Geoscience Reference
In-Depth Information
blown dust in China shows that the region north of the Himalayas was drying out at about
this time too. There is also a change in the sediments off the west coast of Africa, with
an increase in wind-blown dust in the layers. It seems that this corresponds to the start of
the drying out of Africa and the beginnings of the Sahara Desert as the moist clouds were
drawn away towards India. There is a theory that the huge amount of chemical weather-
ing that must have taken place in the eroding Himalayas drew down so much carbon diox-
ide from the atmosphere that, in turn, it may have set the stage for the ice ages of the last
2.5 million years. So perhaps the climatic changes in Africa that provided the evolutionary
pressures that led to the development of modern humans there also have their origins in the
rise of Tibet and the Himalayas.
Swiss roll
Further west than the continental pile-up of the Himalayas, the Tethys Ocean narrowed
to an inlet, but the results of the collision, in this case of Italy and the African plate with
Europe, are similar, if on a slightly smaller scale. The Alps are one of the most studied and
best understood mountain ranges. To the north lies a sedimentary basin which slowly filled
with sediments known as molasse. South of the Alps, in Italy, lies the plain of the River Po,
equivalent to the Ganges plain in India. Between it and the mountains is a series of wedges
of sediment, scooped up from the Tethys Ocean, sediment known as flysch. Then come the
high Alps of Switzerland, made of the crystalline base of the continent, together with intru-
sions of granite from partial melting beneath. Beyond them come a series of very strongly
folded rocks scooped up into giant over-folds called nappes, folding to the north and sag-
ging under their own weight, as if they had been scooped up like whipped cream. These
nappe folds are often so extensive that older rocks are folded up above younger rocks in a
very confusing sequence. As with the Himalayas, there is a series of thrust faults, in places
doubling the thickness of the continental crust.
Cratons
No continent is an island entire of itself. Continents can split apart or join up and merge.
Modern mountain ranges such as the Alps and the Himalayas are just the latest examples of
this. Others are so ancient that they have been worn down almost flat again. The Caledoni-
an range of northwest Scotland and the Appalachians of North America are examples dat-
ing from when a forerunner of the Atlantic closed about 420 million years ago. The modern
continents are patchwork quilts of such features. But the older and thicker a continent be-
comes, the more rigid it grows and the longer it lasts. The most stable cores of continents
that are least affected by tectonic movements are known as cratons, and they make up the
cores of present-day North and South America, Australia, Russia, Scandinavia, and Africa.
