Geology Chapters 4, 5 part of 6 and 7
Igneous rocks and intrusive igneous activity; volcanoes and volcanism
Terms in this set (141)
What is magma composed of?
It is composed of silicon and oxygen with lesser amounts of several other chemical elements.
What are the most important controls on the mobility of magma and lava?
Temperature and composition.
Where does most magma originate in correspondence to Earth?
It originates within the Earth's upper mantle or lower crust at or near divergent and convergent plate boundaries.
Why is the processes in magma that brings about chemical change important?
So magma may evolve from one kind into another.
How do all igneous rocks form?
They form when magma or lava cools and crystalizes, or by the consolidation of pyroclastic materials ejected during explosive eruptions.
What do geologists use to classify igneous rocks?
They use texture and composition.
When do Intrusive igneous bodies called plutons form?
They form when magma cools below the Earth's surface.
What do we know about the origin of the largest plutons?
They are not fully understood yet.
Does the majority of magma reach Earth's surface?
NO. The majority of magma cools and crystalizes underground forming different types of igneous bodies called plutons.
What parts of the continent do igneous rocks make up?
They make up large parts of the continent and all of the oceanic crust, which forms continuously at divergent plate boundries.
What is the definition of magma?
It is any mass of molten rock material below Earth's surface.
What is the definition of Lava?
It is magma that reaches the Earth's surface.
What are lava flows?
It is the magma that does not reach the surface and issues forth as streams of lava.
What are pyroclastic materials?
It is lava flows that is forcefully ejected into the atmosphere as particles. Pyroclastic materials the measure less than 2mm are called volcanic ash.
All igneous rocks derive from?
What are the two separate processes that account for how igneous rocks form?
1) magma or lava cools and crystalizes to form aggregates of minerals.
2) pyroclastic materials both of which are extruded onto the surface are also known as volcanic rocks or extrusive rocks.
When are Plutonic and Intrusive igneous rocks formed?
They are formed when magma cools below the surface, that is, from magma intruded into the crust.
What is another source of magma?
Earth's upper mantle, which is composed of rocks that contain mostly ferromagnesian silicates.
What is the primary constituent of magma?
What does Felsic magma contain?
It is silica rich and contains considerable sodium, potassium, and aluminum, but little calcium, iron and magnesium.(Rhyolite-Granite are the igneous rocks that crystalize and form from Felsic magma.)
What does Mafic Magma contain?
It is silica poor and contains proportionately more calcium, iron, and magnesium. (Bassalt-Gabbro are the aphanatic and phaneritic rocks that crystalize from mafic magma. Bassalt is aphanatic. Gabbro is Phanaritic.)
What does Intermediate magma contain?
It has a composition between felsic and mafic magma.
(Andesite_Diorite are the igneous rocks that crystalize from intermediate magma.)
What is the reason lava and magma retain heat so well?
It is because rocks conduct heat so poorly.
How long do lava flows and pyroclastic flow deposits remain hot for?
They remain hot for months or years.
When do plutons cool?
Depending on their size and depth, they may not cool for thousands or millions of years.
What is viscosity?
It is the resistance to flow.
Does cool magma or lava move more readily than hotter magma or lava?
No it does not. It is the other way around.
What strongly controls magma and lava's viscosity?
Silica content controls magma and lava's viscosity.
What happens when atoms in a solid which are in constant motion are heated?
The energy of motion exceeds the binding forces and the solid melts.
What are magma chambers?
They are the reservoirs where magma accumulates.
Where do magma chambers exist?
They exist below spreading ridges where the crust in thin. They are at a depth of about a few kilometers.
What is the volume of a magma chamber?
It ranges from a few to many hundreds of cubic kilometers of molten rock within the otherwise solid lithosphere.
Do minerals crystalize simultaneously from cooling magma?
Not all of them crystalize simultaneously from cooling magma. They crystalize in a predictable sequence.
Bowen's Reactions Series is supposed to account for what?
It accounts for the derivation of intermediate and felsic magmas from mafic magma.
What are the two branches that Bowen's reaction series consists of?
1) a discontinuous branch
2) a continuous branch
What is the discontinuous branch of Bowen's Reaction Series?
This branch contains only ferromagnesian silicates, where one mineral changes to another over specific temperature ranges. A temperature range has to be reached before a mineral will begin to crystalize.
