Geology #2
igneous rocks
-derive from the mantle
mafic rocks
-found in oceanic crust -comes from magnesium and iron (have silica content 45-52%) creates dark colored rock like gabbro
bimodal volcanism
-different types of lavas erupted at the same place. -continental mantle plume hot spots: mafic magmas make it up into the crust a little bit -but sit in the crust for a long period of time -transfer there heat to the continental crust, melt it: heat transfer melting -have potential to form a supervolcano ex: yellowstone -basalt and felsic volcanics
xenolith
"xeno"=strange "lith"=rock -blocks of country rock incorporated into melt
diversification of magmas
-% partial melting -fractional crystallization -assimilation -magma mixing
earths internal heat sources
-Primordial heat, radioactive decay -magma itself is not a heat source. -energy trapped as heat within the earth from the early earth forming processes (primordial) -heat breaks down to different elements releasing energy (radioactive) -Earth interior=solid (except outer core)
partial melting
-always generates a magma with higher SiO2 (silica content) -silica is one of the elements that goes into the melt early on. -if you melted a rock completely, silica would be the same % as the source rock. -if you only melt 5% of the melt, silica content will be much higher than the source rock. -relatively high silica compared to the source to the same silica content. ultramafic source: low% of pm-->high%pm -low % pm= Felsic -high % pm=Mafic mafic source -low% pm=felsic high%pm= intermediate intermediate source -felsic -oceanic crust is drived by melting from the mantle -how much partial melting is happening determines composition
plutons
-deeper in the crust, part of the rock that doesnt have cracks (lower crust) -intrusions form large blobs, move upward by taking advantage of the ductile nature of the lower crust -deform rock around them to migrate upwards. -blob/rounded shaped intrusions
volcanic gases
-emitted by volcanoes -part of the magma for a long time, dissolved within the magma until some point when they are released. -atoms that tend to form volatile molecules
melting rock
-is not easy! melt originates in the upper mantle (asthenosphere) -mantle source of magma -source rock: peridotite (olivine-rich rock) -partial melt:10-20% (30% maximum) -rock melts very differently than ice because they're made up of many minerals with many melting temperatures. -source rocks only partially melt. -partial melt:10-20% (30% maximum) -composition of the source rock is different than the composition of the melt. -different types:decompression, subduction, heat transfer
magma mixing
-mantle derived melt (basalt) brings heat up to the crust and starts partially melting the crust around it. -potentially have 2 different types of magma forming -mantle derived basaltic magma and crustal magma -mixing of the 2 batches of magma results in a magma with a different composition -magma (mantle derived) and felsic (crustal derived) form together to form an intermediate magma.
types of volcanos
-shield volcanos -cinder cones
assimilation
igneous intrusion intruding sedimentary rock on the continent. -the rock around the intrusion=country rock -intrusion makes its way upward, breaking off chunks of country rock into the magma chamber -chunks melt and chemically react with the magma. -mafic melt + felsic country rock--> intermediate magma. -one of the ways in which magma can migrate up towards the surface.
extrusive igneous rock
igneous rocks cool rapidly at the surface. -marks where lava spits out, ex:volcano
cinder cones
minor Type of Volcano -composed of pyroclastic debris (cinders) -often small and short-lived
magma
molten rock beneath the Earth surface -silica based -have very different behavior than liquids we're used to dealing with (sticky, harder to move around) -rock melts at temperatures ranging from 600-1200 C) -less dense and more buoyant than surrounding rock -generates in Earth's interior -rises closer to the surface, due to the high pressure of the Earth (follows networks of cracks and faults to get there.
lava flows
molten rock that moves over the ground
volcano
mountain that forms around a place where magma exists the earth -buoyant magma exits the surface->lava -magma chamber beneath (consistant source of lava for it)
hawaii
oceanic hot spot -upwelling of hot mantle-heat transfer, depression melting -results in basalt magmas formed by partial melting of peridotite
basalt-andesite
oceanic oceanic
andesite-rhyolite
oceanic-continental -flux melting
products of volcanic eruptions
what comes out of volcano? -volcanic vent -lava flows -pyroclastic debris -gas
where volcanos form
90% are associated with subduction zones -2 types: oceanic-oceanic- mafic/intermidiate contiental-oceanic -continental hotspots -oceanic hotspots -hotspots=decompression and heat transfer of the mantle; only process not plate movement related -mid ocean ridges: decompression of peridotite; end up with mafic magma; result=mafic igneous rocks -continental rifts -hot spots and mid ocean ridges-very little interaction of mantle direct melt with the crust; basalt/mafic rock.
