Geology Reference
In-Depth Information
may cut across the grain of the relief. The Great Dyke
of Zimbabwe is over 500 km long and averages 6-8 km
wide. On occasions, dykes radiate out from a central sup-
ply point to form
cone sheets
(Figure 5.2b).
Necks
and
pipes
are the cylindrical feeders of volcanoes and appear
to occur in a zone close to the ground surface. They are
more common in acid igneous rocks than in basalts. They
may represent the last stage of what was mainly a dyke
eruption. Six dykes radiate from Ship Rock volcanic neck
in New Mexico (Plate 5.1). They were probably only 750
to 1,000 m below the land surface at the time of their
formation.
Sills
are concordant intrusions and frequently form
resistant, tabular bands within sedimentary beds,
although they may cross beds to spread along other
bedding planes (Figure 5.2a). They may be hundreds
of metres thick, as they are in Tasmania, but are nor-
mally between 10 and 30 m. Sills composed of basic
rocks often have a limited extent, but they may extend
for thousands of square kilometres. Dolerite sills in the
Karoo sediments of South Africa underlie an area over
500,000 km
2
and constitute 15-25 per cent of the rock
column in the area. In general, sills form harder members
of strata into which they are intruded. When eroded, they
may form escarpments or ledges in plateau regions and
encourage waterfalls where they cut across river courses.
In addition, their jointing may add a distinctive feature to
the relief, as in the quartz-dolerite Whin Sill of northern
England, which was intruded into Carboniferous sedi-
ments. Inland, the Whin Sill causes waterfalls on some
streams and in places is a prominent topographic feature,
as where Hadrian's Wall sits upon it (Plate 5.2). Near the
north Northumberland coast, it forms small scarps and
crags, some of which are used as the sites of castles, for
instance Lindisfarne Castle and Dunstanburgh Castle.
It also affects the coastal scenery at Bamburgh and the
Farne Islands. The Farne Islands are tilted slabs of Whin
Sill dolerite.
Laccoliths
are sills that have thickened to produce
domes (Figure 5.2a). The doming arches the overlying
rocks.
Bysmaliths
are laccoliths that have been faulted
(Figure 5.2a). The Henry Mountains, Utah, USA, are
a famous set of predominantly diorite-porphyry lacco-
liths and associated features that appear to spread out
from central discordant stocks into mainly Mesozoic
shales and sandstones. The uplift connected with the
intrusion of the stocks and laccoliths has produced
several peaks lying about 1,500 m above the level of
Plate 5.1
Ship Rock (in the mid-distance) and one of its dykes (in the foreground), New Mexico, USA. Ship Rock is an
exhumed volcanic neck from which radiate six dykes.
(
Photograph by Tony Waltham Geophotos
)
