Exam 2 Study Guide Flashcards
What is the difference between magma and lava?
Magma: the molten/partially molten material found beneath the surface of the Earth
Lava: magma once it reaches the Earth’s surface.
Where does magma form in terms of plate tectonics?
Divergent boundaries at the mid-ocean ridge and continental rifts and Convergent boundaries in subduction zones
What are the 3 ways to generate magma?
- Decompression melting
- Addition of volatiles
- Addition of heat
What is decompression melting?
Melting occurs when the body of rock is held at the same temperature, but the pressure is reduced.
Where do we see decompression melting in terms of plate tectonics?
At a mantle plume, mid-ocean ridge, or continental rift
What is the addition of volatiles?
Water lowers the melting temperature of the mantle to generate partial melts.
Where do we see the addition of volatiles in terms of plate boundaries?
Subduction zones
What is the addition of heat?
When some other mechanism generates magma to intrude into the cold crust, it brings the heat with it.
Where do we see the addition of heat in terms of plate boundaries?
Hot spots and continental rifts
What are the 3 ways magma evolves?
- Fractionation
- Assimilation
- Magma mixing
Compare intrusive and extrusive igneous rocks.
Intrusive: formed from magma that cools slowly and the result is that these rocks are coarse-grained,
Extrusive: cool quickly and as a result, these rocks are fine-grained or lack crystal growth.
What are the 5 intrusive environments?
- Dike
- Sill
- Plutons
- Laccolith
- Batholiths
What are the 3 different compositions of magmas (igneous rocks)?
- Felsic
- Intermediate
- Mafic
What are the 6 textures of igneous rocks?
- aphanitic (fine-grained)
- Phaneritic (coarse-grained)
- Porphyritic
- Glassy
- Pyroclastic
- Pegmatitic
What is aphanitic texture?
- fine-grained “invisible”
- Microscopic crystals
- Rapid cooling of lava/magma
- May contain vesicles (holes from gas bubbles)
What is phaneritic texture?
- (coarse-grained) texture
- crystals large enough to see with the naked eye
- slow cooling of magma
- intrusive origin
What is porphyritic texture?
- 2 different sizes of crystals
- Minerals form at different temperatures and rates
- Can indicate two stages of cooling
What is glassy texture?
- Very rapid cooling of molten rock
- Extrusive origin
What is pyroclastic texture?
- extrusive origin
- Various fragments ejected during a violent volcanic eruption
What is pegmatitic texture?
- exceptionally coarse-grained texture
- found in intrusive igneous rocks
- forms in the late stages of crystallization of granitic magmas
Igneous rocks form when:
a melt solidifies at or below the Earth’s surface
The molten rock beneath the Earth’s surface is called ____.
magma
The greatest amount of magma is created at _____________.
divergent plate boundaries
How is magma generated at subduction zones?
Addition of volatiles
Which type of magma contains the lowest amount of silica?
Mafic
_____ is a mafic extrusive igneous rock.
basalt
A _______ is a concordant igneous rock body which runs parallel to the pre-existing bedrock.
Sill
Magma rises toward the Earth’s surface because:
it is less dense than the surrounding rocks
Why are intrusive igneous rocks from a batholith generally coarser-grained than those from volcanic vents?
They are cooled more slowly and thus have more time to grow.
A _____ texture indicates two stages of cooling
porphyritic
Compare explosive and effusive eruptions.
Explosive: Felsic and intermediate lavas thtat can generate deadly pyroclastic flows
Effusive: Mafic lavas with gentle flowing lava due to low viscosity
Compare Fissure and vent eruptions
Fissure: lava emerges from a fracture or cracks in the ground rather than from a central vent
- Common:
+ Where the crust is undergoing extension; continental rifts and mid-ocean ridges
+ Where magma has low viscosity (relatively fluid)
- They are the dominant style of eruption
Vent: magma is extruded at a central vent
How are mafic lava eruptions characterized?
Effusive/less violent
What are the effects of mafic lava eruptions? (lavas and volcanoes)
- Pillow lavas
- Ropy lava (pahoehoe)
- Blocky lava (‘a,’a)
- Shield volcanoes
- Cinder cones
How are felsic and intermediate lavas characterized?
Explosive/more violent
What are the effects of intermediate and felsic lavas?
- Tephra; classified by size
- Stratovolcanoes
- Lava domes
- Continental calderas
What are the major volcanic hazards? (2)
- pyroclastic flow
- lahars
A lava flow whose surface is broken into rough angular fragments is:
a’a
Tephra comes in three sizes; _____ is the smallest
ash
_______ are avalanches of hot ash, rock, volcanic glass fragments, and gas that race down the sides of the volcano during an eruption.
pyroclastic flows
The most abundant volcanic gas is _____.
water vapor
A _____ is a Volcanic mudflow/debris flow.
lahar
What is the most common risk to humans from lava flows?
Property damage.
Which of the following is NOT a common cause of pyroclastic flows?
a. boiling over from an eruptive vent
b. collapse of eruption column
c. collapse of lava domes or flows
d. megathrust earthquakes
d. megathrust earthquakes
T/F: Lahars are only triggered by volcanic eruptions.
False
____ eruptions are typically associated with shield volcanoes.
Effusive
A ____ is a solid rock fragment greater than 64mm in diameter that was ejected from a volcano during an explosive eruption.
block/bomb
What are the classifications of tephra
- Ash <2 mm
- Lapilli 2-64 mm
- Bomb >64 mm
Compare physical (mechanical) and chemical weathering.
Physical:
- mechanical breakdown of rock into smaller fragments
- provides a fresh surface for a chemical attack
- creates sedimentary clasts (sediments) that can become clastic sedimentary rock
Chemical:
- chemical change of the minerals within the rock to forms that are stable in the surface environment
- weakens the rock so it is more susceptible to mechanical weathering
- creates ions in solution that can eventually become sedimentary rock.
What are the 7 types of mechanical weathering?
- Impacts/ Abrasion
- Freeze-Thaw Action (Frost/Ice Wedging)
- Temperature Changes
- Salt Crystal Growth
- Pressure Relief
- Wetting and Drying
- Biological Activity
What causes chemical weathering?
- Lower temperature
- Lower pressure
- Higher free water
- Higher free oxygen
What minerals are more susceptible to chemical weathering?
- Halite
- Calcite