igneous rock classification
intrusive vs. extrusive
lava
molten rock at the Earth surface
decompression melting
type of melting process (special situation of the geothermal gradient) -most abundant type -rock moves to lower pressure rapidly, with little heat loss -result of the solidus and pressure -geotherm= theoretical temperature with depth -solidus= conditions at which rock starts to melt -liquidus= conditions at which rock completely melts -can happen in hot spots; mantle plume, rifts, mid-ocean ridges -main cause of magmetism in the oceanic crust!
subduction melting
type of melting process that generates magma -volcanic arcs (ex:cascades) chain of volcanoes parallel to a subduction zone -lowers the melting temp of rock -addition of fluids to the mantle lowers melting point of peridotite. -lots of water comes off of subducting slab. -as it moves away from the mid ocean ridge and cools off, the crust interacts with the ocean water and ground water; creating different minerals that contain water(crust gets hydrated) -hydrous minerals brought down to higher temps and higher pressures within the mantle where they are no longer stable -mantle wedge; receives the water coming off of subducting slab -lowers temp of peridotite and triggers melting.
exposed deep intrusions
yosemite: granite plutons
tectonic settings of magma generation
4 places on earth where you find volcanoes -plate divergence: mid ocean ridges (basaltic extrusives, basaltic intrustions) -island arcs: oceanic oceanic subduction zone (mafic to intermediate) -oceanic continental subduction/continental arc (felsic rocks-anything from basalt to rhyolite) -hot spots: places in the middle of the plate where you have melting happening and volcanoes (just basalt)
resurgent dome
A dome formed by swelling or rising of a caldera floor due to movement in the magma chamber beneath it. Wikipedia
caldera
A gigantic volcanic depression formed when the magma chamber collapses -10-100kms in diameter -much larger than craters -form where there is evacuation of magma chamber and overburden that collapses into the empty magma chamber. -create a bowl-shape -ex: Crater Lake in Oregon
causes of metamorphism
Anything that makes the protolith minerals uncomfortable. -main conditions that affect a mineral: temperature, pressure, change in the fluids, water, changing the chemical system of the rock. -earth's geothermal gradiant changes temperature the further into the Earth you go. -pressure gradiant also increases the deeper in the Earth you go.
old mountain belt
Appalachians -formed 300 million years ago -looked like the himalayas at first but now they are much smaller/lower. -still attacked by erosion -but you are able to see metamorphic rocks formed deep in the earth if you go there, brought up by lengthy processes of erosion and uplifting. -
vesicles
Common feature in volcanic holes -cavities found in volcanic rock representing trapped gas bubbles -form while rock is still liquid -tells you about the nature of the lava and how much pressure the gas was exerting on the lava around it
orogenic collapse
Derives from Orogeny crust is too thick, flows at depth, cracks near surface
young mountain belt
Himalayas -still growing -collision still occurring so mountains are continuously pushed upwards. -wherever the highest mountains are is where the thickest lithosphere most likely is. -lots of erosion: sediment draining from them and traveling elsewhere -When the mountain is eroded, uplift of the surface takes place: Bring lower rocks higher to replenish what was removed through erosion. -uplift and erosion work on mountains over and over again.
moving rocks out of their comfort zones
Metamorphic...Ways to do so -Burial: more and more sediment piled on top. End up with stuff at the surface of the earth to subside deeper into earth to experience higher pressure/temperature -Heating (igneous intrusions): intrusions bring heat with it, magma coming from deep into earth to surface, heating rocks around it while doing so. -Tectonic Activity (collision, faulting, subduction): shifting and bumping of plates cause rocks to be changed. Subduction, actually thrusting rocks into deeper depths of the Earth. -Orogeny-one of the main ways
bowens reaction series theory
Norman Bowen -looked at resulting rock at different temperatures to see what crystals form. -found sequence of minerals that form as cooling occurs -first to form= olivine -2nd=pyroxene -3rd=amphibole -4th=biotite -theorized that you could produce all the different sources of music through continual crystallization. -you can fractionally crystallize a starting melt and create all the diversity of igneous rock types.
very old mountain belt
Northern Minnesota -rocks that formed in the crustal root are at the surface -not much mountain, but deeply formed igneous and metamorphic rocks that have been brought to the surface.
