Exogenic Processes , Weathering, Erosion, Mass Wasting and Deposition

-------

Exogenic Processes

The geological phenomena and processes which occur on and originate from the Earth's surface. These force molds and gives shape to the Earth. The main exogenic processes include weathering, mass wasting, erosion and deposition.

Weathering

Weathering is the break down or dissolving of rocks and minerals on Earth’s surface by different means such as action of water, temperature changes and other activity.

Types of Weathering

1.)    Physical ( or Mechanical ) Weathering

It is the process in which the rock is physically broken or disintegrated into smaller pieces due to any physical factors or force ( such as wind, water, ice, sun, etc) without any alteration of its composition.

The following are the major processes that lead to the mechanical weathering of rocks:

a. Frost wedging- when water gets inside the joints, alternate freezing and thawing episodes pry the rock apart. The water expansion pushes the rock apart.

b. Salt crystal growth- force exerted by salt crystal that formed as water evaporates from pore spaces or cracks in rocks can cause the rock to fall apart

c. Abrasion – wearing away of rocks by constant collision of loose particles
    
c. Exfoliation/Unloading - peeling away of large sheets of loosened rock materials

d. Biological Activities - caused by an organism’s activity such as plant roots, burrowing animals & humans

e. Friction and impact

f. Temperature changes
    The sudden cooling of a rock surface may cause it to contract so rapidly over warmer rock beneath that it flakes or grains break off.  This usually happens in deserts, where intense daytime heat is followed by rapid cooling after.

 Chemical Weathering

It is the process that decomposes rocks through chemical reactions that alters the composition of the original rock-forming minerals.

The following are the main processes that leads to chemical weathering :

a.    Dissolution – dissociation of molecules into ions such as dissolution of limestone in acidic water. This process is sometimes called carbonation.
Ex. sinkholes, caves ; CO2 + rainwater = Carbonic Acid

b.    Oxidation- when minerals in rocks react with the oxygen dissolved in water
Ex. water + iron + oxygen = iron oxide (rust)

c.   Hydrolysis- reaction between minerals and the water itself. Ex. calcites in caves

c.    Living organisms - Lichens that grow on rocks produce weak acids that chemically weather rock.

d.    Acid rain - compounds from burning coal, oil and gas react chemically with water forming acids. Acid rain causes very rapid chemical weathering



Factors that affect the rate, type and extent of weathering

a. Climate – areas that are cold and dry tend to have slow rates of chemical weathering and weathering is mostly physical; chemical weathering is most active in areas with high temperature and rainfall.

b. Rock type – the minerals that constitute rocks have different susceptibilities to weathering. The susceptibility of minerals (from high to low) roughly follows the inverse of the order of crystallization of minerals in the Bowen’s reaction series. Thus, olivine which crystallizes first is the least resistant whereas; quartz which crystallizes last is the most resistant.

c. Rock structure- rate of weathering is affected by the presence of joints, folds, faults, bedding planes through which agents of weathering enter a rock mass. Highly jointed/ fractured rocks disintegrate faster than a solid mass of rock of the same dimension

d. Topography- physical weathering occurs more quickly on a steep slope than on a gentle one. On a gentle slope, water may stay longer in contact with the rocks, hence chemical weathering is enhanced.

e. Time- length of exposure to agents of weather determines the degree of weathering of a rock. The longer time of exposure to agents of weathering results to higher rate of weathering.

Mass Wasting

Mass wasting is the process of downslope movement of rock, regolith, sand and soil under the direct influence of gravity. It is also known as mass movement or slope movement.

It moves materials from higher to lower elevations. The streams or glaciers can then pick up the loose materials and eventually move them to a site of deposition.

Types of Mass Wasting

They are classified based on what materials are involved, the type of movement, and the rate of movement and are controlled by the slope angle, water saturation, and presence of clay. These are classified into two broad categories: slope failures and sediment flow.

A.) Slope failures

Slope failures are sudden failure of the slope that results to the transport of debris downhill by sliding, rolling, and slumping.

