Chapter 10: Weathering, Soil, & Erosion
Terms in this set (87)
the processes that decompose rocks and convert them to loose gravel, sand, clay, and soil
the processes that pick up and carry off weathered rocks and minerals
reduces solid rock to small fragments but does not alter the chemical composition of rocks and minerals
occurs when air and water chemically react with rock to alter its composition and mineral content
what are examples of mechanical weathering?
pressure release fracturing, frost wedging, abrasion, organic activity, and thermal expansion and contraction
which two additional processes result from combinations of mechanical and chemical processes?
salt cracking and hydrolysis expansion
how do mountain ranges form?
many igneous and metamorphic rocks form deep below Earth's surface
the pressure from the weight of the overlying rock is about 5,000 times that at Earth's surface
over millennia, tectonic forces may rise the granite to form a mountain range
fracturing of a rock that occurs when pressure decreases and rock expands as tectonic forces raise it from a depth of several kilometers and erosion removes overlying rocks
when water expands by 7 to 8 percent when it freezes; this process involves water accumulating in a crack and then freezing and the ice expands
large piles of loose, angular rocks that have the potential to fall at the angle of repose
when rocks, grains of sand, and silt collide with one another when currents or waves carry them along a stream or beach. During these collisions, their sharp edges and corners wear away and the particles become rounded
the mechanical wearing of rocks by friction and impact
if soil collects in a crack in bedrock, a seed may fall there and sprout. The roots work their way into the crack, expand, and may eventually widen the crack
thermal expansion and contraction
rocks at earth's surface exposed to heating and cooling on the daily
they expand when they are heated and contract when they cool
temperature changes rapidly, the surface of a rock heats or cools faster than its interior, and as a result, the surface expands or contracts faster than the interior
the forces generated by these processes may fracture the rock
when minerals dissolve into water
which are the most important processes of chemical weathering?
dissolution, hydrolysis, and oxidation
what causes chemical weathering processes to decompose rocks?
water, acids and bases, and oxygen cause these processes to decompose rocks
how does water dissolve a mineral?
imagine a single atom at the top of a crystal
it is held in place because it is attracted to the other atoms in the crystal by chemical bonds (electrical forces)
at the same time, electrical attractions to the outside environment pull the atom away from the crystal
the result is like a tug-of-war
if the bonds between the atom and the crystal are stronger than the attraction of the atom to its outside environment, the crystal remains intact
if outside attractions are stronger, they pull the atom away from the crystal and the mineral dissolves
ex) when a crystal of halite is dropped in a glass of water, the negatively charged oxygen atoms in the water pull the positively charged sodium ions away from the halite, while the positively charged hydrogen atoms remove the negatively charged chlorine ion
why does halite dissolve in water?
because the attractions between the water molecules and the sodium and chloride ions are greater than the strength of the chemical bonds in the crystal
_______ & ________ dissolve more rapidly when water is acidic or basic
rocks and minerals
what does an acidic solution contain?
a high concentration of hydrogen ions (H+)
what does a basic solution contain?
a high concentration of hydroxyl ions (OH-)
why do acids and bases dissolve most minerals more effectively than pure water?
because they provide more electrically charged hydrogen and hydroxyl ions to pull atoms out of crystals
a weak acid that is formed by atmospheric carbon dioxide dissolving into raindrops and reacting
what makes rain even more acidic?
how are limestone caverns formed?
by the dissolution of limestone. Rainwater picks up carbon dioxide from the air and as it percolates (seeps) through the soil, which turns into a weak acid. This slowly dissolves out the limestone along the joints, bedding planes and fractures, some of which become enlarged enough to form caves
when water reacts with a mineral to form a new mineral with the water as part of its crystal structure
the loss of electrons from a compound or element during a chemical reaction; in the weathering of minerals, this usually occurs when the mineral reacts with molecular oxygen
when salt water seeps through pores and cracks in bedrock
when the water evaporates, the dissolved salts crystallize
the growing crystals exert tremendous forces that loosen mineral grains and widen cracks
a process in which large plates or shells split away like the layers of an onion
why is exfoliation concentrated near earth's surface?
because water and chemical weathering are most abundant close to the surface
a thin layer of loose rock fragments, clay, and sand overlying bedrock
a mixture of mineral grains, organic material, water, and gas
what causes iron to rust?
when it reacts with water and oxygen
what is rusting an example of?
which minerals occur as sulfide minerals in ore deposits?
iron, copper, lead, and zinc
how do chemical and mechanical weathering act together?
after mechanical processes fracture a rock, water and air seep into the cracks to initiate chemical weathering
what two examples of salt cracking
many sea cliffs show pits and depressions caused by salt cracking because spray from the breaking waves brings the salt to the rock
also common in deserts, where surface water and ground water often contain dissolved salts
a mixture of sand, clay, silt, and generous amounts of organic matter; also the most fertile soil!
bits of leaves, stems, and flowers on the soil surface
when litter decomposes sufficiently that you can no longer determine the origin of individual pieces
what does humus do?
