after having solidified. Combined pieces of preexisting rocks, often shattered.
Time Taken For Magma Cooling
The amount of time it takes for a magma to cool depends on how fast it can transfer heat into its surrounding. To compare, when you pour hot coffee into a thermos bottle and seal it, it will stay hot for hours due to the insulation, so the heat is only being cooled by the outside air very slowly.Surround wall rock acts like an insulator for magmas in that it transports heat away from a magma at a very slow rate, so underground magma (in an intrusive environment) cools slowly. In contrast, when coffee is spilled on a table, it cools much faster because it loses heat to direct exposure of the cold air. Similarly, lava that erupts at the ground surface cools quickly due to the air or water surrounding it, it loses its heat very quickly. Three factors that control the cooling time of magma are: The depth of intrusion The shape and size of a magma body The presence of circulating groundwater.
Batholiths
The combination of numerous pluton intrusions in a region creates a vast composite body that may be several hundred kilometers long, and over 100 kilometers wide. An example is the rock that makes up the Sierra Nevada of California, formed by plutons the intruded 145-80 million years ago.
Change in Magma While Cooling
The process of freezing magma or lava is much more complex than the freezing of water into ice, because molten rock contains many different compounds and not just water. During the freezing of molten rock, many different minerals form, including the fact not all of the minerals form at the same time.
Baked Zone
The rim of the heat-altered wall rock.
Intrusive Igneous Rocks
They cool out of sight, underground. Much greater volume than extrusive rocks. Cooling rate is slower than extrusive. Reside in large volume magma chambers and smaller volume tabular bodies/columns.
Large Igneous Provinces (LIPs)
This is an unusually large outpouring of magma, resulting in large amounts of igneous rocks. Consists of mostly mafic, but some felsic examples. Forms when the bulbous head of a mantle plume first reaches the base of the lithosphere. Erupts huge amounts of magma as flood basalts that are of low viscosity, can flow to tens of hundreds kilometers, and accumulate in thick piles. Columbia River Flood Basalts
Factors of Viscosity
Viscosity is dependent on the temperature, volatile content, and silica content of the magma or lava. Hotter magma is less viscous than cooler magma because the thermal energy breaks bonds and allows for the atoms to move more easily. Similarly, magmas/lavas containing more volatiles are less viscous than dry (volatile-free) magmas, because the volatiles also tend to break apart silicate molecule and may also form gas bubbles. Mafic magmas are less viscous than felsic magmas because silicon-oxygen tetrahedra tend to link together in magma to create long molecular chains that can't move past each other with ease, and felsic magma contains more of these chains than mafic magma does. In conclusion, hotter mafic lavas have relatively low viscosity and flow in thin sheets over wide regions, but cooler felsic lavas are highly viscous and may clump into a dome-like mound at the volcanic vent.