1. Slump. It is a type of slide wherein downward rotation of rock or regolith occurs along a curved surface. Slump usually results when the geometrical stability of a slope is compromised by the undercutting of its base, such as by wave action, a meandering river, or construction.

2. Rock fall and debris fall. These are the result of free falling of bodies of rocks that break loose or a mixture of rock, regolith, and soil in the case of debris fall. Rockfalls occur when pieces of rock dislodge from a steep rock face or cliff.

3. Rock slide and debris slide. It involves the quick displacement of masses of rock or debris along an inclined surface.

4. Landslide. A type of mass movement from a sudden rapid event in which large quantities of rock and soil plunge down steep slopes. This includes all downslope movement whether it be bedrock, regolith, soil, or a mixture of these.

B.) Sediment flow

In sediment flows, materials flow downhill mixed with water or air; Slurry and granular flows are further subdivided based on velocity at which flow occurs

1. Slurry flow – water-saturated flow which contains 20-40% water; above 40% water content, slurry flows grade into streams

• Solifluction – common wherever water cannot escape from the saturated surface layer by infiltrating to deeper levels; creates lobes and sheets of debris. The flow of water-saturated earth material over an impermeable surface such as permafrost.

• Debris flow – results from heavy rains causing soil and regolith to be saturated with water; commonly have a tongue-like front; Debris flows composed mostly of volcanic materials on the flanks of volcanoes are called lahars.

• Mud flow – highly fluid, high velocity mixture of sediment and water; can start as a muddy stream that becomes a moving dam of mud and rubble; differs with debris flow in that fine-grained material is predominant;

2. Granular flow

Granular flows contains low amounts of water, 0-20% water; fluid-like behavior is possible by mixing with air

• Creep – slowest type of mass wasting requiring several years of gradual movement to have a pronounced effect on the slope ; evidence often seen in bent trees, and offset in roads and fences. Creep occurs when regolith alternately expands and contracts in response to freezing and thawing, wetting and drying, or warming and cooling.

• Earth flow – involves fine-grained material such as clay and silt and usually associated with heavy rains or snowmelt; tend to be narrow tongue-like features that that begin at a scarp or cliff

• Grain flow – forms in dry or nearly dry granular sediment with air filling the pore spaces such as sand flowing down the dune face

• Debris avalanche – very high velocity flows involving huge masses of falling rocks, debris, soil, and trapped air racing down in very steep mountain ranges. The rock and debris break up and pulverize on impact.

Events that cause mass wasting processes

a. Shocks and vibrations

Earthquakes and minor shocks such as those produced by man-made explosions and passage of heavy trucks on the road

b. Slope modification

This creates artificially steep slope thus it is no longer at the angle of repose

c. Undercutting

Made by streams eroding banks or surf action undercutting a slope

d. Changes in hydrologic characteristics

Heavy rains may lead to water-saturated regolith thereby increasing its weight, and reducing grain to grain contact and the angle of repose;

e. Changes in slope strength

Constant weathering weakens the rock and leads to slope failure;
Vegetation holds soil in place and slows the influx of water;
The tree roots further strengthen slope by holding the ground together

f. Volcanic eruptions

They produce shocks; thereby causing the production of large volumes of water from melting of glaciers during eruption, resulting to mudflows and debris flows

Erosion and Deposition

Erosion is the geological process that transports material by a mobile agent such as water, wind, or ice (usually in the form of glacier).  It is the action of surface processes that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location.

Deposition is the exogenic process that happens after erosion. This process adds sediments, soil and rocks to a landform or land mass.

1. Water deposition
Where a river meets the ocean is called the mouth of the river. Soil carried by a river is deposited at the mouth and new land is formed.  This new soil-rich land is known as a delta.

2. Wind Deposition
Sand dunes are large deposits of sand dropped when the wind stopped blowing. The location of the sand dunes shifts frequently.      

3. Glacial Deposition
When glaciers melt, they drop or deposit the rocks they were carrying.
Forces That Cause Erosion

1.) Water

This is considered to be the main cause of erosion on Earth. Some of the ways that water causes erosion are by :

Rivers and Streams. They create a significant amount of erosion since they break up particles along the river bottom and carry them downstream. One great example of river erosion is the Grand Canyon which was formed by the Colorado River.

