Page 62 - Ultimate Visual Dictionary (DK)

PREHISTORIC EAR TH

Faults and folds STRUCTURE OF A FOLD

Axial plane
THE CONTINUOUS MOVEMENT of the Earth’s crustal plates (see pp. 58–59) Crest
can squeeze, stretch, or break rock strata, deforming them and
Limb Angle of
producing faults and folds. A fault is a fracture in a rock along which plunge
there is movement of one side relative to the other. The movement
can be vertical, horizontal, or oblique (vertical and horizontal). Faults
develop when rocks are subjected to compression or tension. They
tend to occur in hard, rigid rocks, which are more likely to break
than bend. The smallest faults occur in single mineral crystals and are
microscopically small, whereas the largest—the Great Rift Valley in Hingeline
Africa, which formed between 5 million and 100,000 years ago—is more
than 6,000 miles (9,000 km) long. A fold is a bend in a rock layer caused
STRUCTURE OF A FAULT
by compression. Folds occur in elastic rocks, which tend to bend rather
Fault plane Dip of fault plane
than break. The two main types of fold are anticlines (upfolds) and (angle from
synclines (downfolds). Folds vary in size from a few millimeters long to horizontal)
folded mountain ranges hundreds of miles long, such as the Himalayas
(see pp. 62–63) and the Alps, which are repeatedly folding. In addition
to faults and folds, other features associated with rock deformations Upthrow
include boudins, mullions, and en échelon fractures.

FOLDED ROCK Crest of
anticline
Plunge Throw (vertical
Steeply dipping displacement Downthrow
limbs
of fault)
Hade of fault plane
(angle from vertical)
STRUCTURE OF A SLOPE

Strike
Angle
of dip

Strike and Direction
dip are at of dip
SECTION THROUGH FOLDED ROCK right angles
STRATA THAT HAVE BEEN ERODED to each other

Dipping bed Anticlinal Monoclinal
fold fold

Mineral-filled
fault

Upper Carboniferous Millstone Grit Lower Carboniferous Limestone

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