Weathering and Erosion
Weathering – The breakdown or “rotting away” of rocks. This is a static process
(no movement of rocks is involved in weathering).
2 Types of Weathering:
A) Mechanical Weathering – caused when rocks are broken down by
forces. There is no change to the mineral composition.
1) Unloading – The layers of rock on top of deeply buried rocks create
tremendous pressure. Ultimately, the upper rocks are washed away. When
this pressure is removed, the lower rocks form cracks or joints.
This causes the rock to “peel” apart, similar in
appearance to an onion, which is called
exfoliation.
2) Frost Wedging – caused by the freezing, and thus, expansion of
water within the cracks of rocks. This causes tiny cracks to become
large cracks.
3) Plant Wedging – Roots expand in small
rock cracks over time.
4) Extremes in temperature – The outer layer of the rock is heated greatly
by the sun during the day, causing it to expand. At night, the cooling of
the rock causes it to contract. After this expansion and contraction has
been repeated many times, the outer skin of the rock peels away in a way
similar to that of an onion.
5) Feldspar swells when it absorbs water – When the outer feldspar minerals
absorb water, they swell up, and cause exfoliation. This is particularly comon
with granite, and causes spheroidal weathering. (rounded edges)
B) Chemical Weathering – The result of air, water and acids reacting with
minerals that change the composition of the mineral.
O2
H2O
causes oxidation
causes hydration
CO2 + H2O → H2CO3 (carbonic acid)
Examples of chemical weathering:
Feldspars weather to clay
Pyrite weathers to limonite
Calcite weathers to a dissolved solution
Granite is composed of :
Weathers to:
quartz
sand
feldspar
clay
FeMags
clay
Mica
clay
The end result of weathering is soil.
Erosion – The process in which weathered particles are transported from 1 place to
another. These sediments will eventually be deposited somewhere else (Deposition)
Agents of Erosion:
Running Water
Wind
Ocean Waves and Currents
Glaciers
Gravity
1) Gravity – 3 Types of Movement:
Fall – free fall of pieces
Slide – material moves along a well defined surface
Flow – material moves as a viscous fluid
Can occur quickly, or slowly
Creep – slow movement of soil
downhill. (1 mm/yr – 1 mm/day)
Liquifaction – when enough water mixes with soil to make a mixture that flows.
Slump – small mass movement, occurs where the slope is steep, and the ground
becomes saturated with water.
Talus – An accumulation of rock at the base of a cliff, usually from frost wedging
and exfoliation.
Landslide – Fast, large scale movement of rock material downhill.
2. Wind – Capable of transporting only small particles.
Deflation occurs when wind removes
small particles, leaving only large
particles, which then protect the desert
from further erosion.
The result is Desert Pavement.
Desert Pavement
Dust Bowl in Kansas, late 1930’s
Eventually, sand is deposited where the speed of the wind slows down.
Cross bedding is the
result.
Different types of sand dune deposits:
3. Glaciers – 2 types:
a) Continental glacier – large mass of ice that covers large masses of land.
Antarctica and Greenland are completely covered by ice up to 3000
meters thick.
b) Alpine Glaciers – form in the valleys of mountains (Alaska)
Glaciers flow slowly downhill
If the winters are cold and wet, glaciers grow.
When the winters are dry, and the summers hot, the glacier recedes.
Glaciers erode by:
Plucking – lift rocks and transport down the glacier
Abrasion – Rocks being carried down a glacier rub against rocks in the valley.
Features caused by alpine glaciation:
Hanging Wall
U-shaped valley
Depositional Features occur along the glacier, and at the bottom of the glacier.
Lateral Moraine – Deposits of rocks along the edge of the glacier.
Medial Moraine – Deposits of rock in the middle of the glacier. Forms when 2
glaciers merge together.
Terminal moraine – deposit of rocks at the bottom of the glacier.
What type of moraine?
Striations are a characteristic of glaciated valleys
Erratic – Glaciers are capable of transporting large boulders a long distance.
Once the glacier melts, it can deposit large boulders in places they don’t
belong.
Typically, the sediment deposited by a glacier can
vary in size from “flour” to large boulders.
4. Ocean Waves and Currents
Sediment transport is mainly due to long shore drift
If a beach runs out of sand up-shore, the rest of the beach will eventually erode.
Sea Arch is formed when waves erode softer rock at a faster rate. Eventually, a sea
arch collapses, forming a Sea Stack. Spit – caused by longshore drift. Sediments
will be deposited along a bay where the direction of the shore changes.
Baymouth Bar – when a spit grows across a bay
Wave-cut cliff – formed from the cutting action of the surf against the coast. (Bad
place to build a home!)
Barrier islands are often very long spits
5. Running Water – This is the major cause of erosion.
Rivers are erosional if they move fast
over a steep gradient.
Rivers carry sediment by:
Dissolved load –dissolved material
Suspended load – small particles that are
carried down a river suspended in the
water.
Bed load – larger particles that roll down
the bed of a river.
The progression of a river..This is in the rivers early stages, and is erosional.
Rivers become depositional when they slow down.
4 Depositional types of river deposits:
1. Changing directions (Meandering River)
As the river ages, it forms a wide, flat valley, where depostion occurs.
The flat valley is more developed, and the river becomes a meandering river,
forming oxbow lakes.
2. When Rivers overflow their banks, they become depositional.
3. Rivers become depositional when they enter standing bodies of water, called a Delta
4. Alluvial Fan - Formed above water, at the base of mountain ranges,
where the stream gradient changes from steep to flat. This deposits
sediment that is generally very coarse grained.
6. Groundwater:
Important terms: Porous, permeable, zone of aeration, zone of saturation, aquifer
Spring creek. Source of water is from groundwater that comes out of the ground.
Pure artesian water?
This is how a well runs dry.
Caves are the result of limestone
being dissolved by groundwater
containing carbonic acid.
The first part of forming a cave involves the dissolving of the limestone.
The second part of forming a cave involves the decoration of the cave.
Stalactite – cone of rock hanging from the top of the cave
Stalagmite – cone of rock deposit formed on the bottom of the cave.
Column – when a stalactite and stalagmite join
Flowstone – formed from water depositing minerals along the side of a cave.