General remarks on Surface processes

Having talked about some of the basic elements of the Earth system, let's now look at some of the basic processes operating on system the Earth's surface. This is where rock, hydrosphere (oceans) and atmosphere all come together. Also the part of the Earth that we inhabit so many of those processes affect our lives (landslides, floods etc., or have affected our history and evolution (evolution of the landscape and climate).

We have talked about weathering of rocks in this zone to understand how regolith (surface debris including soil) and consequently, sediments are generated. We sai little about how the weathered rock debris is transported.

Four agents of transportation all- systems of which we will examine in turn

Agent system

gravity mass wasting

running water rivers, coastlines (hydrologic cycle)

moving air wind

moving ice glaciers, ice sheets and glaciations

deal with each of these in upcoming lectures

Mass Wasting (Movement) - Landslides

Mass wasting is name given to all those processes where the Earth"s gravity is the dominant agent. Covers what are called landslides, mudflows etc.. Even snow avalanches

Factors influencing mass movement

o Steepness of slopes

o Nature of materials, conslidated or unconsolidated (loose)

o Amount of water

Steepness of slope

Component of weight parallel to slope. Steeper the slope, the greater is this component.

Thus, steeper the slope the more likely material is to move.

Nature of materials

Unconsolidated

Angle of repose

Angle of repose greater for coarser grain size

Angle of repose greater for moist materials, than for dry (surface tension holds grains together)
(but water saturated materials have no strength)

Scree slopes and Talus cones

Consolidated materials

Includes solid rock, but also vegetated soils. Also soils contain clay minerals which are bound together by cohesion.

These infuenced by planes of weakness - bedding planes, joints

Amount of water

Obviously important controls cohesion.

Thus, heavy rainfall or melting snow can saturate unconsolidated material, loss of cohesion in consolidated materials thus triggers mass movement.

______________________

Types (classification) of mass movements

Classify basically according to nature of motion, speed and whether in dominantly rock or unconsolidated.

Slide - rock debris

Avalanche - debris, even snow

Slump

Flow - like a thick fluid - flow (mud flow, earth flow, debris flow depending on comp)

Creep - soil, debris, Bending of bedding, or foliation

Sometimes slow movement almost imperceptible and may be cause of subsidence

What causes and triggers moderate to rapid (ie disastrous) mass movement ?

Anything that steepens slope, changes nature of slope materials introduces water, or changes water/rock interaction, so:

o Over steepening of slopes - either by human engineering or by natural processes (undercutting by rivers etc)

o Heavy rainfall - storms, melting snow,

o earthquakes (because shaking causes liquefaction)

o Changing the properties of the slope materials - for example, de-forestation

Hazard from landslides

Annually a great deal of loss of life and property from mass movement (commonly called landslides). eg. California

Damming of rivers by landslides
eg. Gros Venture (Jackson Hole, Wyoming), Thistle, Utah

Marine Landslides

These do occur and can be very large. Main hazard is that they cause tsunam

Hawaii - flanks of Mauna Loa collapsed and slid eastward up to 5 km

Daytona - few years a large wave caused damage to this resort - thought to have been generated when a large landslide occurred on the continental shelf.