Chapter 4: Magma and Igneous Rocks Flashcards
What is a volcano? Give an example
a vent where molten rock comes out of Earth
Ex: Kilauea Volcano, Hawaii
How are igneous rocks formed?
- Hot (~1,200oC) lava pools around the volcanic vent.
- Hot, syrupy lava runs downhill as a lava flow.
- The lava flow slows, loses heat, and crusts over.
- Finally, the flow stops and cools, forming an igneous rock.
Igneous rock is formed by ______
cooling from a melt
What is magma?
melted rock below ground
What is lava?
melted rock once it has reached the surface
At what temperatures do igneous rocks freeze?
- Igneous rock freezes at high temperatures (T).
- 1,100 °C–650 °C, depending on composition.
What are the two types of igneous rocks?
- Extrusive igneous rocks
- Intrusive igneous rocks
How are extrusive igneous rocks formed?
cool quickly upon exposure of lava (or pyroclastic debris) at the surface
How are intrusive igneous rocks formed?
cool out of sight, underground from a magma.
Which type of igneous rock is found more commonly?
Intrusive igneous rocks are typically found in much greater volume than extrusive igneous rocks.
Which type of igneous rock cools faster?
Intrusive rocks cool at a much slower rate than extrusive rocks
What are lava flows?
streams or mounds of cooled melt
What is pyroclastic debris? What is it composed of?
They are cooled fragments and composed of:
- Volcanic ash
- Volcanic rock (fragmented by eruption)
What is volcanic ash?
fine particles of volcanic glass
Why is the cooling rate of intrusive igneous rocks are slower than extrusive igneous rocks?
- Large volume magma chambers
- Smaller volume tabular bodies or columns
Where is magma formed?
- Magma is not everywhere below Earths crust.
- Magma only forms in special tectonic settings.
Which layer of the Earth does melting of the magma occur?
melting occurs in the crust and upper mantle.
What causes the melting of the magma?
melting is caused by
- pressure release
- volatile addition. (flux melting)
- heat transfer.
How does pressure affect melting? Explain.
Decrease in pressure (P)— “decompression”
- The base of the crust is hot enough to melt mantle rock. - But, due to high P, the rock doesn’t melt.
- Melting will occur if P is decreased.
- P drops when hot rock is carried to shallower depths.
True/False
High pressure melts the mantle rock
FALSE
Melting occurs is the pressure is decreased.
When does pressure decrease?
P drops when hot rock is carried to shallower depths.
How is the hot rock carried to shallower depths?
- Mantle plumes
- Beneath rifts
- Under mid-ocean ridges
How do volatiles affect melting? Explain.
- Volatiles lower the melting T of a hot rock.
- Subduction carries water into the mantle, melting rock.
What are some common volatiles?
Common volatiles include H2O and CO2
Where does hydration melting occurs?
Subduction zones
How does heat transfer affect melting? Explain.
- Rising magma carries mantle heat with it.
- This raises the T in nearby crustal rock, which then melts.
What is the difference between magma and lava?
A.Composition: the former contains more magnesium and iron than the latter.
B.Location: the former is molten rock underground whereas the latter is molten rock at the Earths surface. C.Temperature: the former is hotter than the latter.
D.Grain size: the former has bigger crystals than the latter.
B
In the upper crust of a continent, the geothermal gradient is about . . .
A.350° - 400°C per km
B.150° - 200°C per km.
C.10° - 20°C per km.
D.1° - 2°C per km
C
Which of the following phenomena will cause melting to produce magma?
A.removal of volatiles (H2O and CO2) from the rock B.increasing pressure, while the temperature stays constant
C.Bogus question: the crust floats on a layer of perpetually molten rock, so there is no need to melt rock to “produce” magma
D.transfer of heat from a hot magma into surrounding wall rock
D
Where does igneous activity occur?
Igneous activity occurs in four plate-tectonic settings.
- Volcanic arcs bordering deep ocean trenches
- Isolated hot spots
- Continental rifts
- Mid-ocean ridges
Established or newly formed tectonic plate boundaries
Where does igneous activity NOT occur?
hot spots, which are independent of plates
Where are subaerial volcanoes located?
