Geoscience Reference
In-Depth Information
Ireland, around Benbane Head and Giant's Causeway, where
individual, near horizontal flood basalts form prominent cliffs.
The basalts were extruded on to a land surface near the North
Atlantic mid-ocean ridge, as sea-floor spreading separated
Greenland and Europe 55-58 Ma ago.
Photo: Ken Addison
Plate 10.5
Basin-range structures produced by asymmetri-
by the rising leading edge of rotating fault blocks, separate
two basins occupied by lakes. The rear lake has evaporated,
leaving a saline floor.
Photo: Ken Addison
which often survive as resistant plateaux, as in parts of
India (Deccan, 0·5 M km
2
), the United States (Columbia,
in Washington/Oregon, 0·13 M km
2
) and South Africa
(Karroo). The Karroo flood basalts formerly covered over
5 M km
2
. Tertiary basalts up to 5 km thick were extruded
in the Irish-Hebridean basin of the British Isles during the
formation of the Atlantic Ocean, forming the modern
Antrim plateau and Fingal's Cave landmarks (
Plate 10.6
).
Eruptive
alkali-basalts and other magmas form volcanoes.
The largest concentration forms the East African Rift
Valley complex, with well known individuals such as
Mount Kenya, Kilimanjaro and Ruwenzori over 5 km
high. In Europe the Eifel volcanoes of Germany are
associated with the Rhine graben.
Sea-floor spreading also leaves
passive margins
on
the trailing edge of continents which often form major
escarpments, upwarped either by the initial crustal
elevation producing the rift or a subsequent isostatic or
thermal (epeirogenetic) response. Passive margin
escarpments, reaching elevations of 1·8-3·5 km, are best
developed on the eastern seaboards of southern Brazil
(Serra da Mantiqueira), southern Africa (Drakensburg
range) and Australia (the Great Escarpment), both sides
of the Red Sea and the west coast of India (the Western
Ghats). The Piedmont Fall Line marks a lower, persistent
passive margin on the south-east coast of the United
States. In all cases, escarpments source continental-margin
sedimentation on their seaward side.
BRITAIN
Few modern continental areas illustrate crustal evolution
better than the British Isles. Into their diminutive 150,000
km
2
are crammed rocks representative of half Earth's
history and structures of all three
Phanerozoic
orogens.
Origins are traced through rocks which tell a story of
fragments lost, gained and surviving as terranes were
assembled and dismantled in the long drift across Earth's
surface. Britain's familiar coastline is less than 10 ka old
and dependent on global sea level. The story is elaborated
by three vital strands of the science of
stratigraphy
.
Litho-
stratigraphy and
bio
-stratigraphy reveal the physical and
biological character of past environments and
chrono
-
stratigraphy provides a time scale, based on the decay of
constituent radioactive minerals (see
Chapter 23).
The
early history is very obscure but we have a clearer view of
the past 0·5 Ga, in which fragments originating 60
south
of the equator were joined by others as 'Britain' drifted to
its modern position at 50-60
N. En route, subtropical
Silurian coral reefs were joined by Devonian and Permo-
Triassic desert sands 'sandwiching' Carboniferous
equatorial swamp forests, and the whole was subjected
most recently to Quaternary glaciation (
Figure 10.20
).