Stream - a body of running water, confined to a channel that runs downhill under the influence of gravity.

• Headwaters - upper part of stream near its source in the mountains
• Mouth - place where a stream enters sea,   lake or larger stream
• Channel - a long, narrow depression eroded by a stream into rock or sediment
• Stream banks - sides of channel
• Streambed – channel bottom

Drainage basin - the total area drained by a stream and its tributaries
Tributary - a small stream flowing into a larger one

Drainage pattern - the arrangement, in map view, of a stream and its tributaries
a. Dendritic - drainage pattern resembling the branches of a tree
b. Radial pattern - streams diverge outward like the spokes of a wheel
c. Rectangular pattern - tributaries have frequent 90° bends and join other streams at right angles
d. Trellis pattern - parallel streams with short tributaries meeting at right angles

Stream Erosion

Stream velocity controlled by stream gradient (slope), channel shape and channel roughness.

Streamflow erosion occurs by :
Abrasion, the process of sediments wearing down the bedrock and the banks
Attrition, collision between sediments breaking into smaller more rounded pebbles
Hydraulic action, the force of water against the banks compressing air pockets into cracks, which expand and fracture the rock over time
Solution, the process of dissolving soluble sediment by action of acidic water

Floodplain - flat valley floor composed of sediment  deposited by the stream.
Canyons - large valleys created by a river or stream

Sediment Transportation

Sediment load transported by a stream can be subdivided into:
1. Bed load - large or heavy particles that travel on the streambed.
2. Suspended load - sediment that is small/light enough to remain above the stream bottom by turbulent flow for an indefinite period of time.
3. Dissolved load - dissolved ions produced by chemical weathering of soluble minerals upstream.

Sediment Deposition

Sediments are temporarily deposited along stream course as bars and floodplain deposits, and at/near its end as deltas or alluvial fans

• Bars - ridges of sediment (usually sand or gravel) deposited in the middle or along the sides of a stream.
• Braided streams - contain sediment deposited as numerous bars around which water flows in highly interconnected rivulets.
• Delta - body of sediment deposited at the mouth of a river when flow velocity decreases
• Alluvial fan - large, fan- or cone-shaped pile of sediment that forms where stream velocity decreases as it emerges from a narrow mountain canyon onto a flat plain.

Flood - occurs when water overflows or inundates land that's normally dry. Most common is when rivers or streams overflow their banks.

Ocean or Sea waves. They can cause the erosion on coastlines. The shear energy and force of the waves causes pieces of rock and coastline to break off changing the coastline over time. Waves are caused by wind

Three factors that affects the wave:
1. Wind speed - the greater the wind speed, the larger the waves.
2. Wind duration - the greater the duration of the wind (or storm) the larger the waves.
3. Fetch - the greater the fetch (area over which the wind is blowing - size of storm) the larger the waves.

Types of waves
Constructive waves        Destructive waves
Low energy            High energy
Long wavelength        Short wavelength
Stronger swash        Strong backwash
Built coast/ beach        Erode coast/beach

Waves behaviour
1. Reflection waves - bounce off
2. Refraction waves - change in direction
3. Diffraction waves - bend through an opening
4. Interference - waves affect each other
5. Resonance - slash and fro

Beach - a landform along the coast of an ocean, sea, lake or river; it is formed by constructive waves.

Parts of a Beach
1. Marine terrace - sloping platform that maybe exposed at low tide
2. Beach face - section exposed to wave action
2. Wrack line - organic and inorganic debris is deposited by wave action
3. Berm - a wave-deposited sediment platform that is flat or slopes slightly landward
4. Spit - fingerlike ridge of sediment that extends out into open water formed by long shore drift
5. Bay-mouth bar - a ridge of sediment that cuts a bay off from the ocean which is formed by sediment migrating across what was earlier an open bay
6. Tombolo - a bar of sediment connecting a former island to the mainland

Coast - the area where land meets the sea or ocean.

