Lab 10 - Weathering and Soils.docx
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Dec 6, 2023
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GEOL101 Dynamics of the Earth – Fall 2023
Name:
Emily Thomson
Laboratory 10: Soils and Weathering – Things fall apart
Section:
Learning Outcomes:
● Describe the three general soil formation processes that promote the development of soils
● List “idealized” vertical of soil horizons and describe the general processes that influence their
physical appearance
● Describe various local factors that influence soil order development thickness in a given area
● Identify some major soil orders based on physical characteristics and local environment
Background
Soils are common features of the Earth’s surface. You encounter them daily, but probably give them
little notice or just think of them as the “dirt” in which people grow vegetables or flowers. Yet, without
soils, most if not all of the terrestrial animal life of our planet would not exist! The United States
Department of Agriculture National Resources Conservation Service (NRCS) defines a soil as follows:
The unconsolidated mineral or organic material on the immediate surface of the Earth
that serves as a natural medium for the growth of land plants.
The upper limit of soil is the boundary between soil and air, shallow water, live plants, or
plant materials that have not begun to decompose. Areas are not considered to have soil
if the surface is permanently covered by water too deep (typically more than 2.5 meters)
for the growth of rooted plants.
The lower boundary separating soil from non-soil is difficult to define. Commonly, soil
grades at its lower boundary to hard rock or to earthy materials virtually devoid of
animals, roots, or other marks of biological activity. For purposes of classification, the
lower boundary of soil is arbitrarily set at 200 cm (2 m).
General Soil Formation Processes
There are three main processes that take place at the Earth’s surface, or very near it, that contribute to
soil formation. These processes are weathering, water, and organic activity
Weathering processes
include chemical and physical processes; for example, chemical reactions
occur when minerals come in contact with air and water. Some reactions involve volume expansion,
which in turn produces internal stresses that physically break rock into smaller and smaller bits and
pieces. In short, weathering contributes to the formation or production of new material as loose debris,
mineral grains, and ions in solution.
Water
is a relatively ubiquitous solvent on planet Earth. As rain falls on the Earth, some runs off into
streams and gutters while some infiltrates cracks and holes produced by weathering and biological
activity. As water percolates downward, it carries with it dissolved ions derived from chemical
weathering and clay particles derived from physical weathering. The region within the soil where this
activity takes place is referred to as the
zone of leaching
. Further down the removed clay particles may
be deposited and the dissolved ions may precipitate as new minerals, a region termed the zone of
accumulation.
Organisms
within soils introduce organic matter, break this organic matter down into inorganic
nutrients, and mix and redistribute soil particles. For example, earthworms and microbes physically
mix and break up soil, consume dead organic matter, and release inorganic nutrients back into soils.
Any gardener will tell you their compost bin would not be effective without the help of worms and
microbes. The accumulation of dead organic material is called humus . . . not to be confused with
hummus!
Soil Horizons
As a result of the above three processes, bedrock and regolith (i.e., loose
unconsolidated sediments that overlie bedrock) are often converted into soil
over time. The composition and character of the soil evolves into something
very different than its starting composition. Soil forming processes act
differently at different depths to produce a “soil profile” comprising a number
of “soil horizons.” An idealized soil profile with labeled soil horizons is shown
to the right and the soil horizons are briefly defined below:
O = Organic layer, almost no mineral matter; dark in color
A = Mixed mineral and organic layer, darker in color than lower layers
E = Rich in quartz due to clay removal; zone of leaching; light in color
B = Rich in clays and precipitates; zone of accumulation
C = Fractured/fragmented rock; no accumulation from above; aka regolith
R = Unweathered/unaltered rock parent material (R not shown)
Question 1.
Examine the soil profile below exposed in a grassland. List each horizon in order
from the top horizon to the bottom horizon based on the given descriptions. The scale in the
image is in cm.
1.
O
2.
A
3.
E
4.
B
Question 2.
Now examine the soil profile below from a tropical rainforest environment. List each
horizon in order from the top horizon to the bottom horizon based on the given descriptions
Note the boy for scale.
1.
O
2.
B
3.
C
Question 3.
Comparing the two soil profiles above from grassland and rainforest environments.
How are they similar? How are they different? Which soil horizons are absent from each
location? Hypothesize why a horizon may be missing or did not form? In your hypothesis,
your are encouraged to include a role for water/rainfall and weathering.
