Page 50 - Fundamentals of Geomorphology

THE GEOMORPHIC SYSTEM 33

Box 2.1

ROCKS AND MINERALS

The average composition by weight of chemical ele- include gabbro and basalt. Ultramafic rocks, which
ments in the lithosphere is oxygen 47 per cent, silicon are denser still than mafic rocks, include peridotite and
28 per cent, aluminium 8.1 per cent, iron 5.0 per cent, serpentine. Much of the lithosphere below the crust
calcium 3.6 per cent, sodium 2.8 per cent, potas- is made of peridotite. Eclogite is an ultramafic rock
sium 2.6 per cent, magnesium 2.1 per cent, and the that forms deep in the crust, nodules of which are
remaining eighty-three elements 0.8 per cent. These sometimes carried to the surface by volcanic action.
elements combine to form minerals. The chief miner- At about 400 km below the surface, olivine under-
als in the lithosphere are feldspars (aluminium silicates goes a phase change (it fits into a more tightly packed
with potassium, sodium, or calcium), quartz (a form crystal lattice whilst keeping the same chemical com-
of silicon dioxide), clay minerals (complex aluminium position) to spinel, a denser silicate mineral. In turn, at
silicates), iron minerals such as limonite and hematite, about 670 km depth, spinel undergoes a phase change
and ferromagnesian minerals (complex iron, magne- into perovskite, which is probably the chief mantle
sium, and calcium silicates). Ore deposits consist of constituent and the most abundant mineral in the
common minerals precipitated from hot fluids. They Earth.
include pyrite (iron sulphide), galena (lead sulphide),
blende or sphalerite (zinc sulphide), and cinnabar Sedimentary rocks
(mercury sulphide).
Rocks are mixtures of crystalline forms of minerals. These are layered accumulations of mineral particles
There are three main types: igneous, sedimentary, and derived mostly from weathering and erosion of pre-
metamorphic. existing rocks. They are clastic, organic, or chemical
in origin. Clastic sedimentary rocks are unconsoli-
dated or indurated sediments (boulders, gravel, sand,
Igneous rocks
silt, clay) derived from geomorphic processes. Con-
These form by solidification of molten rock (magma). glomerate, breccia, sandstone, mudstone, claystone,
They have varied compositions (Figure 2.1). Most and shale are examples. Organic sedimentary rocks
igneous rocks consist of silicate minerals, especially and mineral fuels form from organic materials. Exam-
those of the felsic mineral group, which comprises ples are coal, petroleum, and natural gas. Chemical
quartz and feldspars (potash and plagioclase). Felsic sedimentary rocks form by chemical precipitation in
minerals have silicon, aluminium, potassium, calcium, oceans, seas, lakes, caves, and, less commonly, rivers.
and sodium as the dominant elements. Other impor- Limestone, dolomite, chert, tufa, and evaporites are
tant mineral groups are the micas, amphiboles, and examples.
pyroxenes. All three groups contain aluminium, mag-
nesium, iron, and potassium or calcium as major Metamorphic rocks
elements. Olivine is a magnesium and iron silicate.
The micas, amphiboles (mainly hornblende), pyrox- These form through physical and chemical changes
enes, and olivine constitute the mafic minerals, which in igneous and sedimentary rocks. Temperatures or
are darker in colour and denser than the felsic miner- pressures high enough to bring about recrystallization
als. Felsic rocks include diorite, tonalite, granodiorite, of the component minerals cause the changes. Slate,
rhyolite, andesite, dacite, and granite. Mafic rocks schist, quartzite, marble, and gneiss are examples.