Geology Reference
In-Depth Information
The joint pattern consists initially of three sets of more
or less orthogonal joints, but unloading effects pres-
sure release in the top 100 m or so of the batholith
and a secondary set of joints appears lying approxi-
mately parallel to the surface. These joints play a key
role in the development of weathering landforms and
drainage patterns (p. 145). The upwards pushing of a
granite pluton may produce
active gneiss domes
(Ollier
and Pain 1981). These landforms occur in Papua New
Guinea (e.g. Dayman dome and Goodenough dome),
with ancient examples from the USA (e.g. Okanogon
dome, Washington state), and many of the world's oro-
gens. They stand 2,000-3,000 m high and are tens of
kilometres across. Their formation seems to involve the
metamorphosing of sediments to gneiss; the formation
of granite, which starts to rise as a pluton; the arching of
the gneiss by the rising pluton to form a dome of foli-
ated gneiss; and the eruption of the dome at the ground
surface, shouldering aside the bounding rocks.
Lopoliths
are vast, saucer-shaped, and layered intru-
sions of basic rocks, typically of a gabbro-type composi-
tion (Figure 5.1b). In Tasmania, dolerite magma intruded
flat Permian and Triassic sediments, lifting them as a
roof. In the process, the dolerite formed several very
large and shallow saucers, each cradling a raft of sedi-
ments. Lopoliths are seldom as large as batholiths. Their
erosion produces a series of inward-facing scarps. The
type example is the Duluth gabbro, which runs from the
south-western corner of Lake Superior, Minnesota, USA,
for 120 miles to the north-east, and has an estimated
volume of 200,000 km
3
. In South Africa, the Precam-
brian Bushveld Complex, originally interpreted as one
huge lopolith, is a cluster of lopoliths.
Stocks
or
plugs
are the largest intrusive bodies of basic
rocks. They are discordant and are the solidified remains
of magma chambers. One stock in Hawaii is about 20 km
long and 12 km wide at the surface and is 1 km deep.
Dykes, sills, laccoliths, and other
minor intrusions
Smaller intrusions are found alongside the larger forms
and extrusive volcanic features (Figure 5.2a). They are
classed as concordant where they run along the bedding
planes of pre-existing strata, or as discordant where they
cut through the bedding planes. Their form depends
upon the configuration of the fractures and lines of
weakness in the country rock and upon the viscos-
ity of the intruding magma. If exposed by erosion,
small intrusions can produce landforms, especially when
they are composed of rock that is harder than the
surrounding rock.
Dykes
are discordant intrusions, characteristically
1 to 10 m wide, and commonly composed of dolerite
(Figure 5.2a). They often occur in swarms. Along the
coast of Arran, Scotland, a swarm of 525 dykes occurs
along a 24-km section, the average dyke thickness being
3.5 m. When exposed, they form linear features that
Figure 5.2
Minor intrusions. (a) Laccoliths and associated features (dykes, sills, and bysmaliths). (b) Cone sheets.
Source:
Adapted from Sparks (1971, 90, 101)
