Weathering, Soil, and Sedimentary Rocks - The Changing Earth: Exploring Geology and Evolution - page 148

Denver Denver

Topeka

COLORADO COLORADO

KANSAS

Image not available due to copyright restrictions

Santa Fe Santa Fe

OKLAHOMA

Oklahoma City

NEW MEXICO NEW MEXICO

TEXAS
Austin

Explanation
Severe wind erosion
in 1935-1936
Severe wind erosion
in 1938
Severe wind erosion
in 1940
Most severe wind erosion
in 1935 - 1938

a Map of the Dust Bowl of the 1930s.

◗ Figure 1 The dust bowl of the 1930s was a time of
drought, dust storms resulting from wind erosion, and
economic hardship.

c By the mid-1930s, tens of thousands of people were
on relief, homeless, or leaving the Dust Bowl. In 1939,
Dorthea Lange photographed this homeless family of seven
in Pittsburg County, Oklahoma.

Source: a “Map: Extent of Area Subject to Severe Wind Erosion,” p. 30, from Dust Bowl: The Southern Plains in the 1930s, by
Donald Worster. Copyright © 1979, 1982 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc.

fl oodplain, in a stream channel, on a beach, or on the seafl oor,
where physical, chemical, and biological processes impart vari-
ous characteristics to the accumulating sediment. Geologists
recognize three major depositional settings: continental (on the
land), transitional (on or near seashores), and marine, each with
several specifi c depositional environments (

To illustrate the relative importance of compaction and
cementation, consider a detrital deposit made up of mud and
another composed of sand. In both cases, the sediment consists
of solid particles and pore spaces , the voids between particles.
These deposits are subjected to compaction from their own
weight and the weight of any additional sediment deposited on
top of them, thereby reducing the amount of pore space and the
volume of the deposit. Our hypothetical mud deposit may have
80% water-fi lled pore space, but after compaction, its volume
is reduced by as much as 40% (Figure 6.18). The sand deposit,
with as much as 50% pore space, is also compacted, but far less
than the mud deposit, so that the grains fit more tightly
together (Figure 6.18).
Compaction alone is suffi cient for lithifi cation of mud, but
for sand and gravel, cementation involving the precipitation

◗ Figure 6.17).

How Does Sediment Become
Sedimentary Rock?
A deposit of detrital sediment consists of a loose aggregate of
particles: Mud in lakes and sand and gravel in stream chan-
nels or on beaches are good examples. To convert these aggre-
gates of particles into sedimentary rocks requires lithifi cation
by compaction, cementation, or both (

◗ Figure 6.18).