External geological processes and landscapes

UNIT 8. EXTERNAL DYNAMICS OF THE EARTH

I. BASIC CONCEPTSII. TYPES OF LANDSCAPES

INDEX:I. BASIC CONCEPTS (section 1)

Exogenous geological processes: weathering, erosion, transportation and sedimentation.

Landscape shaping.

II. TYPES OF LANDSCAPES (sections 2-5) Aeolian landscapes Glacial landscapes Torrential landscapes Fluvial landscapes Karst landscapes Coastal landscapes

UNIT 8 – I. BASIC CONCEPTS

Weathering is the process of alteration and fragmentation of rocks by the joined action of the atmosphere, water and living organisms.

What is wheathering?

It is a static process: it does not involve movement.

Where does it take place?

There are two main classes of weathering processes:

a) Physical weathering: It’s the breakdown of rocks and minerals into small-sized particles through physical forces.

b) Chemical weathering: It’s the breakdown of rocks through chemical reactions causing changes in their mineral composition.

Living things can affect the rocks both physically (e.g. roots) and chemically (lichens) through their activity. We can consider them as a third type of weathering, called biological weathering.

e.g. Freeze-thaw weathering or frost wedging in high mountain areas.

e.g.: Hydrolysis, Oxidation, Carbonation and Dissolution.

e.g.: Thermal stress weathering in deserts.

Water flows into cracks.The freeze-thaw cycle is repeated, and eventually the rock cracks.

When the water freezes, it expands. The ice acts like a wedge.

Mechanical weathering

Freeze-thaw weathering is typical of mountainous regions where temperature fluctuates below and above the freezing point. Water in the rock cracks, freezes and melts repeatedly overtime, acting as a wedge and causing them to break.

Freeze – thaw weathering:

As a result of this process, great piles of rocky fragments called scree are formed.

Hydrolysis of granite

Granite

Gravel

Hydrolysis processes take place when slightly acid rainwater dissolves some minerals like feldspars and transforms them into clay minerals. Granite is a hard and resistant rock, but becomes weakened and disintegrated when feldspars are broken down by hydrolysis and quartz crystals remain as loose debris, unaffected.

There are some factors which can affect the processes of weathering.

In general, weathering is more efficient when climatic conditions are extreme.

• Hot and wet climate conditions enhance chemical weathering. Why is this?

• Low or changing temperatures and wet conditions promote physical weathering.

• Weathering processes are typically slower in dry regions, whether cold or hot.

What is erosion?

What is transportation?

What is sedimentation?

It is the removal of weathered materials due to the action of water, wind, ice and gravity. It is a dynamic process that involves movement and wears rocks away.

Eroded materials are carried away by gravity, wind, water or ice.

Deposition and accumulation of transported materials in sedimentary basins (depressions in the surface of the land or in the ocean floor). This happens when the agent loses energy.

Landscape shaping

Some landscapes are fascinating: river canyons, coastal cliffs, huge sand dunes, waterfalls… The responsible agents for these breathtaking reliefs are called external geological agents.

The geological agents are liquid water, ice, and air when they move:

• Wind• Glaciers• Surface runoff• Rivers• Groundwater• Oceans

All these agents are powered by the Sun or the Earth’s gravity. The sun heats the air and water and causes the wind and oceanic currents. Gravity forces water and ice to move towards lower areas.

What geological agent has shaped these landscapes?

Landscape shaping takes place through three different processes:

a) Erosion: it is the removal of the weathered rocks away from the site of weathering.

b) Transportation: eroded materials are carried away to another place.

c) Deposition: it’s the laying down of the transported rock

materials. The resulting materials are called sediment.

Erosion, transportation, and deposition shape landscape

creating landforms (ex. valleys, cliffs, dunes, etc.). The relief

of Earth’s surface is the result of destructive processes

tearing down the land, and constructive processes building it

up.

• If erosion exceeds deposition, part of the surface

relief is destroyed. Destructive processes are

dominant and create erosional landforms.

• If deposition exceeds erosion, new relieves are built

up. Constructive processes are dominant and create

depositional landforms.

Are these erosional or depositional landforms?

