Geoscience Reference
In-Depth Information
Major valcano-
tectonic structures
(b)
(a)
Triassic
Marls, sandstones and conglomerates
Carboniferous
Limestones, Sandstones, shales and coal seam
Devonian
Sandstones, shales, conglomerates
(Old Red Sandstone) slates and limestones
Silurian
Shales, mudstones,greywacke, some limestone
Ordovician
Mainly shales and mudstones
Cambrian
Mainly shales, slate and sandstones
Lower Palaeozoic and Proterozoic
Mainly schists and gneisses
Early Precambrian (Lewisian)
Mainly gneisses
Intrusive
Mainly granite, granodiorite, gabbro and dolorite
Volcanic
Mainly basalt, rhyolite, andesite and tuffs
post-Acadian strata
(upper Devonian and younger
Powys Supergroup
(upper Ordovician-Lower Devonian)
Gwynedd Supergroup
(Ordovician)
Dyfed Supergroup
(Cambrian-Lower Ordovician)
Precambrian
Fault
0
50
0
50
km
km
Figure 10.22
The principal rocks and stratigraphic units of Wales (a) and the principal tectonic structures and terranes (b). Each
map is colour-coded separately.
By regarding the familiar atmosphere, hydrosphere
and biosphere as Earth's outer fractionates we can more
easily understand the core, mantle, asthenosphere and
lithosphere inner fractionates with which they are linked.
This lays the foundations for appreciating the morpho-
tectonic and rock cycles which shape our Earth, its
principal architecture of oceans, continents, mountains
and sedimentary basins, etc., and surface geomorphic
processes and landsystems. We see Earth's surface no
longer as just a static outline on the world map but as an
evolving scene whose contemporary components have
come together so recently - to create the world we know
- and are already changing towards the world our
descendants will inhabit.
KEY POINTS
1
Earth acquired its own heat engine and a particular set of planetary raw materials, determined by its position
in the solar system, at its formation. The continuous fractionation or segregation and refinement of these
raw materials, in conjunction with solar radiation at Earth's surface, have created our atmosphere,
hydrosphere and biosphere. Their reaction with fractionation processes in Earth's geological interior and
the slow release of internal energy produce familiar surface physical features.
2
Earth's interior is similarly segregated into an inner core surrounded concentrically by a mantle and crust.
The outer mantle and crust form a cool, light and brittle lithosphere capable of being dislocated and moved
as semi-rigid plates by convection currents in the underlying deformable mantle asthenosphere. This
movement is assisted by partial melting of the asthenosphere in prevailing temperature-pressure
conditions.
3
Continental lithosphere thins and rifts apart over rising mantle plumes and new oceans form as sea water
floods in. Asthenosphere peridotite rises faster than it cools into the rift and forms new, denser oceanic
lithosphere. The ocean enlarges by sea-floor spreading and the divergent plates eventually converge
elsewhere with other plates.