Most subaerial volcanoes on Earth reside in arcs.
Volcanic arcs mark _____
convergent tectonic plate boundaries
How are volcanoes formed on volcanic arcs?
- Deep oceanic trenches and accretionary prisms
- Subducting oceanic lithosphere adds volatiles (water).
- Rocks of the asthenosphere partially melt.
- Magma rises and creates volcanoes on overriding plate.
- Magma may differentiate.
Give some examples to volcanic arcs
- Aleutian Islands
- Japan
- Java and Sumatra
How many hot stops are there in the world?
50-100
Hot spots are independent _____
tectonic plate boundaries
Hot spots may erupt through _______
oceanic or continental crust.
What type of magma do hot spot eruption release from oceanic crust?
mostly mafic magma (basalt)
What type of magma do hot spot eruption release from continental crust?
mafic and felsic (basalt and rhyolite)
What creates a hot-spot track?
burn a volcano chain through overiding tectonic plate
What are continental rifts?
places where continental lithosphere is being stretched
What are the consequences of continental rifts?
- Rifting thins the lithosphere.
- Causes decompressional melting of mafic rock.
- Heat transfer melts crust, creating felsic magmas.
Give an example to continental rift
East African Rift Valley
Most igneous activity takes place at _____. Explain how?
mid-ocean ridges
- Rifting spreads plates leading to decompression melting. - Basaltic magma wells up and fills magma chambers.
- Solidifies as gabbro at depth.
- Moves upward to form dikes or extrude as pillow basalt.
Where does the heat inside (internal heat) the Earth come from?
- Remnant/primordial heat: –> Leftover heat, mainly coming from the core from Earth’s formation
- Ongoing heat generation: –> Decay of radioactive elements in the crust and mantle
Earth slowly loses heat to space.
Eventually, Earth will solidify.
What causes the ongoing heat generation?
Ongoing radiogenic heat production through radioactive isotope decay in the crust and mantle.
What is magma viscosity and flow behaviour like?
Lava flows sometimes race down the side of a volcano, sometimes it builds up into a rubble-covered mound, and sometimes it oozes like toothpaste.
The viscosity (resistance to flow) of a lava depends upon:
- Chemical composition
- Temperature
- Gas content
- Crystal content
Give 3 examples to different lava types and explain what they form
- Basaltic lava forms thin sheets.
- Andesitic lava tends to break up as it flows.
- Ryolitic lava piles up at the vent as a dome.
The surface texture of the basaltic flow, when it freezes, reflects _________
the timing of freezing relative to how fast it was moving.
What are the two types of volcanoes and what are their characteristic features?
Shield volcano
- lava fountain
- lava flow
Stratovolcano
- lava dome
- ash fall
- ash flow
How does shield volcano eruption occur?
low-viscosity lava that erupts from a circular vent or along a crack/fissure
What is the largest volcano on Earth?
Mauna Loa, Hawaii: A Shield Volcano
How does fissure shield volcano eruption occur?
Lava may erupt along a linear tear, (a fissure). Fissure eruptions may display a “curtain of fire.”
What is the shape of Stratovolcano?
Stratovolcanoes are large, cone-shaped volcanoes with steep slopes made of alternating layers of felsic lava, tephra, and debris. Often symmetric, they can acquire odd shapes following landslides, etc.
How does Stratovolcano eruption occur?
Typically form from medium to high viscosity lava (andesitic), often leading to explosive volcanism
What are the fragments that are released by a volcanic eruption called? Give examples to these possible fragments
Volcanoes erupt large quantities of fragments—volcaniclastic deposits.
This material includes pyroclastic debris, pre-existing rock, landslide debris, and lahars (mudflow with volcanic ash).
Give an example to a classic Stratovolcano on Earth
Mt Fuji, Japan
What are the two types of silicate structures? Give examples to each
- Neosilicates (Sheet silicate)–> Olivine and mica
- Inosilicates (Pyroxine, Framework silicate)–> Amphibole, Quartz, Feldspar
How do silicate structures affect the lava viscosity?
silica tends to link into long chains and these increase the lava viscosity