UNIT 8 – II. TYPES OF LANDSCAPES

How does wind shape landscape? Aeolian landscapes

Wind is not a very powerful agent. Aeolian processes are only important in arid environments with sparse vegetation and a large supply of fine and unconsolidated (loose) sediment.

Wind removes and transports only fine particles from the soil surface, leaving the larger ones. When the wind stops or reduces its speed, suspended particles are deposited.

Aeolian erosional processes and landforms:

Process Landform

Deflation (the wind removes fine, loose particles from the

ground)

Rocky deserts or pavements (reg).

Abrasion (suspended particles erode

the rocks close to the surface)

Mushroom-shaped rocks

Aeolian depositional processes and landforms:

Process Landform

Wind reduces its speed and deposits sandy particles

Dunes (ex. Barchans) and dune fields. Sandy

deserts or ergs.

The finest particles of dust travel very long distances. Loess

Gravel desert or desert pavement (Reg)

Deflation and sand column

Mushroom-shaped rocks created by abrasion

Erg Barchan

Loess deposits are formed far away from deserts.

Arches National Park (Utah)

How do glaciers shape landscape? Glacial landscapes

Glaciers are moving ice masses. They form where there is more snowfall in winter than melts during summer. There, snow accumulates over time, and turns slowly into ice which starts moving downhill due to gravity.

They are only in polar regions or in high mountains.

Continental glaciers

Alpine glaciers

Cirque

Tongue

Glacial erosional processes and landforms:

Near the mountain tops, ice causes intense mechanical weathering and erosion, forming very steep relieves with vertical walls and sharp peaks called horns.

CIRQUE

Tarn


Cirques, where snowfall accumulates and turns into ice, are round bowl-shaped depressions. When glaciers disappear, they are often occupied by round lakes called tarns.


Glacial tongues erode the bedrock forming characteristic U-shaped valleys. Ice pulls chunks of rock up into the ice and scrape the underlying bedrock due to abrasion.

Striations are common in rocks of glacial areas.

Glacial depositional processes and landforms:

Glacial erosion produces much rock debris which is dragged and deposited by glacial tongues forming moraines.

Cirque

Tongue

Tarn Steep relief HornHanging valley Melting lake

Moraines

When glaciers disappear they leave very characteristic landscapes in mountainous regions.

How do rainfall, surface runoff and temporary watercourses shape landscape? Torrential landscapes

How do rivers shape landscape? Fluvial landscapes

They are the result of liquid water running off the surface!!Liquid water is probably the most important geological agent in the

world.

How do rainfall, surface runoff and temporary watercourses shape landscape? Torrential landscapes.

In arid regions with scarce vegetation and sporadic but torrential showers, rainfall intensity quickly exceeds soil infiltration capacity and important surface runoff is produced, thus eroding and shaping landscape.

Raindrops cause mechanical weathering and disintegrates soil materials.

Torrential erosional processes and landforms:


Surface runoff can erode and transport big amounts or fertile soil: this is called soil loss.

Sometimes runoff concentrates on certain flow paths and cuts a well defined channel on the hillside, that grows from small gullies to ravines or deep gorges.


GULLIES

GULLIES

RAVINES



GORGES

Between gullies or ravines, there may be tall and thin spires left, called fairy chimneys.


Torrential depositional processes and landforms:

Intense runoff transports a lot of soil particles which are deposited at the end of ravines forming alluvial fans or alluvial cones. Alluvial fans are high flood risk areas.

ALLUVIAL CONES

Gullies

Ravine

Alluvial fan Fairy chimneys

Alluvial cone

How do rivers shape landscape? Fluvial landscapes

UPPER COURSE

- Steep slope-Fast flow- Erosion is dominant- V-shaped valleys- Waterfalls and rapids- Gorges and canyons

MIDDLE COURSE

- Gentler slope- Lower velocity- Erosion and deposition are equally important.

-Transportation is dominant.- Wide flat-floored valleys.- Floodplains and terraces- Meanders and oxbow lakes

LOWER COURSE

- Very gentle slope- Very slow flow- Deposition is dominant- Very Wide floodplains.- Deltas

In wetter regions, where rains are not torrential, but maintained and regular, water forms rivers. Rivers are permanent watercourses, usually freshwater, flowing towards another river, an ocean, or a lake.

