Quiz 3 Flashcards
Metamorphism
To change from one form to another (metamorphic rocks have undergone solid state changes in texture/mineralogy/chemical composition)
Protolith
Parent rock
Relationship between metamorphic rocks and their parent rock
Most metamorphic rocks have the same overall composition as the parent rock (in terms of what minerals are in it). The exception: if water/gases are added, they are carrying dissolved ions within them that can add new elements or remove some (another way to put it: possible loss/accumulation of volatiles like water and carbon dioxide)
Four principle factors that drive metamorphism
Temperature, pressure, fluids, and parent rock composition
Geothermal gradient
Rate at which temperature increases as you go further into the crust
General effects of temperature on metamorphic rocks
Affects a rock’s texture and mineralogy, breaks chemical bonds and alters crystal structure, atoms and ions re-crystallize into new mineral assemblages, many new crystals will grow larger than they were in the parent rock.
Given a specific set of minerals in a metamorphic rock, you can infer the temperature at which the metamorphic rock formed (for index minerals) because the metamorphic changes that occur with temperature follow a predictable and repeatable path
Recrystallization
Mineral grains recrystallize to form new, interlocking grains of the same mineral (and grains typically get larger)
Neocrystallization
Chemical reactions change the original assemblage of minerals into a new, metamorphic assemblage of minerals. This means new minerals, same chemical composition
Confining pressure
Pressure in all directions; produces compact rocks with a greater density because it closes the spaces between mineral grains, but doesn’t cause deformities
Differential stress
Pressure applied more in one or two directions; compression, tension, and thus deformities (folds, faults, foliation; shape of the mineral changes: compressed, stretched, rotated)
Metaconglomerate
Pebbles flattened, elongated, and aligned. Can tell what way stress was applied based on the alignment of the mineral grains
Shearing
Pressure applied in opposite directions, giving an “S” or sigma-shaped grain
Foliation
Alignment of minerals, which happens with minerals that have cleavage. Requires a platey (muscovite, biotite) or elongate (amphibole, pyroxene) cleavage
General effects of pressure on metamorphic rocks
Changes a rock’s mineralogy and texture in a predictable manner. Metamorphic minerals can be compressed, elongated, and/or rotated by being forced into preferred orientations
General effects of fluids on metamorphic rocks
The only way that you can add or remove components during metamorphism, as fluids can carry dissolved ions or remove ions.
Water acts as a catalyst during metamorphism to increase the likelihood of metamorphism (as it makes it easier to break bonds).
Water aids in the exchange of ions between growing crystals.
Four main criteria of metamorphism
Size of their crystals (minerals), how the mineral grain shape is changed, the degree to which minerals are segregated into light and dark bands, and metamorphic grade
Metamorphic grade
How much temperature and pressure a rock experience/how much metamorphic changes rocks have undergone
Differences between low grade, intermediate grade, and high grade metamorphic rocks. Include the location and conditions in which the rocks form.
Low grade: formed in shallower, crustal regions under low temperature and/or low pressure
Intermediate grade: in between the two
High-grade: formed in deeper crustal regions, perhaps as deep as the upper mantle, under high temperature and/or high pressure
Foliated rocks in order of increasing grade
Slate: fine-grained, parent rock is shale
Phyllite: grain size is barely visible, but has a higher proportion of micas (shinier). Parent rock is mud-/clay-rich sedimentary rocks
Schist: medium to course grain, many of them are platey/flakey from mica/muscovite/biotite. Derived from clay and mud sedimentary rocks, passed through a process involving the production of shales, slates, and phyllites as intermediate steps
Gneiss: light-and-dark banding, parent rock can be shale or granite
Migmatite: boundary between metamorphic and igneous due to partial remelting
Isograds
Boundaries between zones in regional metamorphism
What makes non-foliated rocks the way they are?
They don’t experience differential pressure and/or are composed of only one mineral. This means no deformation, large recrystallization
Another name for non-foliated rocks
Granoblastic rock
Examples of non-foliated rocks
Quartzite, marble, hornfels, greenstones, and amphibole
Contact/thermal metamorphism
A hot, igneous body (like a magma chamber or contact with hot groundwater) bakes the surrounding rocks. Has no differential pressure; doesn’t produce foliated rocks
Hydrothermal metamorphism
Hydrothermal fluids can carry dissolved calcium dioxide, sodium, silica, copper, and zinc. Ascending hydrothermal fluids can react with overlying rock, creating new minerals (which may have great economic value).
Can happen in the ocean at mid-ocean ridge zones (where plates are moving apart) where there’s hot, mineral-rich water that can interact with rocks experiencing pressure and temperature changes from the magma upwelling (while the plates move apart).
Burial metamorphism
Happens as a plate subducts (goes underneath another plate), and rocks are metamorphosed due to increase in temperature and pressure as they descend.
Can lead to changes in minerology, texture, composition, and foliation (depending on the stresses as that plate is subducting).
High temperature, high pressure.
Regional metamorphism
Large-scale metamorphism that can also happen with tectonics when there are two continental plates coming together, leading to mountain building (which leads to increased pressure and stress, with some increase in temperature).