gas bubbles
Really wanna escape! -only way to escape is by migrating up liquid and escaping into atmosphere -makes difference is liquid is runny vs. sticky. -sticky: hard to escape, get trapped. low pressure- expand, move upwards, get larger; gases decide to escape and form gas bubbles. -ex:soda, packaging it under high pressure; opening it releases the press high pressure-gases dissolved within the magma, pressure is reduced nearing the surface Analogy for gas bubbles moving through different viscosity: -blowing bubbles in soda vs. milkshake. Milkshake is more difficult because it is more sticky and viscous, bubbles have a harder time traveling through the shake
mid ocean ridge
Tectonic Setting of Magma Generation places where the most magma is being generated. -decompression melting of peridotite (partially melting it at high percentages, ends up with a mafic magma) -Thin crust (minimal assimilation, fractional crystallization) -2 plates moving away from one another; place they actually separate= ridge -most volcanic activity occurs at the ridge. -as plates move apart, crust gets really thin; upwelling of asthenosphere to fill in the space. -as plate moves away from zone of upwelling, mantle cools down
crater eruptions
Type of Eruption -where lava reaches the surface through a pipe-shaped conduit -formed pathway that is used over and over again -volcano made up of layer upon layer of old lava and pyroclastic debris
explosive eruption
Type of Eruption high viscosity magma -build up of gases in magma chamber or vent; eventually, they exert so much pressure that it causes whole thing to explode. -magma blasted out of volcano and fragmented. -more dangerous; unpredictable, more violent; might end up erupting more material due to huge amount of lava, ash, and other particles.
fissure eruptions
Type of Eruption instead of a single vent where lava comes out, there is a crack in the earth where lava dispenses along. -happens because magma wants to make it to the surface where ever it can -rifting and faulting makes this easier to do. -common at rift zones and mid ocean ridges
effusive eruption
Type of Eruption low viscosity magma -lava flows and volcanic gases -bubbles can escape easily -gas is constantly coming out of the volcano (bubbles constantly escaping/not getting trapped) -lava flow flows across the surface as a sheet of fluid lava
basaltic lava flow
Type of Lava Flow -often thin and fluid which facilitates long flow distances -forms long rivers of lava -flow downhill, follow low elevation/easiest path -"sheets" -water fountain: little sputtering of lava -ex: pouring maple syrup -flows are very hot, low silica, and low viscosity
rhyolititic lava flow
Type of Lava Flow dome forms right above the vent, clogs its own vent -lava is only able to travel within the crater -surrounded by pyroclastic material in a ring -tephra core
andesitic lava flow
Type of Lava Flow more viscous, more sticky. -flowable but forms a blob -bulge of lava/bump of lava rather than sheet -can't travel huge distances -impact area right around volcano.
decompression
Type of Melting -most common type on earth solidus slopes at higher temperature, therefore, melting pressure is also higher..
volatile flux
Type of Melting adding water and other volatiles to rock, lowers the temperature of the rock -moves solidus over. -temps of the earth can
heat transfer
Type of Melting local change in the geothermic gradiant -extra heat added in the earth, dramatic increase in temperature -ex: body of magma cooling at the basis of the crust, cooling the rock around it
melting peridotite
Type of Melting results in mafic magma -gives you basaltic/mafic magma
shield volcanos
Type of Volcano -form where there is low viscosity magma -far traveling lava flows. -layer upon layer of basaltic lava -have a low, broad shape
supervolcano
a volcano that has erupted or has the potential to erupt a volume of pyroclastic material greater than 1,000km3 -not volcanos in the traditional sense, so large that you don't see a mountain created. -can see caldera: a large volcanic crater, typically one formed by a major eruption leading to the collapse of the mouth of the volcano. ex: Toba eruption
volcanic vent
actual place where stuff comes out
viscosity
another property of magma similar to magma composition -how resistant a fluid is to flow -affects how magma can move both below and above the surface -some flow easier than others (ex: water vs. honey) -honey has a higher viscosity because it is more resistant to flow -stickier magma the higher the silica content -mafic magma=runny -felsic magma =sticky, don't like to flow. lava flow low viscosity: hotter, less SiO2 (mafic), less trapped volatiles -mafic magma high viscosity: cooler, more SiO2 (felsic), more trapped volatiles -felsic magma; stuck in magma chamber, explosive eruptions. -higher viscosity magmas have a harder time making it to the surface; they stick around underneath volcanoes until so much gas builds up that they forcibly erupt.
fractional crystallization
as magma cools, crystals form -some minerals form sooner than others -the original melt is mafic -after mafic minerals settle out, the remaining magma becomes more felsic -remaining magma is depleated in magnesium and iron but enriched in silicon, sodium, and potassium -crystallizing out olivine and pyroxene crystals leaves being silica rich magma.
flow characteristics of volcanic rock
basalt=low viscosity rhyolite=high viscosity hotter they are the less viscous they'll be
crater
bowl-shaped depression atop a volcano -sometimes full of lava -right after eruption, most of it is evacuated resulting in empty space within the mountain and collapsing of the crater.