What happens as the discontinuous branch of Bowen's Reaction Series continues to cool?
As it continues to cool, reactions take where the minerals continuously change into another form moving it fro one type of magma to another.
What happens if magma cools to rapidly?
The early formed minerals do not have time to react with the melt therefore the ferromagnesian silicates in the discontinuous branch can be in one rock.
What are the only minerals in the Continuous Branch of Bowen's Reaction Series?
Plagioclase feldspars which are nonferromagnesian silicates are the only minerals.
In the continuous Branch of Bowen's Reaction Series, what crystalizes first?
Calcium-rich Plagioclase crystalizes first.
What happens to the continuous branch of Bowen's Reaction Series if it continues to cool?
The magma will continue to cool and the calcium-rich plagioclase will react with the melt, and plagioclase containing porportionatly more sodium crystalizes until all of the calcium and sodium are used up.
Is it common for the magma to cool to fast in the continuous branch of Bowen's Reaction Series?
Yes. Often times the cooling is too rapid for a complete transformation from calcium-rich to sodium-rich plagoiclase to take place.
The plagoiclase forming under the rapid cooling conditions is zoned. What does that mean?
It means that is has a calcium rich core surrounded by zones progresivley richer in sodium.
What happens to iron and magnesium if minerals crystalize simultaneously along the two branches of Bowen's Reaction Series?
Iron and Magnesium are depleted because they are used up in ferromagnesian silicates where calcium and sodium are used up in plagioclase feldspars.
What is the geothermal gradient?
It is when the Earth's temperature increases with depth which relates to the origin of magma.
What happens to the melting temperatures of rocks at depth in Earth?
Rocks at depth are hot but remain solid because their melting temperature rises with increasing pressure.
Pressure below spreading ridges decreases causing what?
It causes the temperature to exceed the melting temperature, because pressure decreases.
What causes a decrease in pressure on the already hot rocks at dept, thus initiating melting?
Plate separation at ridges.
What decreases the melting temperature beneath spreading ridges?
A belt of volcanoes and plutons is a result of what plate movement?
It is a result of an oceanic plate being subducted beneath either a continental plate or another oceanic plate.
What is used to explain the origin of magma at subduction zones?
The phenomenon of partial melting is used to explain the origin of magma at subduction zones.
What are the common and uncommon places volcanism occurs?
The common places volcanism occurs are at divergent and convergent plate boundaries. The uncommon places volcanism occurs at are ocean basins and on continents that are not near either of these boundaries.
What is a hot spot?
It is a localized zone of melting below the lithosphere that probably overlies a mantle plume.
What is a mantle plume?
It is a cylindrical plume of hot mantle rock that rises from perhaps near the core-mantle boundary.
What is crystal settling?
It involves the physical separation of minerals by crystilization and gravitational settling. Once magma forms, it's composition may change by crystal settling.
What is a sill?
It is a tabular or sheetlike concordant pluton.
What is assimilation?
It is a process by which magma reacts with preexisting rock called country rock, with which it come in contact.
What is country rock?
Any preexisting rock that has been intruded by a pluton or altered by metamorphism.
The fact that assimlation occurs is indicated by what?
It is indicated by inclusions, incompletely melted pieces of rock that are fairly common in igneous rocks.
What is magma mixing?
it is the process by whereby magmas of different composition mix together to yield a modified version of the parent magmas.
What is an aphanatic texture in minerals?
It is where individual minerals are too small to be seen without magnification. They usually indicate an extrusive origin.
What is a phaneritic texture in minerals?
It is where minerals are clearly visible. They usually indicate an intrusive origin.
What is a porphorytic texture in minerals?
It is where minerals of markedly different size are present in the same rock.
What happens if lava cools to fast?
It's constituent atoms do not have time to become arranged in the ordered, three-doimensional frameworks of minerals.
What are vesicles?
They area small holes or cavities formed by gas trapped in cooling lava.
What kind of rocks does a pyroclastic or fragmental texture characterize?
They characterize igneous rocks formed by explosive volcanic activity.
What plays an important role in determining the mineral composition of igneous rocks?
The parent magma.
What do the members of a pair of rocks have in common and what don't they have in common?
They have the same composition but different textures.
What can the extrusive and intrusive members of each pair be distinguished by?
They can be distinguished by texture.
