geology midterm 2 Flashcards
physical/mechanical weathering
Where the rocks are physically broken/fractured into smaller pieces
frost wedging
Frost wedging -accomplished when water that seeps into the cracks of rocks expands when it freezes, and this expansion causes the crack to wedge farther apart. Multiple freeze and thaw events can cause cracks to get so big that they connect, and rocks are broken loose from the bedrock.
Salt wedging
occurs when salt crystal grow as salt water that lands on rock near the coast (commonly from ocean spray) evaporates. As the salt crystals grow larger, they can wedge existing fractures in the rock apart
Biogenic wedging
trees and plants wedge their roots in between crystals of rock
Unloading - sub-horizontal fractures
-unloading/exfoliation: the type of fractures seen on half-domes. Rocks on top unload, taking pressure off rock below, allowing that rock to expand.
Abrasion
water carves through rock, water has other pieces of rock and sand that are used as a tool by the water to abrade the bedrock
Chemical weathering
The bonds that are holding the atoms together in minerals are being attacked.
-dissolution, hydrolysis, oxidation
chemical weathering: Dissolution
is when minerals dissolve in weakly acidic water
chemical weathering: Hydrolysis
Hydrolysis is the process by which water and weak acids attack the bonds in minerals to produce clay minerals (which have water in their crystalline structure and are aluminum‐ rich) as well as ions dissolved in water
chemical weathering: oxidation
Oxidation is the chemical process in which ions in the mineral structure leave the crystalline lattice to bond with oxygen atoms from the atmosphere
climate & weathering
Wet, warm environments favor chemical weathering processes, cold and dry environments tend to favor physical weathering processes.
Products of weathering:
Clasts - Weathering produces clasts (pieces of rock that can range in size from boulders to pebbles to sand and microscopic mud) and ions dissolved in water.
Ions - dissolved in rainwater, moved into streams, transported to oceans - making it more salty
soil
Soil - Weathering is also closely tied to the production of soil. Soils are often rich in clay minerals.
Soil profile: rock that’s been weathered and mixed with plant debris
clastic sedimentary rocks
Rivers deposit bits and pieces of rocks, turning them into clastic sedimentary rock.
bedding
Bedding
When sedimentary beds are tilted, we can infer that the beds must have been deformed after they were deposited.
Sometimes sedimentary beds can get so deformed that they are turned upside‐down! Sedimentary structures can help inform geologists of the original direction of ‘up.’ Original ‘up’ is important because up indicates the age of the rocks: older on the bottom, younger on the top
graded bed
A bed that gradually changed from coarse to fine. This is how you check if the bed is right side up. It should be fine grained on top.
sedimentary provenance
where the clasts came from. Tells us about areas around the basin.
chemical sedimentary rock:
when water evaporates and leaves behind minerals like salt, forming chemical sedimentary rock
Chemical sedimentary rocks: formed by ions
chemical sedimentary rock: evaporite
inorganic. A chemical sedimentary rock that forms when dissolved ions precipitate because the water evaporates.
chemical sedimentary rock: limestone (reef):
organic. Formed when biologic processes cause dissolved ions to precipitate from water. For example, many plants and animals in coral reefs use ions from sea water to form their shells/tests.
chemical sedimentary rock: deep sea life
organic chemical sedimentary rocks can form in deep ocean environments where the primary lifeform that accumulates on the ocean floor are plankton tests (shells.) Foraminifera make their tests out of calcite. Diatoms and radiolarians make their shells out of silica.
Sedimentary lithification:
The process of taking loosely consolidated clasts or chemical precipitates and turning them into sedimentary rocks. This is a two step process.
1) Compaction: as the rocks get buried deeper, the rocks at the bottom get squished and compacted, sticking sand grains together
2) Cementation: reducing the space between the grains, cementing them together so they can’t come apart
depositional setting:
the environment in which the loose grains are being deposited.
depositional setting: fluvial/river
-associated with alternating layers of conglomerate, sandstone, mudstone, and sometimes coal.
depositional setting: deep marine
-associated with alternating layers of mudstone and plankton shells (diatomite/chert/ chalk)
depositional setting: shallow marine
-associated with sandstone that is deposited at the beach, mudstone that is deposited just beyond the breaking waves, and limestone that forms on the outer/deepest portion of the shallow marine setting.
sedimentary basin
-sedimentary basin: the low point where all of the sediment collects and layers
rift basin
subsidence in rift basins occurs in extensional settings (divergent boundaries) as the crust thins and the surface sinks.
flexural basin
subsidence in flexural basins occurs at convergent boundaries as the land is bent (or flexes) downward due to the weight of growing mountain belts, making space for the sediment to come in.
transgressions
-Sea level rises
-Transgressions that form as sea level rises are documented by stratigraphic columns of shallow marine rocks that indicate increasing water depths with time (e.g., sandstone below mudstone below limestone).
regression
-Sea level falls
-Regressions that form as seal falls are documented by stratigraphic columns of shallow marine rocks that indicate decreasing water depths with time (e.g., limestone below mudstone below sandstone).
metamorphic rocks
are rocks that have changed mineralogy and/or texture due to changes in pressure and temperatures.
Driving force: temperature
Forces water out of the crystalline structure
Increased temperature forms new minerals that have no water in their crystalline structure (composition)
Causes minerals to grow bigger (texture)
High temperature favors larger crystals
Mineralogical change: water out
The metamorphic process starts the wet-melting process for igneous rocks
protolith & metamorphic rocks
-Metamorphic protolith refers to the composition of a metamorphic rock before it was metamorphosed.
-Because different rocks have widely variable initial compositions, metamorphism of adjacent rocks with different protoliths but at the same pressure and temperature conditions will yield visibly different metamorphic rocks
-Two protoliths that are particularly important for the study of metamorphic rocks are mafic protoliths (e.g., basalt and gabbro), and pelites (mudstone).
geotherm
describes the temperature and pressure as they increase deeper into our planet. Temp and pressure are the two main variables determining when a rock melts