Lecture 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?

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.

Clay Soils• Soils that are high in clay have unique properties:

• Very sticky when wet and very hard when dry.

• 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