(MODULE 9.1) SOIL FORMATION Flashcards
geologists call soil ___.
regolith
A layer of weathered, unconsolidated material that contains organic matter and is capable of supporting plant growth
soil
soil consists of ___.
grains with water and air in the voids between grains
soil layers are called ___ and can be distinguished from one another by appearance and chemical composition
soil horizons
the uppermost layer that consists entirely of organic material
O horizon
a dark-colored soil layer that is rich in organic matter and high in biological activity, both in plant and animal
A horizon
organic acids and carbon dioxide produced by decaying plants in the topsoil that percolate downward from topsoil
E horizon
the E horizon is also called the ___.
zone of leaching
produced from downward movement of water in the E horizon and carries the dissolved minerals, and fine-grained clay minerals as well, into the soil layer below
B horizon
the B horizon is also called the ___.
zone of accumulation
the bottom of the soil horizon is unweathered parent material, also known as ___.
bedrock
what are the factors affecting soil formation?
1.
2.
3.
4.
5.
- parent material
- slope
- living organisms
- climate
- time
how does time affect soil formation?
time affects the soil through weathering; the longer the soil is exposed to weathering, the more soil accumulates
soils that remain in-place when the rock weathering process is faster than the transport process
residual soils
residual soils have characteristics that depend on its ___
parent rock
the engineering properties of residual soils range from poor to good, generally improving with ___ due to ___.
depth; high density causing it to be more compact.
examples of residual soils
1.
2.
3.
- decomposed granite
- saprolite
- laterite
sandy residual soil from granitic rock
decomposed granite
the general term for residual soils that are not extensively weathered and still retain much of the structure of the parent rock
saprolite
residual soil cemented with iron oxides, which gives it a high dry strength
laterite
residual soils are often seen in ___
mountainous areas
such soils are formed when rock weathers at one side and the particles are moved to another location
transported soils
transporting agents of transported soils:
1.
2.
3.
4.
- gravity
- running water
- glaciers
- wind
transported soils are found in ___
spots far from their original spot
soils transported downslope by gravity
colluvial soils
types of downslope movement: ___ and ___
slow and rapid
slow downslope movement is called ___
creep
rapid downslope movements include ___ or ___
landslides or mudflows
gravity deposits tend to be ___ and ___.
loosely compacted and exhibit little change in the general character of soil material
colluvial soils are often found at ___
the bottom of a slope
soils transported by moving water
alluvial soils
alluvial soils are also known as ___ or ___
fluvial soils or alluvium
alluvial soils are found in ___
the vicinity of rivers
alluvial soils often contain ___ which are important in the development of water supply wells and in geo-environmental engineering
extensive groundwater aquifers
most alluvial soils have ___ and provide ___ for buildings and other structures
moderately good engineering properties; fair to good support
a soil that is very permeable; allows water to pass through easily
aquifer
what condition would alluvial soils have in order to be considered not good soil?
a mixture of coarse sand, loose conditions, and water saturations, which leads to liquefaction
alluvial soils deposited and accumulates for a long time
alluvium
when a river’s velocity decreases substantially, coarser soil particles settle, forming submerged, flat, triangular deposits known as ___
alluvial fans
larger particles, such as sand and gravel, tend to settle more quickly to form ___
natural levees
alluvial soils deposited beneath lakes
lacustrine soils
the sustainability of lacustrine soils as foundation support ranges from ___ to ___.
poor to average
alluvial soils deposited underwater, formed in the ocean
marine soils
a special type of marine deposit formed where rivers meet larger bodies of water, and gradually build up to the water surface
deltas
the engineering properties of marine soils are often ___ due to its silt and clay being very soft.
poor
transported soils resulting from the action of glaciers
glacial soils
glaciers grind down the rock and soil, transporting the materials over long distances, causing the deposits to contain a ___
mixture of minerals from different sources
soil deposited directly by glacier
till
soil that was bulldozed by the glacier then deposited in ridges or mounds
ablation till
the ridges and mounds found in ablation till deposits are called ___ and are loose and easy to excavate
moraines
glaciers move with respect to the ___.
seasons
during the winter season and the summer season, what do glaciers do?
during the winter season, they move forward; at summer, they detract
soil caught beneath the glacier
lodgement till
the lodgement till is ___ under the weight of the ice.
heavily consolidated
a lodgement till is sometimes called ___
hardpan
while a lodgement till provides excellent support, it is also ___
very difficult to excavate
when glacier melts, they generate large quantities of runoff that erodes till and deposits it downstream as ___
glaciofluvial soil
glaciofluvial soil is also called ___
outwash
glaciofluvial soil deposits are ___ than till and an excellent source of ___ for use in concrete aggregates
more uniform than till; sand and gravel
the fine-grained portion of the till often remains suspended in the runoff water until reaching a lake or the ocean where it finally settles to the bottom. these are called ___
glaciolacustrine and glaciomarine soils
glaciolacustrine and glaciomarine soils are ___ and are thus prone to problems with ___.
soft and compressible; shear failure and excessive settlement
soils transported by wind
aeolian soils
transportation through wind generally produces ___ soils due to the ___ of wind
very poorly graded; strong sorting power
aeolian soils are usually ___, and thus have only ___.
very loose; fair engineering properties
aeolian sands can form horizontal strata, which are interbedded with alluvial soils, or they form irregular hills called ___
sand dunes
a loamy deposit formed by wind
loess
due to the high deposition mode of loess, it typically has a ___.
very high porosity
loess is fairly ___, but becomes ___.
strong when dry; weak when wet
loess can be stable when cut to steep slopes where water infiltration is ___. it is unstable when the slope is ___ and water is ___.
minimal; flatter; able to enter the soil
