Download Soil From Rocks - Department of Soil, Water, and Climate

Lecture 1a
Soil 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?
Soil is
• The soil profile
• And the soil we
pick up to feel
consisting of sand,
silt, clay particles
and decomposed
organic matter.
Soil 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?
Rocks
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
Rock Weathering
• Rock weathering
Takes place in-situ
(in place)
• It differs from
erosion which
involves removal
of material away
from a site.
Physical 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
Physical 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.
Chemical 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.
Chemical 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.
Chemical 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.
A 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.
Chemical Weathering
• Common chemical
weathering
processes are:
hydrolysis,
dissolution, and
oxidation.
• Chemical
weathering tends
to weaken rock,
thereby making it
easier to break.
Weathering
• Physical and chemical
weathering occur
together.
• Physical breaks rocks
into pieces so more
surface is exposed to
chemical weathering
which breaks it down
further.
Weathering
• 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.
Weathering
• 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
Weathering 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.
Sands from around the world
• Italy
• Hawaii
• Madeline
Is.
Wisconsin
• St. Peter
Sandstone,
St. Paul,
MN
Silt 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.
Silt 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.
Glacial 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)
LOESS
• 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.
Silt 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.
Loess 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.
• What a road cut!
Road Cuts
• Dyad
Loess 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.
Clay 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.
• Soils that are
high in clay
have unique
properties:
• Very sticky
when wet and
very hard when
dry.
Clay Soils
• 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.
Hiking in Snowdonia N.P. – Central Wales, UK