Page 135 - Fundamentals of Geomorphology

118 STRUCTURE

The joint pattern consists initially of three sets of more for 120 miles to the north-east, and has an estimated
3
or less orthogonal joints, but unloading effects pres- volume of 200,000 km . In South Africa, the Precam-
sure release in the top 100 m or so of the batholith brian Bushveld Complex, originally interpreted as one
and a secondary set of joints appears lying approxi- huge lopolith, is a cluster of lopoliths.
mately parallel to the surface. These joints play a key Stocks or plugs are the largest intrusive bodies of basic
role in the development of weathering landforms and rocks. They are discordant and are the solidified remains
drainage patterns (p. 145). The upwards pushing of a of magma chambers. One stock in Hawaii is about 20 km
granite pluton may produce active gneiss domes (Ollier long and 12 km wide at the surface and is 1 km deep.
and Pain 1981). These landforms occur in Papua New
Guinea (e.g. Dayman dome and Goodenough dome), Dykes, sills, laccoliths, and other
with ancient examples from the USA (e.g. Okanogon minor intrusions
dome, Washington state), and many of the world’s oro-
gens. They stand 2,000–3,000 m high and are tens of Smaller intrusions are found alongside the larger forms
kilometres across. Their formation seems to involve the and extrusive volcanic features (Figure 5.2a). They are
metamorphosing of sediments to gneiss; the formation classed as concordant where they run along the bedding
of granite, which starts to rise as a pluton; the arching of planes of pre-existing strata, or as discordant where they
the gneiss by the rising pluton to form a dome of foli- cut through the bedding planes. Their form depends
ated gneiss; and the eruption of the dome at the ground upon the configuration of the fractures and lines of
surface, shouldering aside the bounding rocks. weakness in the country rock and upon the viscos-
Lopoliths are vast, saucer-shaped, and layered intru- ity of the intruding magma. If exposed by erosion,
sions of basic rocks, typically of a gabbro-type composi- small intrusions can produce landforms, especially when
tion(Figure5.1b).InTasmania,doleritemagmaintruded they are composed of rock that is harder than the
flat Permian and Triassic sediments, lifting them as a surrounding rock.
roof. In the process, the dolerite formed several very Dykes are discordant intrusions, characteristically
large and shallow saucers, each cradling a raft of sedi- 1 to 10 m wide, and commonly composed of dolerite
ments. Lopoliths are seldom as large as batholiths. Their (Figure 5.2a). They often occur in swarms. Along the
erosion produces a series of inward-facing scarps. The coast of Arran, Scotland, a swarm of 525 dykes occurs
type example is the Duluth gabbro, which runs from the along a 24-km section, the average dyke thickness being
south-western corner of Lake Superior, Minnesota, USA, 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)