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
if a large mass of halite or calcite is placed in water
the minerals completely dissolve until the water cannot hold any more of the dissolved ions, then dissolution stops (the portion that did dissolve, however, dissolves completely with no residue left behind)
incongruent dissolution
partial dissolution of a mineral resulting in a new mineral phase
example of incongruent dissolution
orthoclase to kaolinite (clay) - some of the ions of orthoclase are dissolved, some are rearranged in the new mineral (kaolinite) and water has been incorporated into the new mineral
? can enhance weather of some minerals
acid
acid is
simply a hydrogen ion H+
nature supplies its own acid by
dissolving carbon dioxide in water
CO2 + H20 -> H^+ + HCO3^-
in most cases, if water is actually part of the reaction, added acid will
increase the rate of reaction
sulfur and nitrogen emissions from burning fossil fuels (particularly coal) can
increase the acid content of rain (known as acid rain)
ancient sculptures made of limestone or marble (both calcite) heave weather must faster due to
acid rain
oxidation
alteration of a mineral by removing electrons, usually by oxygen
if electrons are removed, the charge on an ion changes, which means
the chemical formula must be altered to achieve charge balance
physical and chemical weathering ? each other
enhance
chemical weathering weakens the rock, which results in
faster physical weathering
common minerals that undergo incongruent dissolution and oxidation to produce clays & oxides
muscovite, K-feldspar, biotite, plagioclase, amphibole, pyroxene, olivine
common minerals that undergo congruent dissolution and totally dissolve
quartz, calcite, halite
weathering rates are increased by
- contact with water
- contact with air (CO2 and O2)
- higher temperature
weathering is greatest in what kind of climates?
hot and humid
solids also increase weathering by
retaining moisture and facilitating root action
soil
weathered rock plus organic material (humus); typically layered
the layers of soil are referred to as
horizons
entisols
soils without layering
vertisols
contain expandable clays and layers mix
inceptisols
young soils, poorly formed layering and weathering
aridosols
desert soils, saline or alkaline soils
mollisols
calcium rich soils high in humus
spodosols
light gray, leached A horizon, organic material carried to B horizon
alfisols
acid soils with clay rich subsoils
ultisols
more weathered alfisols
oxisols
intensely weathered and leached soils
histosols
wetland soils
andisols
volcanic ash soils
gelisols
permafrost soils
A horizon
upper layer (zone of leaching); ranges from topsoil to bottom of root zone, generally rich in organic material (humus) rainwater dissolves minerals (leaching)
B horizon
zone of deposition; little organic matter, dissolved minerals from A-horizon may precipitate (Fe and Al oxides, calcite), often contains weathered rock fragments
C horizon
weathered rock
soil layers are not always
alphabetical (e.g., R, H, E, O)
dry climate’s effect on A horizon
thin, mostly unweathered
dry climate’s effect on B horizon
caliche formation
temperate climate’s effect on A horizon
thick, rich in humus, mildly weathered
temperate climate’s effect on B horizon
minor precipitates
tropic climate’s effect on A horizon
very little humus, highly leached
tropic climate’s effect on B horizon
insoluble Fe and Al minerals that do not leach
best quality soil climate & characteristics
temperate; rich in humus and not highly leached
intermediate quality soil climate & characteristics
arid; not highly leached, but poor in humus
poor quality soil climate & characteristics
tropical; highly leached
how can tropical soils be poor if they support lush jungles
all the nutrients are in the plants, not the soil
why do subsistence farmers only grow crops for a few years before the crops start to fail and they have to clear a new plot of land?
the soil is poor and only the dying plants are the source of nutrients for growing plants
