Deformation, Mountain Building, and the Continents - The Changing Earth: Exploring Geology and Evolution - page 256

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Figure 10.13 Domes and Basins

K Cretaceous Younger

Belle Fourche R Triassic

C Carboniferous

Oldest
exposed rocks

K

Sundance

C Cambrian

PC Precambrian

R

R

K

C

Rapid City

C

Newcastle PC

C

K

This geologic map, a surface view only, uses
colors and symbols to depict rocks and geologic
structures in the Black Hills of South Dakota. You can
tell this is a dome because the oldest rock layers are
exposed at its center.

b

Dome

Notice that in a dome, the oldest exposed rocks are in the
center and all rocks dip outward from a central point.

a

Youngest
exposed rocks

Basin

In a basin, the youngest exposed rocks are in the center
and all rocks dip inward toward a central point.

c

Strike-slip faults are characterized as right-lateral or
left-lateral, depending on the apparent direction of offset. In
Figure 10.16d, for example, observers looking at the block
on the opposite side of the fault from their location notice
that it appears to have moved to the left. Accordingly, this is a
left-lateral strike-slip fault. If it had been a right-lateral strike-
slip fault , the block across the fault from the observers would
appear to have moved to the right. The San Andreas fault
in California is a right-lateral strike-slip fault, whereas the
Great Glen fault in Scotland is a left-lateral strike-slip fault
(see Geo-inSight on pages 262 and 263).

As we mentioned in the introduction, the study of geo-
logic structures is important in the exploration for minerals,
oil, and natural gas. In addition, geologic engineers evaluate
geologic structures in the planning stages for many projects,
especially if they are in tectonically active areas (see Geo-
Focus on pages 252 and 253).

DEFORMATION AND THE ORIGIN
OF MOUNTAINS
Mountains form in several ways, but the truly large moun-
tains on continents result mostly from compression-
induced deformation at convergent plate boundaries. Before
discussing mountain building, though, we should define
what we mean by the term mountain and briefly discuss
the types of mountains. Mountain is a designation for any
area of land that stands signifi cantly higher, at least 300 m,
than the surrounding country and has a restricted summit
area. Some mountains are single, isolated peaks, but more

Oblique-Slip Faults The movement on most faults is pri-
marily dip-slip or strike-slip, but on oblique-slip faults ,
both types of movement take place. Strike-slip movement
might be accompanied by a component of dip-slip, giving
rise to a combined movement that includes left-lateral and
reverse, or right-lateral and normal (Figure 10.16e and see
Geo-inSight on pages 262 and 263).