insulates the soil from excessive heat and cold and reduces water loss by evaporation
also retains soil nutrients and makes them available to plants
also soaks up so much moisture that humus-rich soil swells after a rain and shrinks during dry spells. this alternating shrinking and swelling loosens the soil, allowing roots to grow into it easily
also oxidizes in air and decomposes
layers of a well-developed soil
the uppermost layer of soil; contains mostly litter and humus, with a small proportion of minerals
a mixture of humus, sand, silt, and clay
What are the combined O and A horizons called?
what is the most critical element for growth of plant proteins?
what does phosphorous do for plant growth?
important for metabolism and development of cell membranes
what does sulfur do for plant growth?
required for synthesis of plant vitamins and proteins
what does potassium do for plant growth?
maintains plant cell permeability
what does magnesium do for plant growth?
important for the production of chlorophyll
what does calcium do for plant growth?
important for plant cell walls
what are the basic building blocks of all organic tissue? (3)
carbon, oxygen, and hydrogen
what are the names of the layers of the ground?
B Horizon (subsoil; high concentration of organic matter)
C Horizon (weathered bedrock)
subsoil; roots and other organic material grow in it, but total amount of organic matter is low
high concentration of organic matter; consists of partially weathered rock that grades into unweathered parent rock
downward movement of water and dissolved ions
the action by which water is pulled upward through small pores by electrical attraction to the pore walls
during a raintstorm, water seeps through the A Horizon, dissolving soluble ions such as calcium, magnesium, potassium, and sodium
in arid regions, rainstorms typically are of short duration and little rain falls
consequently, when the rain stops, capillary action and plant roots then draw most of the water back up toward the surface, where it evaporates or is taken up by plants
as the water escapes, many of its dissolved ions precipitate in the B horizon, encrusting the soil with salts, this type of soil is what we're looking at
a hard cement comprised of mineral calcite, as a result of deposition of enough calcium carbonate; brought to the surface after it rains
what are the 6 factors that control soil characteristics?
parent rock, climate, rates of plant growth and decay, slope aspect and steepness, time, and transport of soil materials
ex) when granite decomposes, the feldspar converts to clay and the rock releases quartz as sand grains. if the clay leaches into the B horizon, a sandy soil forms. in contrast, because basalt contains no quartz, soil formed from basalt is likely to be rich in clay and contain only small amounts of sand; nutrient abundance also depends in part on the parent rock
which factors of climate affect soil formation?
rainfall and temperature affect soil formation
how does earth's climate affect soil formation?
rain seeps downward through soil, but several other factors pull the water back upward. roots suck soil water toward the surface, and water near the surface evaporates; also capillary action
when intensively irrigated salts accumulate until plants cannot grow
a type of soil in the B horizon that accumulates and is comprised of aluminum, iron, and some silicon
a type of soil that is comprised of only insoluble aluminum and iron
the world's main type of aluminum ore
the downslope movement of earth material, primarily caused by gravity
a general term for mass wasting and for the landforms created by mass wasting
what are some examples of transported soils?
the soils of river flood plains and deltas, and the rich windblown soils of central China and the American Great Plains
how is soil created?
weathering decomposes bedrock, and plants add organic material to the regolith to create soil at earth's surface
in a healthy forest, what percent of rainwater is soaked into the soil and what percent of it returns to the atmosphere?
25% of rainwater soaks into the soil or escapes as runoff
75% of it returns to the atmosphere through transpiration and evaporation
landslides: how does the steepness of the slope affect the possibility of one to occur?
if frost wedging dislodges a rock from a steep cliff, the rock is less likely to roll down a gentle hillside
landslides: how does the type of rock and orientation of rock layers affect the possibility of one to occur?
slope's stability could cause the landslide (how it's oriented); would want to avoid getting close to the angle of repose
landslides: how does the nature of unconsolidated materials affect the possibility of one to occur?
if the angle goes beyond the angle of repose, it could occur
the angle of repose
the maximum slope or steepness at which loose material remains stable
landslides: how does water and vegetation affect the possibility of one to occur?
excess water lubricates the slope and adds weight to it; a vegetated slope is more stable than a similar bare one because the roots hold soil together and plants absorb the water
landslides: how do earthquakes and volcanoes affect the possibility of one to occur?
earthquake: by shaking an unstable slope
volcano: eruption may melt snow and ice near the top of the volcano; the water then soaks into the slope to release a landslide
during this, loose, unconsolidated soil or sediment moves as a fluid
movement of a coherent block of material along a fracture
rapid, free falling motion of parts of a slope or the whole thing
the slow, downhill flow of rock or soil under the influence of gravity
a slurry of mud and rocks that is formed from when heavy rain falls on unvegetated soil, and the water saturates the soil
occurs when blocks of material slide downhill over a gently curved fracture in rock or regolith
during this, bedrock slides downslope over a fracture plane; the rock breaks up as it moves and a turbulent mass or rubble tumbles down the hillside
where are the best soils formed?
Great Plains: relatively dry area
ex) Great Plains of the U.S.
Ukraine Pampa in Argentina
where are the worst soils formed?
Tropical Rainforest: wet area