What are ultramafic rocks composed of?
What is a pegmatite igneous rock?
It refers to a particular texture rather than a specific composition, but most pegmatites are composed mostly of quartz, potassium felspar, and sodium rich plagioclase, thus corresponding closely to granite.
Some pegmatites are mafic or intermediate in composition meaning they are what kind of igneous rocks?
It means they are baggro and diorite pegmatites.
What is rhyolite Tuff?
It is the consolidation of ash, which forms pyroclastic tuff which is silica rich and light colored.
What is welded tuff?
It is ash paricles that are fused together.
What are both obsidian and pumice varities of?
They are volcanic glass.
What are plutons?
They are intrusive igneous bodies.
What are the three types of plutons?
3) irregular (massive)
What does a concordant pluton mean?
It means that plutons can have boundries that parallel the layering in the country rock.
What are discordant plutons?
It is the plutons with boundaries that cut across the country rock's layering.
How do dikes and sills differ?
Dikes are discordant and Sills are concordant. They are both tabular or sheetlike igneous bodies.
What are dikes intruded into?
They are intruded into preexisting fractures pr where fluid pressure is great enough for them to form their own fractures as they move upward.
What are sills intruded into?
They are intruded into sedimentary rocks, and piles of volcanic rock. They are intruded between layers in country rock when the fluid pressure is great enough for the magma to actually lift the overlying rocks.
What can some volcano eruptions be caused from?
They can be caused from the injection of sills.
What is a laccolith?
It is the overlying rocks bowing upward forming an igneous body.
What can cause laccoliths?
A sill inflating can cause laccoliths.
What does a laccolith look like?
It has a flat floor and is domed up in its central part giving it a mushroom-like geometry.
What is a volcanic pipe?
It is the conduit connecting the crater of a volcano with an underlying magma chamber.
What is a volcanic neck?
An erosional remnant of the material that solidified in a volcanic pipe.
What is a batholith?
It is an irregularly shaped discordant pluton with at least 100 km^2 of surface area.
What is stopping?
It is a process where rising magma detaches and engulfs pieces of country rock.
Magmas form in a number of geologic environments including?
Magmas can form in hot spots, divergent plate boundaries and subduction plates.
What are the three main parts of magmas?
1) high temperature mix of crystalizing solids (minerals),
it can be related to cambels chicken noodle soup. The meat or noodles can be the crystalizing minreals, the broth can be the liquid and the steam can be the volatile.
Igneous rocks look different depending on what?
They look different depending on where they are during the crystalizing stage.
Which one is formed in the highest temperature, igneous metamorphic or sedimentary rocks?
What are the two ways magmas can crystalize?
1) Rapid Cooling 1100-1200 degrees celcius (aphanatic fine grained texture)
2) Slow Cooling (Phaneritic coarse grained texture)
What determines the rate of cooling and what the rocks will look like?
Where magma crystalizes determines their rate of cooling and the rate of cooling determines what the igneous rock will look like.
Who published Bowen's findings?
Bowen had died at a young age and his coworkers published his findings crediting him.
Does a mineral have to reach all three types of magma in order to crystalize?
No it could be at any type during crystalization and suddenly be ejected onto the surface as lava.
When magmas spread out parallel on a plain of weakness what is that called?
What is the definition of volcanism?
It is where magma and associated products ascend through the crust and extrude onto the Earth's surface. (the magma ascends through fracturing that occurs.)
What can erupt from volcanoes that can fall to Earth forming pyroclastic igneous rocks?
Ash and ejecta.
What kind of eruptions are the most and least dangerous?
Lava flows are the least dangerous and explosive eruptions are very dangerous.
What is the best known volcanic catastrophy?
In 79 A.D. Mount Vesuvius erupted destroying the thriving Roman communities. Pompei was buried in 3m of pyroclastic materials. Herculaneum was buried in 20m of pyroclastic materials. It has erupted 80 times since.
What does it mean if a volcano is active?
It means that it has erupted during historic time.
(there are about 550 active volcanos.)
What does it mean if a volcano is dormant?
It means that it has not erupted during historic time but could in the future.
What are extinct or inactive volcano's?
Volcano's that show no sign of ever erupting.
What happens to the pressure when magma rises toward the surface for a volcanic eruption?
The pressure is reduced and the contained gases begin to expand. This could expand big enough to cause an explosion (eruption).
