Exogenic Processes of the Earth

CO4 GEO01 -Earth Science

Mass Wasting

is the downhill movement of masses of bedrock, rock debris, or soil driven by the pull of gravity

Mass Wasting

with proper planning, perhaps the most easily avoidable of all major geologic hazards.

Gravity

the driving force for mass wasting

Gravity

Normal Force

Shear Force

Shear Resistance

Gravity

Shear resistance < shear force = landslide

Steep Slopes

shear forces maximized by gravity

Shear Strength

resistance to movement or deformation

Shear Strength

Saturated soil has reduced shear strength due to increased pore pressure.

• However, a small amount of water in soil can prevent downslope movement.

• Like building a sandcastle.

Mass Wasting Triggers

Seismic (earthquake) activity

Heavy Rainfall

Construction

Lack of Vegetation

no roots to hold rock/soil in place.

Classification of Mass Wasting

Rate of movement

• < 1cm/year – >100 km/hour

Classification of Mass Wasting

Type of material

• Solid bedrock or debris (unconsolidated material at Earth’s surface)

Classification of Mass Wasting

Type of movement flow, slide, or fall

Creep (or soil creep)

very slow downslope movement of soil

Creep

Major contributing factors include water in soil and daily freeze – thaw cycles.

Creep

Can be costly to maintain homes, etc., on creeping ground as foundations, walls, pipes and driveways crack and shift downslope over time.

Flow

descending mass moves downhill as a viscous fluid

Earthflow

debris moves downslope, slowly or rapidly, as a viscous fluid

Solifluction Permafrost

flow of water-saturated soil over impermeable material

• Common in colder climates

Debris Flow and Mudflow

flowing mixture of debris and water, usually down a channel

• Mudflow is only soil and water

Debris Avalanches

very rapid and turbulent

Falls

material free-falls or bounces down a cliff

Rockfall

a block of bedrock breaks free and falls or

bounces down a cliff

talus

Commonly an apron of fallen rock fragments (_____) accumulates at cliff base

Slides

descending mass remains relatively intact, and descends along well-defined surfaces

Translational Slide

movement along plane parallel to motion.

Rotational Slide (Slump)

movement along a curved surface

Rockslide and Rock Avalanche

the rapid sliding of a mass of bedrock along an inclined surface of weakness

Underwater Landslides

• Turbidity Currents

• Can create a tsunami

Preventing Rockfalls and Rockslides on Highways

• Remove Loose material

• Stitch slopes together

Preventing Mass Wasting of Soil

• Construct retaining wall with drains

• Don’t oversteepen slopes during construction

• Remove all rock that is prone to sliding

• Add vegetative cover

• Cover roads

The Hydrologic Cycle

the movement and interchange of water between the sea, air, and land

Evaporation

solar radiation provides energy

Precipitation

rain or snow

Transpiration

evaporation from plants

Runoff

water flowing over land surface

Infiltration

water soaking into the ground

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

bottom of the channel

Floodplain

flat valley floor composed of sediment deposited by the stream

Valleys

most common landform on Earth

Valleys

formed by stream erosion

Drainage Basin

the total area drained by a stream and its tributaries

Tributary

a small stream flowing into a larger one

Divide

ridge or high ground that divides one drainage basin from another

Continental Divide

separates the streams that flow into the Pacific from those that flow into the Atlantic and Gulf of Mexico

Drainage Pattern

the arrangement, in map view, of a stream and its tributaries

acute angle

Most tributaries join the main stream at an ____ ____, forming a V or Y pointing downstream

Dendritic

drainage pattern resembling the branches of a tree

Radial Pattern

streams diverge outward like the spokes of a wheel, such as on conical mountains

Rectangular Pattern

tributaries have frequent 90° bends and join other streams at right angles

Trellis Pattern

parallel streams with short tributaries meeting at right angles

Velocity

• Maximum velocity near center of channel

• Higher stream velocities promote erosion and transport of coarser sediments

• Floods involve increased velocity and erosion

Factors Affecting Stream Erosion and Deposition

• Gradient (slope)

