Unit 2 - Erosion and Deposition

Erosion and Deposition

• Erosion and Deposition help to form many of the features we see in the world

• Eg:

  • Features along rivers such as bank erosion (the weathering away of a stream or river)

• This is because of the way in which the agents of erosion and deposition act on the natural world

• The five agents of erosion and deposition are:

  • Gravity

    • The force that pulls things down

    • The underlying agent for all four other agents

    • Cause erosion by mass-wasting, which is the down slope movement of rock and sediments where transporting agents like streams and glaciers cand then pick up the material and move it to even lover elevations

  • Water (Rivers)

  • Glaciers (Moving Ice)

  • Sea waves

  • Wind

Erosion

• The process by which soil and rock that has been broken down by weathering is moved from one area of the earth’s surface by natural processes, like wind or waterflow, and then deposited in another location

• Transportation

Deposition

• Occurs when eroded material is deposited in a new location

• The end of the process that started with weathering

• Depositing (Placing in the final location)

Rivers

• From the river source (where the river starts flowing) to the ocean passes through 3 stages:

  • Youth stage

  • Mature stage

  • Old age stage

• Each is characterised by features of erosion and deposition

Youth Stage of Rivers

• The beginning of a river

• This river flows quickly and with a lot of energy

• Smaller and has waterfalls (Steep slopes)

• It cuts a narrow channel through mountains

• The youth stage is characterised by:

  • Rapid vertical erosion (Downward erosion that deepens a river channel in the ground)

  • Mountains

  • Steep slopes

  • V-Shaped Valleys

  • Waterfalls

  • Rapids (Fast moving parts of a river caused by a steep descent in the riverbed)

Mature Stage of Rivers

• A wide valley with a wide floor

• More lateral (side) erosion resulting in a U-Shaped valley

• Gentle slopes

• A narrow flood plain (Level land along the banks of a stream)

• Meanders (Bends in a river)

Old Age Stage of Rivers

• The relief around the river becomes very flat

• River flow is very slow

• Meandering (Bends in a river) courses are dominant

• Deposition is dominant because the flow is slow and more meandering often causing an oxbow lake (U-Shaped Lake that is formed on a river plain when a river channel is cut off from a main stream) to develop

The sea (Erosion, Deposition and Wave Action)

• Wave action has an enormous amount of power that causes erosion and deposition

  • Deposition associated with wave action often cause spits and barrier islands.

    • Spits - When waves break along the shore and form sand bars that extend away from the shore

    • Barrier Islands - Form when the land along the shore is flat or slopes gently. Act as the first layer of defence against storms

  • Forms beaches

    • Happens when rivers carry sand, soil and rocks that comes from erosion of mountains and land areas of the continents to the shore

• As a result, there are many features associated with wave action

  • Erosion by wave action is primarily caused by:

    • Hydraulic action

      • The force of the water on the cliffs or coastal rocks. The force of water has the power to break the rocks apart

    • Corrasion (Or abrasion)

      • When rocks and pebbles are flung by waves wear the coastal rocks away

    • Attrition

      • When eroded rock particles are worn down, becoming small and rounded

    • Corrosion (erosion by chemical action)

      • When certain rocks and minerals such as chalk and lime are dissolved in water

Features asosiated with wave action

• A Stack

  • An isolated pillar of rock that has been separated from a headland by coastal erosion

  • It is formed by the collapse of an arch

• An Arch

  • A natural bridge formed where a cliff is subjected to erosion by the sea

• A stump

  • A low out crop of rock formed by the erosion of a coastal stack

• A Sea Cave

  • A hollow opening in the base of the sea cliff formed by waves acting on weak parts of the rock

• A Wave Cut Platform

  • A flat surface at the base of a sea cliff formed by wave erosion

• A Headland

  • A narrow piece of land that projects from a coastline into the sea

• A Cliff

  • A steep rock formed by wave erosion along costal landforms

  • Cliffs are formed by processes the start with the erosion of rock at the foot of the slope when it is acted on by powerful waves

  • This is called abrasion (The scraping or wearing away of rock by small stones when they rub against the rock)

• A Bay

  • An area of the ocean, sea or lake enclosed by land but with a wide mouth that allows access to the sea

Moving Ice

• Moving ice is called a glacier

• Moving ice causes erosion by abrasion and plucking

  • Abrasion is the scraping process caused by the impact of rocks caried in ice upon the bedrock

  • Plucking is the process by which blocks or pieces of bedrock under the glacier are pulled from the surface as the glacier moves

• Features of glacier erosion includes:

  • Horns

    • Pyramidal peaks formed when cirques erode a mountain from 3 or more sides

  • Aretes

    • Narrow saw-like ridges found in glaciated areas

  • Cirques

    • Bowl-shapped depressions found at the head of a glacial valley

Glacial Deposition

• Called till or moraine

  • Till is boulder clay or other sediments deposited by a glacier

  • Moraine is the till deposited along the edges of a glacier

    • Terminal (End) Morine is deposited at the furthest advance of a glacier

    • Recessional (Retreats) moraine is deposited when a glacier retreats

    • Lateral (Side) moraine is a narrow belt of debris deposits along the side of the glacier

      • When 2 glaciers move at the same time their lateral moraines can merge to form a medial moraine

Wind erosion

• Wind has the ability to erode, transport and deposit material

• It is not a very effective method of erosion on its own, but when wind picks up sand particles its power to erode becomes effective

• Two important processes in wind erosion are abrasion and deflation

  • Abrasion is when sand particles carried by the wind wear down on rocks

  • Deflation is when wind removes loose sand particles from the ground to produce a lowered surface level

Wind erosion Features

• Mushroom logs (or pedestals)

  • When the upper parts of the rocks are less eroded than the bottom leaving a mushroom shape

• Hammadas (Bare bedrock areas exposed by wind erosion)

• Deep groves

• Depressions

• Tapered hills

• Yardangs (Landforms about 10m high and 100m long aligned to the prevailing wind, steep slopes facing the wind direction)

Sand Dunes (4 types)

• Wind depositions mainly results in the formation of sand dunes

• A sand dune is a ridge of windblown sand

Four common dune forms:

• Barchans

  • Occurs when there is a limited

    supply of sand and constant wave direction

  • Gentle windward slopes

  • Steep slip face

• Winding traverse

  • Form in areas where there is plentiful supply of sand

  • Gentle windward slopes

  • Steep slip face

• Longitudinal (or liner)

  • Run parallel to the wind directions

  • Variable wind direction

  • Equal slopes

• Blowout (or parabolic)

  • Occurs where vegetation anchors

  • Gentle windward slopes

  • Steep slip face