The soil profiles are similar because they both start with horizon O which is an organic rich layer with
little to no minerals. They also both have the horizon B which are zones of accumulation, but there are
only depositions of clay in image 2. They are different because they contain different horizons because
one is a grassland and one is a rainforest, they have different components. Image one is lacking
horizons C and R, and Image two is lacking horizons A, E, and R. I think this has to do with the
environment such as the rainforest being much more exposed to rainfall that way different horizons are
being washed away by the water weathering.
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Soil Forming Factors
Farmers and farming may be the words you associate with soils; however, ranchers, foresters, and
geologists are also interested in soils and how they form. All of these professions understand that soils
are not the same from one location to the next, and that they may differ in composition, thickness, and
texture. Crops, trees, and grass may grow well in one soil and not grow well or at all in another soil.
These differences reflect a variety of soil forming factors..
Climate
: Higher rainfall and warmer temperatures accelerate chemical weathering and soil
formation.
Bedrock
(R horizon): Soils can form on any rock type (aka the R horizon) and the local rock
type will strongly influence soil composition. For example, a soil forming on basalt would be
higher in iron than a soil forming on a quartz-rich (and therefore iron-poor) sandstone.
Unsurprisingly, all other things being equal, soils will tend to form faster on unconsolidated
material (e.g., volcanic ash) than hard bedrock (e.g., granite)
Slope
: Also unsurprisingly, all things being equal, more gentle slopes will tend to promote
and support more soil formation.
Time
: Soils can form in one to tens of years in protected warm, moist environments whereas
they can take thousands of years to form in exposed cold, dry environments..
Vegetation
: Plants add and extract different amounts or organic matter and nutrients to and
from soils. Some plants have shallow roots while others have deep roots, and deeper roots
tend to help keep soils intact by limiting erosion.
Question 4.
Which soil forming factor(s) likely played the biggest role in producing the soil
profile in Question 2 based on the information you were given (i.e., it formed in a tropical
rainforest and is composed of the horizons portrayed in the given image)?
I think climate likely played the biggest role in the soil profile because rainforests have higher rainfall
so this accelerates chemical weathering and soil formation.
Question 5.
Your answer to Question 4 may or may not have include climate and vegetation,
but let’s assume it did. Now explain in three or four sentences why you think climate and
vegetation played such a big role in the production of the profile. If your answer to Question 4
mentioned more factors then climate and vegetation include those in your answer as well.
My answer included climate which highly affects the soil production profile because it rains a lot in a
rainforest. Rain is a part of the chemical weathering process such as acid rain. Vegetation is also a
huge factor because of the physical properties of a rainforest. There are huge and tall trees so a lot of
greens and the decomposition of the leaves creates more soil and gives the trees more nutrients. Also
not a lot of sunlight reaches the ground.
Soil Order Classification
Soil scientists worldwide have struggled to develop a classification scheme that everyone agrees
upon. In the United States, we use the U.S. Comprehensive Soil Classification System, shown below,
which defines twelve “soil orders” based on physical characteristics and formation environment. For
this laboratory, we will only focus on the three bolded soils, which represent soil end-members (i.e.,
young versus old soils) and the soil in which the majority of our food is grown.
Soil Orders of the U.S. Comprehensive Soil Classification System
Alfisol
Rich in subsurface clay accumulation and abundant nutrients; forms in humid forests.
Andisol
Forms in volcanic ash
Aridisol
Low in organic matter with carbonate horizons; forms in arid environments
Entisol
No obvious horizons; relatively recently formed
Gelisol
Underlain with permanently frozen ground
Histosol
Very rich in organic debris; forms in swamps and marshes
Inceptisol
Moist with poorly developed horizons;
relatively recently formed
Mollisol
Soft, black, and rich in nutrients; forms in subhumid to subarid grasslands
Oxisol
Very weathered; Al-and Fe-oxide rich, nutrient poor; forms in tropical regions
Spodisol
Acidic, nutrient poor, ashy, Al- and Fe-oxide rich; forms in humid forests
Ultisol
Very mature, strongly weathered soils, nutrient poor
Vertisol
Clay-rich so can swelling when wetted; shrinking and cracking common when dry
Entisols
are common in arid regions, have no distinguishable horizons, and are essentially unaltered
from their starting composition, which can vary from loose sediment to solid bedrock. In contrast,
Ultisol
are red clay rich soils produced by long periods of intense weathering in a warm and wet
climate, such as in the southeastern United States.
Mollisols
are the foundation for much of the
agriculture across the United States, and are characterized by a thick, dark surface horizon produced
through progressive addition of organic material. Mollisols are particularly common across Illinois,
Iowa, Kansas, Montana, Nebraska, North Dakota, Oklahoma, and South Dakota.
