Environmental Engineering Reference
In-Depth Information
Chapter
10
Earth's geological structure and processes
Rock material forms Earth's largest constituent by far and has undergone continuous
transformation since it condensed from a collapsing interstellar gas and dust cloud 4·6
billion years (4·6 Ba) ago. The rock volume of 1·083 × 10
12
km
3
and mass of 5·977 × 10
24
kg compares with 1·4 × 10
9
km
3
and 1·40 × 10
21
kg of global water and 5·13 × 10
18
kg of
atmospheric gases. The relatively small size of our planet, just 40,000 km in
circumference, is highlighted by passenger jets capable of circumnavigation in forty-eight
hours and orbiting satellites in ninety minutes. The average radius of 6,371 km from
surface to centre is equivalent to the distance from London to Chicago. Yet the mass,
character and age of Earth's rocks can be hard to comprehend. So, too, are its origins in
astro-geological processes which formed our solar system, through the gravity
concentration of matter from a supernova explosion
c
. 6 Ba ago. What interest, then,
should geographers have in planetary processes dominated by imponderable origins,
astronomic time scales, tiny
geothermal
energy flows compared with solar irradiation of
the atmosphere and vast but almost entirely concealed material reserves? How far do they
influence human habitat and lives at Earth's surface? This chapter explains long-term,
large-scale geological processes which form Earth's dynamic foundations.
Plate
tectonics
provides its unifying theme, linking Earth's early evolution with the
geologically recent
neotectonic
emplacement of its principal global landforms.
Subsequent reshaping by surface
geomorphic
processes is the subject of later chapters.
Table 10.1 shows their far-reaching effects and time scales of operation.
ORIGIN AND DYNAMICS
Infant Earth, dominated by heat-generating accretion around a dense core, would be
unrecognizable today. After an initial hot phase, cooling formed an outer crust violently
pockmarked by outgassing of volatile gases and pulverized by
planetesimals
and other
space debris. This dramatic
Hadean
aeon, named after Hades - the underworld of Greek
mythology - was short-lived and Earth's essential structure was in place 4·4 Ba ago.
Meteorite impacts, occasionally large enough to form craters, and cooling accompanied
by volcanic activity still occur but on a reduced scale. Most geological activity is now
confined to the crust and upper mantle within 150 km of the surface. The present form of
the continents and oceans is less than 200 Ma old, which allows us to concentrate on just
7 per cent of global rock mass and 4 per cent of Earth history.
In the intervening 4·2 Ba continuous but uneven cooling developed a process of
crustal evolution which acts as the radiator to Earth's internal engine. New crust forms
over hot spots and old, cold crust sinks and is recycled elsewhere, actively venting
geothermal energy as well as passively emitting it to space. Mobile crust in transit
between these zones takes the form of semi-rigid plates, and their boundaries coincide
with global-scale landforms, earthquake and volcanic belts. Crustal formation
differentiates between lighter, granitic continental rafts 'floating' above heavier, basaltic
