Geology Reference
In-Depth Information
Plate 5.2
Whin Sill, a dolerite intrusion in Northumberland, with Hadrian's Wall running along the top.
(
Photograph by Tony Waltham Geophotos
)
the Colorado Plateau. Eroded bysmaliths and laccoliths
may produce relief features. Traprain Law, a promi-
nent hill, is a phonolite laccolith lying 32 km east
of Edinburgh in Scotland. However, the adjacent tra-
chyte laccolith at Pencraig Wood has little topographic
expression.
Phacoliths
are lens-shaped masses seated in anticlinal
crests and synclinal troughs (Figure 5.3a). They extend
along the direction of anticlinal and synclinal axes and,
unlike laccoliths, which tend to be circular in plan, are
elongated. Eroded phacoliths may produce relief features.
Corndon Hill, which lies east of Montgomery in Powys,
Wales, is a circular phacolith made of Ordovician dolerite
(Figure 5.3b).
( ) Phacoliths
a
()Corndon Hill phacolith
b
Ashes and
andesites
Dolerite
Sediments
Volcanoes
Volcanoes erupt lava on to the land surface explosively
and effusively. They also exhale gases. The landforms
built by eruptions depend primarily upon whether rock
is blown out or poured out of the volcano, and, for
effusive volcanoes, upon the viscosity of the lava. Explo-
sive or pyroclastic volcanoes blow
pyroclastic rocks
(solid fragments, loosely termed
ash
and
pumice
) out
of a vent, while effusive volcanoes pour out
lava
.
NW
SE
Figure 5.3
Phacoliths. (a) Occurrence in anticlinal crests
and synclinal troughs. (b) Corndon Hill, near
Montgomery in Wales, an eroded phacolith.
Source:
Adapted from Sparks (1971, 93, 94)