Question 6.
Based on the above descriptions of entisol, ultisol, and mollisol, which best fits the
image in Question 1 of the grassland profile?
mollisol
Question 7.
Which best fits the image in Question 2 of the tropical rainforest profile?
ultisol
The map below shows the global distribution of soil orders based on the U.S. Soil Classification
System. Notice entisols are displayed in aqua blue, ultisols in yellow, and mollisols in green.
Question 8.1:
The locations of entisols are shown in an aqua blue color in the soil orders map.
What one or two other soil type(s) are often found adjacent to entisols?
Aridisols
Question 8.2:
Look at the
Soil Orders of the U.S. Comprehensive Soil Classification System
table above for the descriptions of the soil(s) in your answer to Question 8 above. Does it make
sense that they would all be found alongside entisols? Why or why not, and in what climate
region are these soils typically found?
Yes it makes sense because they are similar in climate regions and geographical locations
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Question 8.3:
Using the given latitudes in the map above, circle the combination of latitudes
below that best describes where the majority of entisols seem to occur.
A. 30°N and 60°N
C. 15°N and 45°N
B. 15°N and 15°S
D. 15°S and 45°S
Question 9.1:
Ultisols are shown in yellow in the soil order map above. What are the one or two
other soil type(s) that can be found adjacent to ultisols?
Oxisols
Question 9.2:
Look at the
Soil Orders of the U.S. Comprehensive Soil Classification System
table above for the descriptions of the soil(s) in your answer to Question 9 above. Does it make
sense that they would all be found alongside ultisols? Why or why not and in what climate
region are these soils typically found?
it makes sense because they are both weathered and in poor nutrient regions, they are typically found
in tropical locations
Question 9.3
Using the given latitudes in soil order map, circle the combination of latitudes
below that best describes where the majority of ultisols seem to occur.
A. 30°N and 60°N
C. 15°N and 45°N
B. 15°N and 15°S
D. 15°S and 45°S
Question 10.1:
Based on the global soil map on the next page, mollisols and alfisols tend to
form adjacent to one another, and alfisol is the state soil of California (really, we have a state
soil!). Does this make sense based on their descriptions in the
Soil Orders of the U.S.
Comprehensive Soil Classification System
table? Why are these soils important to you as a
human?
These soils are both rich in nutrients and this and this is important to me as a human because i like in
california and it is important to have nutritious soil to grow plants fruits and vegetables to be able to put
healthy nutrients into my body
Question 10.2:
Describe where in North America these soils form (i.e., are they only in the US?).
Where are the closest mollisols and alfisols to San Diego? What does that mean about where
we most likely get our food?
They don't just from in the US but a lot of North America including Canada.We get a lot of our food from
Mexico, specifically Baja is the closest to San Diego.
Question 11.1:
The global map to
the right shows arid regions in
tan and light brown. Circle the
latitudes where the majority of
arid regions are found.
A. 30°N to 60°N
B. 15°N to 15°S
C. 15°N to 45°N
D. 15°S to 45°S
Question 11.2:
Does your answer to Question 11 match your answer to Question 8.3?
Yes my answer matches
Question 12.1:
In general, when people refer to the tropics they mean the area around the
equator. In the map above, wet regions are shown in dark green. Circle the latitudes below that
best describe where the majority of tropical (wet) regions occur on Earth.
A. Between 30°N and 60°N
C. Between 15°N and 45°N
B. Between 15°N and 15°S
D. Between 15°S and 45°S
Question 12.2:
Does your answer to Question 12.1 match your answer to Question 9.3?
Yes my answer matches
Question 13:
Notice that there are wet regions found on Earth other than those found at the
equator. Take a moment to locate these in the global map above based on the predominance of
green. With these locations in mind, take a look at the global soils map. What soil type (entisol,
ultisol, or mollisol) tend to be located in these wet areas?
Mollisols tend to be located in wet areas
Question 14:
In three or four sentences, summarize where on Earth entisols, ultisols, and
mollisols tend to form; include words like, arid regions, tan, wet regions, green, and latitude in
your answer.
Entisols have no obvious horizons and are generally newer, tan, dry, and arid regions. Ultisols have
strongly weathered soils and poor nutrients found in the tropics, and Molisols tend to form in sub humid
to subarid grasslands, and wet areas.
Question 15:
Why should you care about mollisols?
They are important because they are where most crops are grown with very fertile soils and this is
important in producing crops.
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