• Coastal Landforms – Cliff retreat
A wave cut platform is the platform that’s left behind as the cliff retreats.

• Headlands and Bays
Headland and bays form where there are alternating bands of resistant (hard rock) and less resistant (soft rocks) along a coast.
Coves
Cove is a wide, circular bay with a narrow entrance. They form where there’s a band of hard rock along a coast with a band of softer rock behind it.

2.) Wind

Wind is one type of erosion that occurs especially in dry areas. Wind can erode by picking up and carrying loose particles and dust away (called deflation).

It can also erode when these flying particles strike the land and break off more particles (called abrasion).

Soil Movements Due to Wind Erosion

Suspension This moves fine particles (less than 0.1 mm) of dirt and dust over long distances.

Saltation This process moves soil particles (0.1 – 0.5 mm) across a surface by a series of short bounces along the surface of the ground, and dislodging additional particles with each impact. It is the primary means of soil movement.

Creep It occurs when larger soil particles (0.5 – 1 mm) slide and roll over an area and meet particles that have been through saltation.

3.) Glaciers

Glacier is defined as a moving body of ice on land that moves downslope or outward from an area of accumulation. They are giant rivers of ice that slowly move carving out valleys and shaping mountains.

Glaciers move to lower portions and elevations by means of plastic flow because of the great stress on the ice at depth, and basal slip. This is facilitated by meltwater which acts as lubricant between the glacier and the surface over which it moves.

The velocity of a glacier is lowest near the base and where it is in contact with valley walls; but the velocity is highest near the top center of the glacier.

It is part of a subsystem of the hydrosphere known as the cryosphere.  The cryosphere is those portions of Earth's surface where water is in solid form, including sea ice, lake ice, river ice, snow cover, glaciers, ice caps.

Types of glaciated terrain on the Earth’s surface

a. Alpine glaciation is found in mountainous regions.
b. Continental glaciation exists where a large part of a continent (thousands of square kilometers) is covered by glacial ice.
c. Valley glacier is a glacier that is confined to a valley and flows from a higher to a lower elevation.
Geological features created by glaciers:

a. Arête - steep ridge formed by two glaciers
b. Cirque - bowl shaped landform in the side of a mountain.
c. Horn - pointy shaped mountain peak.
d. Moraine - accumulation of material called till left behind by glacier.

4. Groundwater

Is the water found underground, in the cracks and spaces, in soil, sand and rocks.

Three types of groundwater
a. Meteoric water - water derived from precipitation; rain droplets that seeps down into spaces between the rocks.
b. Connate water - water that contains many mineral components that were trapped in the pores of sedimentary rocks.
c. Magmatic water - water which was formerly chemically bound up and has been released by heating in volcanic processes.

Subsurface water - water in a soil mantle that is divided into two zones:

1. The unsaturated zone (zone of aeration) - the soil pores are only partially saturated with water.
These have three subzones:
    a. Soil water zone - water readily available for plants
    b. Intermediate zone - use in irrigation
    c. Capillary fringe - layer in which groundwater seeps up        from a water table by capillary action to fill pores.

2. The saturated zone (zone of saturation) which includes ground water.
This is classified into 4 categories:

a. Aquifer - an underground layer of water-bearing permeable rock. ex. deposits of sand and gravel

Types of Aquifer
1. Unconfined Aquifer (phreatic aquifer) - When water can flow directly between the surface and the saturated zone of an aquifer.
2. Confined Aquifer (artesian aquifer) -  A water-    bearing subsurface stratum that is bounded above and below by formations of impermeable, or relatively impermeable soil or rock.

b. Aquiclude - a solid, impermeable area underlying or overlying an aquifer; it can absorb water but cannot transmit it in significant amount. Ex clay

c. Aquifuge -  An impermeable body of rock which contains no interconnected openings and therefore neither absorbs nor transmits water. Ex. massive compact rock

d. Aquitard - A bed of low permeability adjacent to an aquifer; may serve as a storage unit for groundwater, although it does not yield water readily. Ex. sandy clay