lava dome
bulge of lava at the vent of a volcano
granite
coarse grained (intrusive) felsic rock
oceanic-continental subduction zones
continental arcs -volatile flux melting of peridotite in the overriding crust -Lower PM-->intermediate composition melts -thicker crust to travel through-magma pools in mid-crust -Assimilation, fractional crystallization enrich melt in Silcia content -Felsic igneous rock
magma evolution
creating a diversity of compositions processes that affect composition of magma: -partial melting -fractional crystallization -assimilation -magma mixing -ALL PROCESSES MAKE A MAFIC MELT MORE SILICA RICH -processes are encouraged by plate tectonics -these processes give us a buoyant, granitic , continental crust.
causes of melting
decompression heat transfer volatile
exposing igneous features
deep seated magma chamber, plutons, dikes and sills all arive to the surface due to erosion
phenocrysts
early formed crystals -started forming in the magma chamber and then the crystallization process was interrupted and magma was brought to surface. -commonly found tucked inside vesicles
basalt
fine grained (extrusive) mafic igneous rock
columnar jointing
forms due to contraction of rapidly cooled igneous rock (lava flow or shallow intrusion) -cools at the surface somewhat slowly -typically found in mafic extrusives (Ex:basalt)
pyroclastic debris
fragments blown out of a volcano -clastic in this context: pieces of rock -pyro-fire -Rhyolitic volcanoes: explosive eruptions-->pyroclastic material -Basaltic volcanoes:effusive eruptions (little pyroclastic debris) -Fragments of lava that solidify when they hit the air-classified by grain size -Ash (<2mm) rapidly erodes, soft material -Lapilli (2-64mm) -Bombs (>64mm) maintains football shape, literally like a bomb, exploding while still molten and flying through the air.
sill
go along with the layering -typically horizontal -occur in the the upper level of the crust where cracks can actually form
metamorphism
happens deep in the earth form very slowly, over long periods of time -can be over millions of years very different from the other types of rocks -depends on Pressure and Temperature!eh -rocks trap/record their highest temperatures (peak conditions the rock experienced) -rocks must be moved out of their comfort zone Takes precursor rocks and turns it into something else -igneous and sedimentary into "new" or "different" rocks ex: limestone-->marble -mostly changed by its texture, mineral grains change their shape/orientation. Part of the rock cycle
diorite
higher percentage of dark mineral than granite -coarse grained
melting
how you create/generate magma
crystallization
igneous rocks form by crystallization of minerals from a magma 1. the magma cools 2. chemical bonds can form- crystals grow. -minerals sit in the magma and add more atoms in an orderly fashion -over time, all the magma is used up, atoms that were in that liquid states have migrated to a mineral structure -interlocking texture of minerals forms; used up all the liquid. crystal size is determined by cooling rate -rapid cool: ex:volcano; atoms don't have time to situate themselves into the perfect spot; form tiny crystals everywhere. (produces FINE crystals slow cooling: atoms have more time to find the best surface to rest upon. (produces COURSE crystals)
how melt moves through the crust
lower density than the rock around it, therefore, it wants to reach the surface. -has to travel through solid rock to get there. -breaks off chunks of the country rock and moves into the created space. -blocks of country rock incorporated into melt=xenolith -removal of blocks allows the melt to move
peridotite
made up of mostly olidine -green in color -rich in iron, magnesium, and calcium -low silica content -considered ultramafic ultramafic course grained rock -found in the mantle -doesn't form in the crust
igneous intrusions
magma cuts across/intrudes the pre existing rock -igneous rocks cool slowly underground -cooling at a higher temperature
composite cone
main Type of Volcano -typical steep sided mountains formed by volcanic material -higher silicate content magma -composed alternating layers of andesitic/rhyolitic lava and pyroclastic debris to make a higher and higher moutain ex: Mt. St. Helens.
intrusions classified by shapes
melt takes advantage of preexisting weakness in the rock to move towards the surface -take advantage of zones of weakness. (bedding, -lower crust=ductile behavior -upper crust= cracks, joints, faults;good places for magma to flow into; tabular/planar shaped intrusions. -dike vs. sill
intraoceanic subduction zone
olivine (Mg2SiO4)-->Serpentine (Mg2Si2O5(OH)4) -oceanic crust is hydrated -water interacts with the rock, converting the basaltic minerals to minerals that contain water -during subduction water is driven off: flux melting -volatile flux melting--> partial melting of peridotite -lower temperature means higher silica content -fractional crystallization increase silica content -intermediate igneous rocks
iceland plateau
one large plateau of basalt, constructed along mid-atlantic ridge and rift zone.
