Review

Weathering

and Soil

What are the two types of Weathering?

• Mechanical and Chemical
• Mechanical weathering does not change the composition of rocks, but chemical weathering does.

Mechanical Weathering

What are four methods that contribute to Mechanical Weathering?

• Ice Wedging
• Collision
• Organisms & Root Pry
• Unloading

Ice Wedging

• Water enters cracks in rocks, freezes, and expands. This process will break rocks down.

COLLISION & ABRASION

• When rocks bump into one another and break into smaller fragments
• Fall because of gravity
• Sand blown by wind

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• Carried by a stream

Organisms & Root Pry

• Roots grow in cracks in rock. As they grow and expand, they break the rock.

• Animals dig into rocks

Unloading

Chemical �Weathering

• Water dissolves minerals
• Acids
• From Roots and decaying organisms
• Water reacts with chemicals to create Carbonic Acid, Nitric Acid, Sulfuric Acid
• Oxidation - reaction of minerals with oxygen changes them [Iron → Rust]

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Climate and Weathering

• Mechanical Weathering occurs in areas the freeze and thaw
• Chemical Weathering is most common in warm, moist areas.

Differential Weathering

Impact of Weathering-

• smaller rocks weather faster than large ones
• jagged rocks become rounded (corners/edges weather faster)

-Different minerals weather at different rates

(mountain ridges are usually made of more resistant minerals than valleys)

-Rocks break down into their components

Ex. Granite

Feldspars become clays, Quartz become sands...

Humus

• Rich, dark organic matter

Leaching

• The process where water dissolves minerals and carriers them deeper into the ground.

Soil Profile

A Horizon (topsoil)- darkest, richest, contains most organic matter

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B Horizon (subsoil or zone of accumulation)- less organic matter, more minerals

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C Horizon (zone of partially decomposed bedrock)- rock is breaking down

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Bedrock- unweathered rock

Impermeable layer

Permeable layer

Zone of saturation/ Aquifer

Water Table

The cone of depression of one well can cause problems for other wells.

This well has run dry because it is

within the cone of depression of the other well.

SPRINGS a place where water flows out of the ground because the water table meets the surface

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HOT SPRINGS a spring where the water has been heated by “geothermal” processes (Hot rocks or magma below ground)

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GEYSERS A hot spring that erupts periodically

�ARTESIAN WELLS well where water under pressure rises to the surface

Stalactites- they hang from the “T”op

Stalagmites-

they look like

a capital “M”

Column- when they join

CAVE FEATURES and KARST

Sometimes the water will evaporate and deposit the calcite as distinct features within the cave.

� Review

Agents of Erosion and Deposition

Four Agents of Erosion:

• Gravity
• Glaciers
• Wind
• Running Water

Running Water

Rills and Gullies

Meanders

Alluvial Fans

Deltas

Gravity

• Mass movement of large amounts of sediments.
• Rockslide
• Mudflow
• Slump
• Creep

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• Rocks break free by mechanical weathering and drop down a slope.
• They leave a pile of deposits called TALUS

Mudflow

• Water mixes with sediments and flows rapidly down hill.
• Common in dry areas when they get rain
• Deposits a cone-shaped pile of mud

Section of a steep slope loosens and drops, leaving a CURVED SCAR and a PILE OF SEDIMENTS as a deposit.

Creep

• Very Slow movement of soil.
• Caused by alternate freezing and thawing.

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• No real deposits, but there are several types of evidence.

Bent TREE TRUNKS and tilted fence posts and telephone poles

Wind

• Wind erosion and deposition greatest in dry areas with little vegetation (deserts, beaches, and plowed fields)
• No moisture to weigh down sediments
• No vegetation to hold sediments down
• No vegetation to block flow of wind

EROSION- Deflation

• Light sediments are picked up and carried away, while larger pebbles “pavement” are left behind

EROSION- Abrasion

• Sediments carried by the wind collide or “sandblast” rocks and other objects.

Evidence of Wind Erosion�- strange, smooth rock features

DEPOSITION- Loess

• Fine wind blown sediments are deposited and packed tightly
• Good, fertile farmland (Great Plains)

DEPOSITION- Dunes

• Moving piles of wind-blown sediments, may form in calm area behind other objects.

Dunes, continued

Wind picks up and carries sediments

Wind loses strength behind crest of dune and deposits sediments

Glaciers

• As snow builds up over time it turns to ice
• When the ice is thick enough, it will begin to flow

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Glacier types

• There are two types of glaciers
• Valley (or Mountain)
• Continental

Glaciers as Agents of Erosion

Plucking- weathering breaks rock around the glacier and the rocks move with the glacier

Moraines- edges/ends of glaciers carry sediments

Glacial Deposition

• Till- large fragments dropped from the end of a glacier

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• Outwash- fine sediments deposited from the melt-water running out from a glacier

Glacial Deposition�Eskers and Erratics

Curvy piles of sediments

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Random Blocks left by glaciers

Copyright 2005 by Andrew Alden, geology.about.com, reproduced under educational fair use.

Evidence of Glaciers

Striations- grooves scraped in a rock as a glacier moves over it

large striations are called glacial grooves

Evidence of Glaciers

U-shaped Valleys- glaciers erode on all sides

Evidence of Glaciers

Cirques- rounded valleys at the top of a glacier

Arete ���and ���Horn

Sharp ridge

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Sharp point