Are volcanic gases dangerous?
Yes. Most of them dissipate into the atmosphere really fast however on some occasions they have been fatal.
What is the term for volcanic smog that residents of Hawaii came up with?
It is vog.
Why do lava flows rarely cause fatalities?
They rarely cause fatalities because they move so slow and because they are fluid, they follow existing low areas. Once a volcano erupts, determining its path it easy leaving plenty of time for evacuation.
What is a lava tube?
it is a tube beneath the solidified surface of a lava flow through which lava moves also the hollow space left when the lava within a tube drains away.
When does a conduit known as a lava tube within a lava flow form?
It forms when the margins and upper surface of the flow solidify.
What is a skylight?
When part of the roof of a lava tube collapses and people can look in and observe or access can be gained t an inactive lava tube.
What are the two types of lava flows?
1) Pahoehoe (pah-hoy-hoy): ropy surface like taffy
2) aa (ah-ah): rough, jagged angular fragments
What is a pressure ridge?
It is where pressure on the solidified crust of a still-moving lava flow causes the surface to buckle.
What are columnar joints?
It is where mafic lava flows have a distinctive pattern of columns bounded by fractures.
What is pillow lava?
Bulbous masses of basalt resembling pillows formed when lava is rapidly chilled under water.
How are interconected pillows formed?
They are formed when the remaining fluid from inside one pillow lava breaks through the crust of the pillow where it repeats the process resulting in interconnected pillows.
What is an ash flow?
it is when a cloud of ash and gas flows along or close to the land surface. They can move fast and cover vast areas.
What are other things volcano's can errupt?
They can errupt lapilli-pyroclastic materials that measure from 2-64mm, blocks-angular pieces of rock ripped from volcanic conduit, and bombs- which have a twisted streamlined shape which shows they were ejected as globs of magma.
What is a volcano?
It is a hill or mountain that forms around a vent where lava, pyroclastic materials, and gases erupt.
What do all volcano's have beneath the surface?
They all have conduits leading to a magma chamber beneath the surface.
What is a crater?
It is a circular depression at the summit, which most volcanoes have.
What are calderas?
They are larger rimmed depressions on volcano's. They form following voluminous eruptions where a magma chamber drains and the mountains summit collapses into the vacated space. ex(crater lake)
What is a shield volcano?
It is a dome-shaped volcano with a low, rounded profile built up mostly by overlapping basalt lava flows.
Does the lava in shield volcano's pose threats to humans?
No the lava usually rises to the surface with little explosive activity posing little danger to humans. They are usually quiet eruptions.
What is the longest recorded volcanic eruption in shield volcanoes?
It is the Kileuea volcano in Hawaii which has been erupting since January 3, 1983.
What kind of volcanoes are the most common in ocean basins?
What is a cinder cone?
It is a small, steep sided volcano made up of pyroclastic materials resembling cinders that accumulate around a vent.
Where is a common place for cinder cones to form?
Many cinder cones form on the flanks or within the calderas of larger volcanoes and represent the final stages of activity, particularly in areas of basaltic volcanism.
What is a composite Volcano?
It is a volcano composed of lava flows and pyroclastic layers, typically of intermediate composition and mudflows.
What rock do Composite Volcanoes form after cooling?
What does lahar mean?
A mudflow composed of pyroclastic materials such as ash.
How do lahar's form?
They form when rain falls on unconsolidated pyroclastic materials creating a muddy slurry that moves downslope.
How do Composite Volcanoes differ from Shield volcanoes and cinder cones?
They differ in shape and composition.
Where was the worlds largest eruption since 1912?
Mount Pinatubo in the Pillippines erupted on June 15 in 1991 where huge quantities of gas and ash were discharged. 722 deaths.
What are lava domes?
They are bulbous steep-sided mountains formed by viscous magma moving upward through a volcanic conduit.
Which eruptions are some of the most violent and destructive?
Lava dome eruptions.
What are basalt plateaus?
They are plateau built up by horizontal or nearly horizontal overlapping lava flows that erupted from fissures.
What are fissure eruptions?
They are a volcanic eruption in which lava or pyroclastic materials issue from a long, narrow fissure (crack) or group of fissures.
What are pyroclastic sheet deposits?
They are vast, sheetlike deposits of felsic pyroclastic materials erupted from fissures.