Earth Sci Midterm 2

At what temperature does low-grade metamorphism occur?

150 degrees Celsius

At what temperature does high-grade metamorphism occur?

800 degrees Celsius

At what range of temperatures does melting and remelting occur in igneous systems?

700-1400 degrees celsius

What are igneous rocks?

Rocks that “freeze” from molten magma or lava

What is magma?

Molten rock beneath the surface 

What is lava?

Molten rock that reaches the surface 

What are intrusive igneous rocks(plutons)?

rocks formed from the solidification of magma below the Earth’s surface 

has large crystals since there’s more time for them to develop

What are extrusive igneous rocks?

rocks formed from the solidification of magma at the Earth’s surface, as either cooled lava or pyroclastic debris

has small crystals since there’s less time for them to form and expand

What are the three components of Magma?

Solid, liquid, and gas

What are solids?

Solidified mineral crystals are carried by the melt, dominated by silica and oxygen

What is liquid?

The melt itself has mobile ions

What is gas?

Magmas contain abundant dissolved volatile gas 

Dry Magma: scarce volatiles(dissolved gas)

Wet Magma: up to 15% volatiles 

What does ultramafic mean?

Silica content is less than or equal to 45%, makes up mantle composition

What does mafic mean?

Silica content is between 45-55%, named for having lots of magnesium and iron 

What does intermediate mean?

Silica content is between 55-65%, rock consists of dark and light colored minerals

What does felsic mean?

Silica content is greater than or equal to 65%, primarily consists of light colored minerals

Magma chamber

A large volume of magma that has risen and accumulated within the crust

Country rock

Wall rock, any crustal rock that was older than and intruded by an igneous body

Intrusive contact

Boundary between the igneous intrusion and the surrounding country rock 

Xenolith

Fragment of rock distinct from the igneous rock in which it is enclosed

What are the heat sources for magma formation?

The Earth’s core and the decay of radioactive elements

What is the geothermal gradient?

The rate at which temperature increases with increasing depth 

What is decompression melting(decrease pressure)?

When a body of hot mantle rock moves upward, and the pressure is reduced, lowering the temperature at which rocks melt

The melting temperature of a mineral increases as pressure increases

What are volatiles?

Any substance(such as H2O, CO2) that evaporates relatively easily

What is flux melting?

When volatiles are added, they significantly decrease a rock’s melting temperature

What is heat transferring melting?

When magma from the mantle rises into the crust, the heat it carries raises the temperature of the surrounding crustal rock and makes it melt

What is melt composition based on?

Composition of the source rock, partial melting, assimilation, magma mixing, and fractional crystallization

What is partial melting?

The process in which only part of the original rock melts to form magma

Minerals w/lowest melt temperature melt first, while those w/higher melt temperatures remain solid

Silica minerals melt first  

What is assimilation?

The process of magma contamination wherein blocks of wall rock fall into a magma chamber and dissolve

What is magma mixing?

When different magmas blend in a magma chamber, and occasionally they don’t mix well

Result: a combination of the characteristics of the two magmas 

What is fractional crystallization?

As magma cools, the early crystals settle by gravity

Result: change in the melt composition, which is why older melts are more felsic 

What is viscosity?

The resistance to flow, which also affects the speed at which molten rock flows

Dependant on temperature, volatile content, silica content, crystal content, a hotter melt is less viscous than a cooler melt

What happens when a melt is cooled?

Igneous rocks are created

What are the factors that control the cooling of a melt?

Depth of the intrusion, morphology(shape and size) of the magma body, presence of circulating water(presence of groundwater can accelerate cooling)

What is a dike?

A tabular, discordant intrusive structure, body is NOT parallel to any layering in the country rock

What is a sill?

A tabular, concordant intrusive structure, body IS parallel to any layering in the country rock

What is a laccolith?

A concordant intrusive structure, like a sill, but the central portion is thicker and domed upward

What are pluton?

Blob-shaped bodies of magma that crystallized at a considerable depth within the crust

What is batholith?

Large discordant plutons formed by multiple coalesced plutons

What is stock?

Small discordant plutons

What is a diapir?

A buoyant, light-bulb-shaped ‘blob’ of geologic material that rises upward through the crust, deforming the overlying rock 

What are flood basalts?