• Channel Shape and Roughness

• Discharge

Discharge

volume of water passing a particular point in a stream over time

Stream Erosion

Streams cut their own valleys, deepening and widening them over time and carrying away the sediment

Hydraulic Action

ability to pick up and move rock and sediment

Solution

dissolving of rocks

Abrasion

grinding away of stream channel by the friction and impact of the sediment load

Potholes

eroded into streambed by the abrasive action of the sediment load in the stream

Bed Load

large or heavy particles that travel on the streambed

Traction Load

large particles that travel along the streambed by

rolling, sliding or dragging

Saltation Load

medium particles that travel by bouncing along

Suspended Load

small/light sediment that remains above the stream bottom by turbulent flow for an indefinite period of time

Dissolved Load

dissolved ions produced by chemical weathering of soluble minerals upstream

Bars

sediments temporarily deposited along stream course

Placer Deposits

concentrated heavy sediment

Braided Streams

contain sediment deposited as numerous bars around which water flows in highly interconnected rivulets

Braided Streams

• Resembles braids of hair or rope

• Common for streams carrying a lot of sediment

Meanders

Rivers that develop pronounced, sinuous curves

Cut Banks

Water flows faster along the outside of bends causing erosion and created

Point Bars

Flows slower along the inside, depositing ____ ____ on the insides of the meanders

Meander Cutoffs

may form when a new, shorter channel is cut through the narrow neck of a meander (as during a flood)

Floodplains

broad strips of land built up by sedimentation on either side of a stream channel

Floodplain Sediments

are left behind as flood waters slow and recede at the end of flood events

Natural Levees

Main channel has slightly raised banks with respect to the floodplain

Delta

body of sediment deposited at the mouth of a river when flow velocity decreases

Delta

• Surface marked by shifting distributary channels

• Shape of a delta depends on whether its wave- dominated, tide - dominated, or stream – dominated

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

Alluvial Fan

• Well – developed in desert regions, such as the southwestern U.S.

• Larger fans show grading from large sediments nearest the mountains to finer sediments farther away

Valleys

Different valley morphologies depend on the erosional processes that created them

Downcutting

process of deepening a valley by erosion of the streambed

V-shaped Valleys

typically form from downcutting combined with mass wasting and sheet erosion

baseless

Streams cannot erode below their ___ ___

Graded Streams

have concave- up longitudinal profile, lack rapids and waterfalls, represent a balance between available sediment load and transport capacity

Lateral Erosion

widens stream valleys by undercutting of stream banks and valley walls as stream swings from side to side across the valley floor

Headward Erosion

the slow uphill growth of a valley above its original source by gullying, mass wasting, and sheet erosion

Stream Terraces

step – like landforms found above a stream and its floodplain

Stream Terraces

• Occurs when river rapidly cuts downward into its own floodplain

• Represents relatively sudden change from deposition to erosion

• Can be caused by rapid uplift, drops in base level, or climate changes

Incised Meanders

• Retain sinuous pattern as they cut vertically downward

• May be produced by profound base level changes, as when rapid tectonic uplift occurs.

Flooding

When water levels rise and overtop the banks of a river, _____ occurs

Flooding

• Natural process on all rivers.

• Described by recurrence intervals.

• Can cause great damage in heavily populated areas.

• High velocity and large volume of water causes flood erosion.

• Slowing of waters as flood ends causes flood deposits to be deposited in the floodplain.

Flooding and Urbanization

• Urbanization creates many impermeable surfaces which increases runoff flooding

• Water is delivered to streams faster which increases peak discharge and hastens occurrence of flood.

Flash Floods

• local, sudden floods of large volume and short duration

• Typically triggered by heavy thunderstorms.

Dams

designed to trap flood waters in reservoirs upstream and release it gradually over time

Artificial Levees

designed to increase capacity of river channel and works well until stream overtops levees, leading to extremely rapid flooding and erosion