Transcript Weathering and Erosion

Weathering – the chemical and physical breakdown of rocks at
or near Earth’s surface.
Two types of weathering:
1) Chemical weathering – breakdown of rock through a
change in mineral or chemical composition.
ex: when iron combines with oxygen to make rust
(oxidation)
ex: the effect of water on minerals (water is a
universal solvent)
2) Physical weathering – breakdown of rock into smaller
pieces without a chemical change.
ex: frost action – water going through
temperature changes, breaks the rock apart
ex: abrasion – when rocks grind against each
other
Factors that Affect the Rate and Type of
Weathering
 Exposure – generally the closer a rock is to the surface,
the faster it will weather
 Particle size – when rock particles are smaller, the total
surface area per unit volume exposed to weathering is
greater.
Factors that Affect the Rate and Type of
Weathering Cont.
 Mineral composition – different minerals have
different physical and chemical properties
 Climate – chemical weathering mostly occurs in warm,
moist climates. In cold climates, frost action is most
common (physical weathering).
Soil Formation
 Physical and chemical weathering processes are
important in the formation of soil. Soil is the mixture
of rock particles and organic matter on Earth’s surface
that supports rooted plants.
 Soils develop horizontal layers with distinctive profiles
 Soils can be transported from other areas by wind,
moving water, or glaciers.
Erosion
weathering of rock particles that are transported as sediments
 Erosion, shapes and lowers Earth’s surface
 Involves a transporting system which includes:
1) an agent of erosion, such as a stream, glacier,
wave, current, wind or human activity
2) the sediments being moved
3) a driving force (gravity)
 Energy from the Sun plays an important role: Sun’s
energy drives the water cycle, which produces
precipitation and running water
Gravity Erosion
Gravity is the driving force behind erosion
Gravity can pull rocks and sediments downhill, this is
called mass movements
Running Water Erosion and Streams
 Running water is the most common agent of erosion
 Running water erosion that is confined to a channel is
called streams
 A smaller stream that flows into a larger stream is
called a tributary
 Streams carry sediments in different ways:
1) dissolved minerals are carried in solution
2) solid sediments are carried in suspension
3) larger sediments are carried by rolling, sliding
or bouncing on the stream bottom (results in
rounding of sediments by stream abrasion)
Running Water Erosion and Streams
Continued
Over time, streams carve deeper channels and a
V-shaped valley forms
The area of land drained by any one stream is called its
watershed or drainage area.
Stream Velocity
 Three factors that determine average velocity:
1) gradient (slope of the stream)
2) discharge (volume of water in the stream)
3) stream channel shape (the shape of the bedrock or loose
materials that confine the stream)
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If the stream has a wide, flat stream channel, then there is a large
surface in contact with moving water (more friction)
If the stream has a semicircular stream channel, the velocity will be
greater
Velocity changes depending on the path of the stream:
-if the stream is straight, the greatest velocity exists in the
center of the stream
-if the stream curves, the greatest velocity exists on the outside
of the curves
A stream with greater velocity can carry larger sediment particles
Stream Velocity
Velocity changes depending
on the Shape of the Stream
Sediment Load changes
depending the Stream’s velocity
Evolution of a Stream

Streams slowly change their characteristics over time.
 In its uppermost part, the stream, starts as a finger lake.
 In early stages there are many abrupt changes resulting in waterfalls and
rapids
 Farther down the stream, more water is added to the stream (from
tributaries and ground water) to increase velocity
 As the stream grows larger, it begins to shift and meander (bend or curve)
 If there is flooding, a lake may form from a meander and/or floodplain
 Near the end or mouth of a stream there is often a fan-shaped delta.
Wind Erosion
* Two main points –
1) deflation
2) sandblasting (abrasion)
* Deflation takes place in areas with small, loose
sediments exposed to the atmosphere. Winds blow
away the loose sediments, lowering the land surface.
* Sandblasting occurs when winds blow sand or silt
grains and other objects. The pelting by the grains can
erode, or abrade rocks.
Glacial Erosion
 A glacier is a naturally formed mass of ice and snow
that moves downhill on land under the influence of
gravity
Mountain glaciers are formed in high mountain
valleys
Ice-sheet or continental glaciers cover huge landmass
regions
The glacier acts like a fluid, it moves fastest in the
center than on the sides
If more snow and ice accumulates than wastes away,
the glacier advances
If more snow and ice wastes away then accumulates,
the glacier retreats.
-When a glacier moves over land, rocks and other materials
beneath in freeze into the ice and are dragged along
-These sediments that are dragged along, scratch the surface
beneath them
-This action creates glacial grooves and glacial parallel
scratches
-The direction of exposed glacial grooves and scratches shows
the direction of former glacial movement
- The wide, thick ice of a glacier erodes its valley walls to form
a U-shaped valley
Wave and Current Erosion
 Waves and currents act as agents of erosion
 When waves enter shallow water near the shore, they
drag against the bottom. The waves then become
unstable and water rushes toward the shore as
breaking waves or surf.
 These waves continue to pound against the shore line
usually at an angle.
 Due to this angle, the water flows in one direction
along the shore. This is called a longshore current.
 Over time, parts of the shore erodes away
 In the zone of breaking waves, deposited sediments
can create a series of mounds called sandbars.
Wind
Erosion
Glacial
Grooves Striations
Glacial Flow
Longshore
current