Title: Lecture 1a Soil From Rocks
1Lecture 1aSoil From Rocks
- Where does Soil Come From?
- Soil is everywhere!
- But how does it develop?
- What causes one soil to be productive and another
to be poor? - What are the rocks doing in this soil?
2Soil is
- The soil profile
- And the soil we pick up to feel consisting of
sand, silt, clay particles and decomposed organic
matter.
3Soil Texture
- Soil texture is determined by separating the
amount of sand, silt and clay in a soil and
determining the of each. We will learn about
this in Week 2. - This mineral part of the soil or the Sand, Silt
and Clay particles are from the weathering of
Rocks and Minerals. - Where have you been aware of the rocks that were
near the surface of the earth?
4Rocks Weather to Soil
- Weathering is the process by which all rocks at
the earth's surface get broken down. - Weathering occurs by both chemical
(decomposition) and mechanical processes
(disintegration).
epilithic
endolithic
5Rock Weathering
- Rock weathering Takes place in-situ (in place)
- It differs from erosion which involves removal of
material away from a site.
6Physical Weathering
- Physical weathering - Rocks get broken into
pieces but its chemical composition remains
unchanged. - Processes of Phys.Weathering
- 1) Freeze / thaw weathering - occurs when
temperature freezes at night and rises during the
day. Water expands when frozen which forces rocks
open. - 2) Biological weathering _ Roots of plants grow
into cracks and force cracks open.
chasmolithic
7Physical Weathering Processes (cont.)
- 3) Exfoliation or Unloading - when rock at
earth's surface is worn away. - After a rock that has formed deep in the earth is
exposed at the surface it expands and gradually
breaks into sheets.
8Chemical Weathering
- Chemical weathering -Rock broken down by chemical
change - water always plays a part. For Example - Carbon dioxide dissolves in rain water forming
carbonic acid which dissolves limestone rock
which is carried away in solution as calcium
hydrogen carbonate. - Chemical weathering is faster for limestone than
sandstone and is speeded up by heat.
9Chemical Weathering
- Chemical weathering occurs fastest at the sharp
edges of rocks as they have a large surface and
less volume so the chemical reactions are faster.
- Gradually the sharp edges become rounded.
10Chemical Weathering
- Chemical weathering produces clays on which
vegetation can grow. - A mixture of dead vegetation, clay, rock
fragments of sand and silt size particles
produces soil.
11A residual soil from limestone
- This soil is weathering from limestone bedrock.
- Limestone is CaCO3
- The 18 inches of soil present here are from the
impurities in the rock and the additions of soil
particles by wind to the surface of the soil. - Prairie grass roots gave the soil a dark color.
12Chemical Weathering
- Common chemical weathering processes are
hydrolysis, dissolution, and oxidation. - Chemical weathering tends to weaken rock, thereby
making it easier to break.
13Weathering
- Physical and chemical weathering occur together.
- Physical breaks rocks into pieces so more surface
is exposed to chemical weathering which breaks it
down further.
14Weathering
- Weathering is controlled largely by climate. The
more water available, the more likely that
chemical processes can proceed. - Additionally, in warm temperatures chemical
weathering can proceed even faster.
15Weathering
- In arid climates weathering processes occur very
slowly because of the lack of water. - Mechanical weathering will be the dominant
process in arid climates - however, because physical weathering relies on
chemical weathering, it will also be quite slow
16Weathering of rocks produces soil particles
- By looking at the sand grains, we can determine
the kind of rocks that physically weathered to
make the sand. -
17Sands from around the world
- Italy
- Hawaii
- Madeline Is. Wisconsin
- St. Peter Sandstone, St. Paul, MN
18Silt Grains- The intermediate size soil particle
- In this sediment sample, the grains are a little
smaller than in the sand photos. They look
bigger due to magnification. - Silt-sized particles have diameters between 0.05
mm to 0.002 mm - Most Silt grains are quartz because the less
resistant minerals have been completely broken
down. - Silt feels very smooth.
19Silt in Soap?
- Glacier Silt Scrubbie
- A moderately abrasive bar made with silt from the
Mendenhall Glacier, Designed to remove dirt, help
cut grease, and leave hands smooth.
20(No Transcript)
21Glacial Flour
- After the glacier grinds up the rock into silt
size particles, the wind can pick up the glacial
flour (silt) and blow it around. - The resulting deposit of silt is called LOESS
(sounds like us)
22LOESS
- Loess in the U.S. is derived from glacial
outwash. The loess was blown directly
from glacial deposits, and also
carried by rivers to be blown off the flood
plains. - The loess in W. Wi. E. MN is thick and is
absent in E.Wi. cent. Mn partly
because the loess
blanket
naturally tapers, but also because most areas
without loess were still ice-covered at the time,
so loess was
intermingled with other materials.
23Silt in Water
- Silt in water will remain suspended until the
water is moving very slowly. - Silt causes water turbidity and can harm fish and
stream quality.
24Loess in Mississippi
- The loess deposits at Vicksburg Mississippi are
believed to be 10,000 to possibly 100,000 years
old. - Fossils of vertebrate animals have been found
buried in the loess and include mastodon, horse,
tiger, bear, deer, and bison.
25Road Cuts
- Describe to your neighbor the road cut you saw
this weekend and on your dyad sheet.
26Loess in China is indirectly derived from glacial
outwash, which is carried into the desert
lowlands by streams. Wind transports loess from
the deserts, and loess is in turn redeposited by
the rivers.
27Clay Particles The smallest soil particle
- In this sample, the flakey nature of clay-sized
sediments is evident. - Clay is the last of the sediments to be deposited
by a stream due to its small grain size. - Clay size particles are made by a recombination
of minerals or synthesis from elements, not from
grinding up of silt particles. - Clays have diameters that are smaller than 0.002
mm.
28Clay Soils
- Soils that are high in clay have unique
properties - Very sticky when wet and very hard when dry.
29- What are the rocks doing in this soil?
- The rocks are waiting to be broken down into sand
grains, later into silt grains, and maybe
eventually chemically weathered into clay
particles if the weathering process has enough
time and the right environment.
30The End