Geology 101 Quiz 6 Flashcards
sedimentary rocks form in one of two ways
(1) by accumulation and lithification of particles or (2) by precipitation from solution
in either case of formation, sedimentary rocks form by accumulating or precipitating what?
existing particles or ions
weathering
the process of breaking down rocks both physically and chemically
physical weathering
refers to physical breakage of rocks and minerals. Smaller pieces retain the same chemical and mineral structure
chemical weathering
refers to the chemical alteration of minerals by either complete dissolution, or by reactions that partially dissolve a mineral and leave a new mineral phase in its place
physical weathering: breaking rock down into smaller pieces can be done in two ways
fracturing and abrasion
fracturing
creating a break to separate one piece into two or more pieces
joints
fractures with no displacement on either side of the break
faults
fracture where the rock on one side of the break has moved relative to the other
abrasion
wearing away the surface of a rock by striking or scraping with other rock fragments
causes of fracturing (large scale processes to smaller scale)
- tectonics
- uplift and erosion
- cooling
- undermining
- ice wedging (or frost wedging)
- roots
tectonics
fracturing causes by tectonic plates moving
uplift and erosion
as overlying rock is removed, the reduced pressure allows the exposed rock to expand - since expansion is always uneven, the rock fractures
special cases of uplift and erosion can
produce layers of rock breaking away like layers on an onion (called exfoliation or spheroidal weathering)
cooling
as newly solidified igneous rock cools, it contracts; contraction is always uneven, and fractures develop
undermining
if erosion cuts away lower rock resulting in overhang, the rock will eventually fracture and fall
example of undermining
the Old Man in the Mountain (New Hampshire) gone in 2003
ice wedging (or frost wedging)
water expands when it freezes and can exert an incredible force; if water fills a crack and freezes, the expanding ice can widen the crack or cause the rock to completely split
roots
small roots can grow into a small fracture; as the root grows, it can widen the fracture or split the rock open
causes of abrasion (large scale processes to smaller scale)
- tectonic faulting
- glaciers
- landslides
- water carried rocks
- wind blown sand
tectonic faulting
where two plates slide past each other, rock can be pulverized and smeared along the fault
glaciers
glaciers trap blocks of rock in the ice that grind over rocks beneath the glacier; this process can grind rock so fine it is referred to as rock/glacial flour
landslides
as rocks fall, they strike other rocks and both break and abrade
water carried rocks
water borne rocks strike other rocks and slowly abrade the surfaces
wind blown sand
in water, sand may travel around a rock without striking it due to the cushioning effect of the water; in air, the cushioning effect is reduced, and blowing sand strikes and abrades the surface - same principle as a sand blaster
fracturing and abrasion can reduce a mineral down to a powder, but it will not produce
most of the clay minerals commonly found throughout this area
chemical weathering is typically achieved by either
dissolution or oxidation reactions
two ways dissolution can occur
congruent and incongruent dissolution
congruent dissolution
complete dissociation of the atoms in the molecule
examples of congruent dissolution
halite: NaCl -> Na^+ + Cl^-
calcite: CaCO3 -> Ca^2+ + CO3^2-
when a small crystal of halite or calcite is placed in water
it completely dissolves; no residue is left behind