The Andes and Himalaya mountains, as well as the Swiss and Austrian Alps, were formed this way.
Ranges from low temperature to high pressure, all the way to high temperature
Shock/impact metamorphism
Occurs when an asteroid/comet impacts the earth’s surface. Moving as fast as 100k mph, the energy is transferred into heat energy and shock waves as it smashes into the earth, resulting in shocked textures and very quick metamorphism
High temperature, low pressure
The difference between weathering and erosion
Both processes are major geologic processes that can significantly shape the earth’s surface; the difference between them is transport!
Both processes can act at the same time.
Weathering
The general process by which rocks are broken down on the Earth’s surface (the chemical and physical breakdown of rocks).
Rocks react with the hydrosphere, atmosphere, and biosphere.
Erosion
The set of processes that loosen and transport soil and rock downhill/downwind (the process that breaks down and transports)
Regolith
Loose blanket of mineral grains, rock fragments, and organic matter; made from bedrock by weathering. Most of the surface of Earth is covered with this.
Salt wedging
In arid climates, dissolved halite or gypsum precipitates in the small pores and joints between grains, wedging them apart on a small scale, analogously to ice.
Types of physical weathering
Jointing, exfoliation, frost/salt wedging
Jointing
Erosion removes material and exposes deep, crustal rocks that are hot and under high pressure. These crustal rocks cool and expand, causing fractures called “joints”
Exfoliation
Buried rocks compressed by weight overlying material; when the overlying material’s removed, the rock cracks in onion-like “exfoliation” layers
Talus/scree
Big piles below a cliff
Sorting indicates what? Give an example of well-sorted and poorly-sorted environments.
Sorting indicates the consistency of environmental energy.
A well-sorted environment could be a beach, and a poorly-sorted environment could be an alluvial fan
Common chemical weathering reactions
Dissolution, hydrolysis, and oxidation
Factors affecting chemical weathering (and how)
Composition: Si-rich minerals are more stable/less prone to weathering
Water: many chemical reactions involve water
Temperature: rates of chemical reactions are faster at higher temperatures
What does it mean for a water molecule to be polar?
One side is positively charged and one side is negatively charged, making it a good solvent
Dissolution
Some minerals dissolve, and acidity enhances this effect. Examples: halite, gypsum, calcite
Reversing this process forms caves, as calcite precipitates
Hydrolysis
Water breaks cation bonds in silicate minerals to make dissolved cations and alteration residues (like clay minerals, which are the weathering product of silicate minerals, as well as iron oxides/rust)
Leaching
Dissolution of primary minerals
Hydration shell
Small ions with a high charge tend to hold onto some water molecules
Hydrolysis reaction
Can split a water molecule into a cation (H+) and an anion (OH-)
Primary minerals vs secondary minerals
Primary minerals: minerals present in bedrock
Secondary minerals: minerals formed as a result of chemical weathering (such as Fe, Al oxides like hematite; clays)
How do soils promote weathering?
Soils retain water (which promotes chemical weathering)
Soils support plant life (which promotes physical and chemical weathering)
Positive feedback system
How can climate enhance chemical weathering and physical weathering?
Chemical weathering is enhanced when climate’s warm, there’s a lot of water, and there’s plants (chemical weathering hardly occurs in desert environments)
Physical weathering is enhanced when there’s more wind, there’s freeze/thaw cycles, and there’s plants
Factors that lower and increase the geothermal gradient respectively
Subduction of cooler oceanic plate and rising magma
Two factors that increase with the intensity of metamorphism
Crystal size and courseness of foliation
Porphyroblasts
Very large crystals that may be developed during the recrystallization process. Only happens to certain minerals (like garnet, staurolite, and andalusite); other minerals (like muscovite, biotite, and quartz) typically form a large number of small crystals.
Mainly concerns schist.
Classification of foliated rocks vs non-foliated rocks
Foliated rocks are classified my texture
Non-foliated rocks are classified by mineral composition
Amphibolite’s parent rocks
Basalt, gabbro
Hornfels’s parent rock and how it’s produced
Shale; contact metamorphism
Metamorphic environments (types of metamorphism)
Contact/thermal, hydrothermal, burial, regional, shock/impact, fault zone
Marble and quartzite can only be formed during what type of metamorphism
Regional metamorphism
Bedrock
Solid rock attached to the crust; relatively rare to find exposed bedrock on Earth’s surface
Sediment
Regolith that has been transported and deposited
Physical weathering vs chemical weathering
Physical: mechanical breaking/disintegrating (busting, cracking, grinding)
Chemical: reacting with water (dissolving, making new minerals)
Oxidation
Reaction in which an element combines with oxygen; important for iron-rich minerals like olivine and pyroxenes
Biological weathering
Organisms like plant roots, fungi, lichens, and bacteria are often important chemical weathering agents
Relationship between climate and weathering
More CO2 in atmosphere = more carbonic acid and more weathering
Increased temperatures or more exposed rocks will increase weathering and thus reduce the amount of CO2 in the atmosphere
Over millions of years, weathering is an important control of atmospheric CO2
