GEOMORPHIC MATERIALS AND PROCESSES - Fundamentals of Geomorphology

Geology Reference

In-Depth Information

and chelation. The chemical and mechanical action of

animals and plants bring about biological weathering.

Rock weathering manufactures debris that ranges in size

from coarse boulders, through sands and silt, to col-

loidal clays and then solutes. Weathered debris may

move downslope under its own weight, a process called

mass wasting. Gravity-driven mass wasting is determined

largely by the relationships between stress and strain in

Earth materials, and by the rheological behaviour of brit-

tle solids, elastic solids, plastic solids, and liquids. Mass

movements occur in six ways: creep, flow, slide, heave,

fall, and subsidence. Half-mountain-sized mass move-

ments are the subject of gravity tectonics. Water runs over

the land surface, through the soil and rock (sometimes

emerging as springs), and along rills and rivers. Streams

are particularly effective landform-makers. They conduct

material along their beds, keep finer particles in suspen-

sion, and carry a burden of dissolved substances. They

wear away their channels and beds by corrosion, cor-

rasion, and cavitation, and they erode downwards and

sideways. They lay down sediments as channel deposits,

channel margin deposits, overbank floodplain deposits,

and valley margin deposits. Episodes of continued depo-

sition and valley filling (alluviation) often alternate with

periods of erosion and valley cutting. Ice abrades and

fractures rock, picks up and carries large and small

rock fragments, and deposits entrained material. Glaciers

carry rock debris at the glacier base (subglacial debris),

in the ice (englacial debris), and on the glacier surface

(supraglacial debris). They also deposit sediment under,

on, and by the side of the moving ice. Meltwater issu-

ing from glacier snouts lays down proglacial sediments.

Wind erodes dry, bare, fine-grained soils and sediments.

It is most effective in deserts, sandy coasts, and alluvial

plains next to glaciers. Wind erodes by deflating sedi-

ments and sandblasting rocks. Particles caught by the

wind bounce (saltation), hop (reptation), 'float' (suspen-

sion), or roll and slide (creep). Wind deposits particles

by dropping them or ceasing to propel them along the

ground. Weathering and wave erosion destroy coast-

lines, while sediment deposition, reef-building corals,

and

Eroded materials eventually come to rest. Deposition

occurs in several ways to produce different classes

of sediment: clastic (solid fragments), chemical (pre-

cipitated materials), or biogenic (produced by living

things). Sediments accumulate in three main environ-

ments: the land surface (terrestrial sediments); around

continental edges (shallow marine sediments); and

on the open ocean floor (deep marine sediments).

Humans are potent geomorphic agents and currently

move more material than natural processes. Mining

and construction, agricultural practices and land-

use, and dam building have significant impacts upon

sediment fluxes.

ESSAY QUESTIONS

1 Evaluate the relative importance of

factors that affect weathering.

2 How does ice flow?

3 Compare and contrast sediment

transport by wind and by water.