Weathering, Erosion and Deposition

Shaping the Earth’s Surface

Weathering

• the process of breaking down rocks into smaller fragments

Erosion

• the transport of rock fragments from one location to another

Deposition

• the settling of rock fragments in a new location

Weathering

Breakdown of earth materials, brought about by weather at the earth’s surface

There are three types of weathering:

• Physical

• Chemical

• Biological

Physical Weathering

rocks are broken down by physical means (there is no change in the chemicals making up the rock)

Ice Wedging

•Water fills cracks in rocks, then freezes

•As water freezes, it expands and pushes the rock apart.

Wetting and Drying

• Clays swell and shrink, causing rocks to fall apart.

Wind and water effects

•Wind and water move sand and small rocks around, rubbing them against each other until they gradually wear into smaller fragments.

Chemical Weathering

Rocks are broken down by chemical reactions, which causes the rocks to change their composition and become weaker.

• Chemicals in rain, snow and air break down some minerals in rocks.

• Some minerals such as feldspar, hornblende and augite react with water to form clays (hydrolysis).

• Oxygen from air combines with iron in some minerals to form rust (resulting in red soil - oxidation).

• Many minerals, such as limestone and marble, dissolve easily in weak acids such as rainwater. Rainwater is usually slightly acidic because carbon dioxide dissolves in it to form carbonic acid.

• Acid rain forms stronger acids which can speed up the rate of chemical weathering.

Biological Weathering

If physical or chemical weathering has been caused by a plant or animal, the process is sometimes referred to as biological weathering.

• Plant seeds or roots may grow in tiny cracks in rocks.

• As the plant grows, it cracks the rock.

• Burrowing animals move rocks around and can expose new rock surfaces to weathering.

• Rotting vegetation forms acids which also lead to chemical weathering.

Weathering and Minerals

• Quartz is very resistant to weathering (doesn’t react with water, oxygen or acids, and has no cleavage).

• Other rock-forming minerals (feldspar, horneblende, mica, augite, calcite) are easily mechanically weathered into small fragments and chemically weathered into clay.

• Calcite, gypsum and halite can also be dissolved in water.

Weathering and Rocks

• IGNEOUS ROCKS weather faster in wet climates due to ice wedging and hydrolysis. They generally weather into clay minerals with quartz pebbles and sand.

Weathering and Rocks

• SEDIMENTARY ROCKS are only as durable as the cements that hold them together; calcite cement weathers quickly while silica cement is VERY durable.

Weathering and Rocks

• METAMORPHIC ROCKS are resistant to mechanical weathering, but susceptible to chemical weathering; for example, marble is a great building material in a dry climate, but not a wet one!

• RATE OF WEATHERING DEPENDS ON:

– type of rock

– amount of rock surface exposed

– climate (warm and wet vs. cold and dry)

SOILS

• loose, weathered rock and organic material in which plants with roots can grow.

• Composed of sand, silt and clay.

• Composition varies by location

• Always a precious resource!

• Parent material – rock from which the soil is formed

• Residual soil – soil formed from underlying bedrock

• Transported soil – deposited by wind, rivers or glaciers.

Types of Soil

• Sandy soil allows water to pass quickly through it

• Clay-rich soil holds water, forming a thick muc

• Soil with some clay and some sand is called loam

• Loam is usually the best soil for farming

Soil Profile

• Cross-section of earth which shows the layers of soil

• 3 distinct zones in mature soils

1) A- Horizon: Topsoil • dark coloured (gray or black)

• contains humus (remains of plants and animals broken down by bacteria)

2) B-Horizon: Subsoil • Lighter-coloured (red or brown)

• Often has more clay than topsoil, since rainfall has washed it downwards

3) C-Horizon: Weathered rock • Mixture of soil and weathered rock directly above

bedrock

Erosion, Deposition and the Landscape

• Erosion is the movement of weathered rock from one place to another.

• Causes of erosion include moving water, moving ice (glaciers), wind, and gravity (i.e., landslides).

moving water, glaciers and wind can also deposit the materials they pick up

Deposition is when eroded rock material stops moving and settles (making “deposits”).

Landscapes formed by Moving Water

• sediment (bits of weathered rock) can be carried long distances by moving water before being deposited at another location

• Over millions of years, rivers can transport billions of tonnes of sand, gravel and mud.

• Running water, using sand and pebbles as cutting tools, wears away at the river bed in a process called abrasion.

• Rivers abrade the land on its sides as it flows, making a deeper and deeper valley over time.

River Valleys

• Young river valleys are narrow, fast-flowing and straight.

• Old river valleys have eroded sides and are v-shaped

• When a river crosses a flat region, it slows down and develops wide, shallow valleys

• Rate of erosion of the river valley depends on:

– Slope of the valley

– Amount of water

– Speed of the water

– Type of rock in the valley

Floods and Flood Plains

• A flood plain is the flat land beside a river that is covered by water during flooding

• As flood water drains away, fine mud from the floodwater is deposited on the flood plain.

• The buildup of mud produces a rich soil which can be valuable for agriculture

• Dams and raised banks (dykes) are often used to prevent flooding, but if water rises too high the flood may break through them

Deltas

As rivers flow into lakes and oceans, the flow of water suddenly slows down

Sediments are deposited when the water slows

The sediments build up at the mouth of the river and fan out into a giant triangle shape called a delta

Landscapes formed by Glaciers

• 2/3 of Earth’s fresh water is trapped in large masses of snow and ice called glaciers

• glaciers form when more snow falls than melts every year, causing it to build up over time

• Glaciers “advance” when ice builds up faster than it melts so that the ice creeps further down into a valley

• Glaciers “retreat”when temperatures begin to shrink glaciers. NOTE: they don’t actually ‘retreat’…the leading edge melts away.

Types of Glaciers

• Large ice sheets, called continental glaciers, cover both the Earth’s poles, in Greenland and Antarctica

• Alpine (or valley) glaciers form in mountain areas, and slowly slide down the mountain due forces of gravity

Ice Ages

• Periods of time when the climate was cooler than it is today

• Ice sheets and glaciers covered large areas of Europe and North America

Erosion

Glaciers

• as glaciers move, they scrape and erode the ground beneath them

• scratch marks, called striations, are left on rocks over which a glacier passed.

Erosion Features

Cirque

Bowl-shaped valley at the head of a glacier

Arête

Narrow ridge (high ground) between two cirques

Horn

Pyramid-shaped peak located between three cirques

U-Shaped Valley

Glaciers grind out large valleys as they are pulled downhill by gravity

Hanging Valley

U-shaped valley cut off by a bigger valley created by a larger glacier

Fiord

A narrow inlet of ocean between steep cliffs carved by glaciers

Deposition

moving water and glaciers can also deposit the materials they pick up

Erratics

• Large boulder deposited on the ground by a glacier

Eskers

• Winding ridge of material deposited by a stream running under a glacier

Moraines

• Ridge of rocky material deposited by a glacier. Can be found at the sides and farthest advance of a glacier.

Outwash

• Material deposited by water from melting glaciers

Landscapes shaped by Gravity

• sediments on slopes slowly work their way downwards by the force of gravity – “creep”

• if wet soil and rock on a slope becomes too heavy, landslides may result

Landscapes Shaped by Wind • Air currents can carry rock particles with it.

• The size of the particles that can be moved depends on the speed of the wind

• Wind erosion is most common in dry climates and in flat, open areas