Voluminous quantities of mafic magma that have erupted or intruded 

What does having a aphanitic texture mean?

rock is fine-grained, grains are less <1 mm, and it’s extrusive(volcanic)

What does having a phaneritic texture mean?

rock is coarse-grained, grains are greater than 1mm, and it’s intrusive 

What does having a pegmatite structure mean?

rock is extremely coarse-grained, grains are greater than 1cm also 1mm, and is intrusive meaning that magma is is enriched in volatiles)

What does having a porphyritic texture mean?

rock has two rates of cooling, large crystals(phenocrysts) form underground before being erupted

What are phenocrysts?

coarse-grained crystals enclosed in a groundmass of finer-grained crystals

What does having a pyroclastic texture mean?

trapped gasses released during a volcanic eruption lead to lava being blasted out of the vent as pyroclasts 

What is tephra?

the accumulation and cementation of pyroclastic debris

What is tuff?

rocks that come from a lot of tephra

What are vesicles?

cavities in extrusive rock created by gas bubbles that were in the lava when it solidified

What is orogenesis?

mountain building

What are the characteristics of young mountains?

high, steep, and grow upward

What is the main characteristic of middle-aged mountains?

they’re dissected by erosion

What are the characteristics of old mountains?

they’re deeply eroded and often buried 

What is elastic strain?

reversible change in shape

What is plastic strain?

irreversible change in shape

At depth above 10-15 km rocks behave _____

brittlely

At depths below 10-15 km rocks behave ____

ductile

What is confining stress?

when objects experience the same stress on all sides, but little to no deformation

What is compression?

object is squeezing((greater stress in 1 direction), which tends to thicken material

What is extension(tension)?

object is pulled apart(greater stress in 1 direction), which tends to thin material vertically 

What is shear stress?

when blocks of rock slide past one another, crust doesn’t thicken or thin 

What are joints?

planar rock fractures with no offset

What are folds?

layers that are bent by slow plastic flow and can be deformed ductilely into curves

What is foliation?

planar metamorphic fabric

What is displacement?

the change in location

What is rotation?

the change in orientation

What is distortion?

the change in shape

What are faults?

fractures offset by movement across the break

What are scarps?

surface exposed along a falling or rising fault block

What does dip-slip mean?

up and down movement, blocks move parallel to fault plane dip 

What does strike-slip mean?

side-to-side movement, blocks move parallel to fault plane strike 

What does oblique-slip mean?

a combination of dip-slip and strike-slip

What is a hinge?

the portion of maximum curvature of a fold

What is a limb?

the less curved “sides” of the fold 

What is the axial plane?

the imaginary surface defined by connecting hinges of successfully nested folds

What are the causes of mountain building?

substantial uplift and plate tectonics 

How do plate tectonics caused mountain building?

• Convergent plate boundaries create mountains

  • Subduction-related volcanic arcs grow on overriding plate 

  • Accretionary prisms (off-scraped sediment) grow upward

  • Compression stacks thrust faults on the far side of the mountain belt

Where do volcanoes occur?

covergent plate boundaries and hot spots

What is a volcano?

a landform constructed by the extrusion of lava or the ejection of rock fragments from a vent

What is a conduit?

a passage/pathway from a magma chamber to the Earth’s surface through which magma and erupt

What is a vent?

an opening through which a volcanic eruption takes place

What is a crater?

a basin-like indent where stuff is erupted up around it

What is an eruption?

an episode when a volcano extrudes lava or pyroclastic debris from its vent

What is lava flow viscosity dependent on?

• Composition

• Temperature 

• Gas content 

• Crystal content

What is the degree of violence associated with w/volcanic activity determined by?

• Amount of gas in lava

• how easily the gas in the lava can escape into the atmosphere 

• Lava viscosity 

What is effusive eruption?

an eruption that is dominated by low viscosity lava 

What is explosive eruption?

driven by the expansion of gases in a viscous magma and dominated by the generation of solid volcanic fragments

What is pahoehoe lava?

mafic(basaltic) lava characterized by a ‘ropy’ or ‘billowy surface’

What is ‘a’a lava?

mafic lava characterized by a ‘jagged’ and ‘rubbly’ surface

What is a spatter cone?

a small, steep sided cone built by lava ‘spattering’ out of a vent  

What is a lava tube?

a tunnel-like conduit for lava that develops during the late stages of solidification of a pahoehoe-type flow

What is pillow lava(pillow basalts)?

lava extruded underwater cools rapidly forming pillow-shaped, rounded masses that are closely fitted together

What is columnar jointing?

parallel, mostly six-sided vertical columns

What happens during an eruption?

• Felsic magma traps volatiles and water

• When the magma reaches the surface, pressure from above is reduced 

• Trapped gases expand

• Boom!

• More explosive means more felsic 

What are pyroclastic flows?

high-speed gravity-driven avalanches of hot ash, rock fragments, and gas that move down the sides of a volcano during explosive eruptions 

What are lahars?

volcanic ash readily moved by water with a debris flow 

What are lava domes?

steep-sided, dome-shaped, or spine-shaped masses of volcanic rock formed from viscous lava that solidified in or immediately above a volcanic vent