orogeny
one way to move rocks out of their comfort zone -term for Mountain Forming! -orogenic belt, another word for mountain belt. -most mountains formed by collision of two plates being forced together. -crust is thickened as it pushes vertically higher. -what we see at the surface is mountains but there could be more rock in the asthenosphere... -for every mountain belt there is an orogenic route: crust thrusted beneath the world, thickening the lithosphere and pushing the asthenosphere out of the way. -all sorts of processes work to raise the lithosphere/even it out and create a gentler profile -orogenic collapse: crust is too thick, flows at depth, cracks, near surface. -mountains are attacked by erosion processes which try to work in attempt to create a perfectly flat surface of the Earth. -This is how intrusive igneous rocks and metamorphic rocks rise back towards the surface: through aspects attacking the mountains.
solidus curve
part of the geothermal gradient -shows us the temp at which peridotite will melt at different pressures
rift
places where there is stretching of the lithosphere; one plate ripping apart -ex: stretching gum/dough=thinner crust -lithosphere flows in to fill in that gap; can in some cases be enough to trigger decompression melting
mantle plume
plume of hot asthenosphere, stops at the mechanical boundary at the base of the lithosphere -rooted in the lower part of the mantle/emerge from core mantle boundary -thermal uplift of the lithosphere -decompression melting of asthenosphere and heat transfer melting of the lithosphere -front of the plume bulges outward-plume head
exhumation
process of bringing the metamorphic rock back - the process by which a parcel of rock approaches Earth's surface.
black smokers
really hot water; black because it has really tiny grains of minerals that are forced out with the water -formed at the sea floor from lots of water and cracks being thermally expanded -sea water trickles down the cracks and heats up very quickly (high thermal gradient); it changes its acidity, becomes very acidic -Wants to rise once it gets very hot and therefore buoyant; dissolves metals -Returns to the surface where its cool again and releases the metals in a black smoke. -some of the metals that are dissolved and than precipitated are economically valuable (ex: copper and iron) -can mine ancient deposits that were formed in this way.
felsic rocks
really only found on continental crust -can only form as a result of plate tectonic processes -unique to earth as far as we know -comes from feldspar and silica (high silica content 66-76%)
intermediate rocks
really only found on continental crust -can only form as a result of plate tectonic processes -unique to earth as far as we know -middle (silica content roughly 52-66%)
mantle plume
region of really hot, buoyant mantle that moves from the lower mantle and migrates up to lower pressures because of its lower density -as it does that, really hot rock can melt because it has a higher temp than the rock around it -how you get really large volumes of magma
source rock composition
rock that partially melts to create magma -10-20% partial melting of peridotite (mantle)--> basalt. -main melting process that happens within the earth -partial melting of gabbro (mafic lower crust)--> diorite, granite. (intermediate to felsic magma)
exposed shallow intrusions
ship rock: dike and volcanic neck devils tower: volcanic neck -overtime, outside of volcano eroded away, leftover differential erosion feature.
geothermal gradient
shows how temp increases with depth -temperature increases with depth because rock is a poor conductor -melting temperature of rock increases with pressure and depth -geotherm and solidus curve. -under normal conditions: temp of the earth is always lower than the temp of melting (why there is a solid mantle)
dike
tabular intrusions that cut across layering -typically vertical
different compositions of melt
tell us about the source of the magma and what that magma went through -silica content (peridotite has very low) -grain size classification (fine vs. coarse) -least evolved to most evolved -different magma compositions have names -mafic: comes from magnesium and iron (have silica content 45-52%) creates dark colored rock like gabbro -felsic (or silica): comes from feldspar and silica (high silica content 66-76%) -intermediate: middle (silica content roughly 52-66%) -ultramafic: peridotite (silica content <45%) -diorite and granodiorite (know these two!)
protolith
term used to describe what the rock was before it was metamorphosed. -the rock that came before-the unmetamorphosed version of the rock. -When looking at metamorphic rock its interesting to think of the rock it was before. -helps figure out the path that rock has taken. ex: limestone is the protolith of marble ex: shale is the protolith of Gneiss
volatile flux melting
the melting associated with the addition of carbon dioxide(CO2) or water to rocks is called
cyrstalization
turning magma back into igneous rock
heat transfer melting
type of melting process -hot magma or rock rises rapidly and transfers heat to surrounding colder rock. -mantle plume deep in the mantle generates melting in the asthenosphere -melt migrates up and pools at the base of the rock