The upper course:

The river flows fast down a steep slope. Erosion is dominant. Rivers cut a narrow V-shaped valley. Waterfalls, rapids, gorges and canyons are common too.

WATERFALLS

The middle course:The velocity of the water decreases as the slope gets gentler. Erosion and deposition are equally important. Transportation is dominat. The river forms a wider flat-floored valley, meanders, and a well-developed floodplain.

The middle course: When the river erodes its bed deeper, it creates a new floodplain. The “hanging” old floodplain is called a fluvial terrace.

FLOODPLAIN

FLUVIAL TERRACES

The lower course:

The slope flattens out and the water flow gets slower. Deposition is dominant. Valleys are very wide, with large floodplains. Most materials are deposited at the mouth where a delta may appear.

TORRENTIAL LANDSCAPE

FLUVIAL VALLEY (MIDDLE COURSE)

How does groundwater shape landscape? Karst landscapes

Depending on soil characteristics and on rainfall intensity, a certain amount of rainwater infiltrates the ground, thus becoming groundwater. Groundwater fills the pores and grooves in the rocks forming aquifers.

When acidic water containing carbon dioxide gets into rock cracks, some minerals, such as calcium carbonate or calcite (found in limestone) and gypsum can be dissolved. The rock is chemically weathered.

Then many erosional landforms, such as caves and sinkholes, are formed.

Acidic water gets into the rock joints and dissolves calcium carbonate, creating caves.

When the roof of a cave has been intensely weathered, it collapses and a sinkhole appears.

SINKHOLE and CAVE FORMATION

Sinkholes

If the water discharges some of its dissolved carbon dioxide, then calcium carbonate dissolved into water precipitates forming depositional features.

In caves, dripping water forms:

stalactites (conic deposits which grow downward from the cave ceiling)

stalagmites (which grow upward from the cave floor).

In karst landscapes, disappearing streams and reappearing springs are also common.

Stalactites and Stalagmites

Above-ground depositional features

Disappearing Streams and Springs

SinkholeCaves Stalactites

Stalagmites

Disappearing stream

Reappearing spring

How do oceans shape landscape? Coastal landscapes

Coastlines are very dynamic features, changing shape continually. In coastal landscape, moving ocean water is the geological agent involved.

There are three types of ocean water motions, which affect coastal landscapes:a) Waves: They are created by wind blowing the surface of the ocean. Waves strike the shore repeatedly and disintegrate the rocks mechanically. They are also very effective agents of erosion, transportation and deposition.

Destructive waves: If the backwash is stronger than the swash, the dislodged particles are removed (erosion).

Constructive waves: If the swash is stronger than the backwash, deposition occurs.

So they also transport materials along the shore.

b) Tides: They are the cyclic rise and fall of seawater due to the Moon’s and Sun’s gravitational attraction. They intensify the action of waves: their erosive power increases at high tide and deposition mainly takes place at low tide.

c) Longshore currents: They are gradual movements of water parallel to the shore. They appear when prevailing wind blows regularly at the same angle to the coastline. They can transport many materials along the shore (beach drift).

Coastal erosional processes and landforms:

Wave action over the most exposed parts of the shore shapes vertical rocky walls called cliffs. Cliffs retreat progressively over time, giving rise to a growing flat rocky surface called wave- cut platform.

This is the process:

Wave erosive action can also create other amazing landforms such as arches, caves and stacks.

Coastal erosional processes and landforms:

Depositional landforms:

Constructive waves normally deposit these sandy particles in sheltered areas, like bays. The accumulation of sand from erosion forms beaches.

Depositional landforms:

Longshore currents also transport and deposit big amounts of sand. They can form spits: extended stretches of beach material that project out to sea and are joined to the mainland at one end.

Currents can also form tombolos: sand bars that connect a rocky island to the mainland.

Spit Tombolo

Beach

Cliffs

ArchStack

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External geological processes and landscapes