Geology Reference
In-Depth Information
during glacial episodes at high latitudes (Partridge 1998).
The uplift of the Tibetan Plateau and its bordering
mountains may have actively forced climatic change by
intensifying the Asian monsoon (through altering sur-
face atmospheric pressure owing to elevation increase),
by creating a high-altitude barrier to airflow that affected
the jet stream, and by encouraging inter-hemispherical
exchange of heat (Liu and Ding 1998; Fang
et al
. 1999a,
b). These forcings seem to have occurred around 800,000
years ago. However, oxygen isotope work on late Eocene
and younger deposits in the centre of the plateau suggests
that this area at least has stood at more than 4 km for
about 35 million years (Rowley and Currie 2006).
Recent research shows that local and regional climatic
changes caused by uplift may promote further uplift
through a positive feedback loop involving the extrusion
of crustal rocks (e.g. Molnar and England 1990; Hodges
2006). In the Himalaya, the Asian monsoon sheds
prodigious amounts of rain on the southern flanks of
the mountains. The rain erodes the rocks, which enables
the fluid lower crust beneath Tibet to extrude towards the
zone of erosion. Uplift results from the extrusion of rock
and counterbalances the erosion, which reduces the land-
surface elevation. Therefore, the extrusion process keeps
the front range of the Himalaya steep, which encourages
heavy monsoon rains, so completing the feedback loop
(but see Ollier 2006 for a different view).
Carbon dioxide
is a key factor in determining mean
global temperatures. Over geological timescales (millions
and tens of millions of years), atmospheric carbon diox-
ide levels depend upon the rate of carbon dioxide
input through volcanism, especially that along mid-
ocean ridges, and the rate of carbon dioxide withdrawal
through the weathering of silicate rocks by carbonation, a
process that consumes carbon dioxide. Given that carbon
dioxide inputs through volcanism seem to have varied
little throughout Earth history, it is fair to assume that
variations in global chemical weathering rates should
explain very long-term variations in the size of the atmo-
spheric carbon dioxide pool. So what causes large changes
in chemical weathering rates? Steep slopes seem to play
a crucial role. This relatively new finding rests on the
fact that weathering rates depend greatly on the amount
of water passing through the weathering zone. Rates
are highest on steep slopes with little or no weathered
mantle and high runoff. In regions experiencing these
conditions, erosional processes are more likely to remove
weathered material, so exposing fresh bedrock to attack
by percolating water. In regions of thick weathered man-
tle and shallow slopes, little water reaches the weathering
front and little chemical weathering occurs. Interestingly,
steep slopes characterize areas of active uplift, which also
happen to be areas of high precipitation and runoff. In
consequence, 'variations in rates of mountain building
through geological time could affect overall rates of global
chemical weathering and thereby global mean tempera-
tures by altering the concentration of atmospheric CO
2
'
(Summerfield 2007, 105). If chemical weathering rates
increase owing to increased tectonic uplift, then CO
2
will
be drawn out of the atmosphere, but there must be some
overall negative feedback in the system otherwise atmo-
spheric CO
2
would become exhausted, or would keep on
increasing and cause a runaway greenhouse effect. Nei-
ther has occurred during Earth history, and the required
negative feedback probably occurs through an indirect
effect of temperature on chemical weathering rates. It is
likely that if global temperatures increase this will speed
up the hydrological cycle and increase runoff. This will, in
turn, tend to increase chemical weathering rates, which
will draw down atmospheric CO
2
and thereby reduce
global mean temperature. It is also possible that varia-
tions in atmospheric CO
2
concentration may directly
affect chemical weathering rates, and this could provide
another negative feedback mechanism.
The idea that increased weathering rates associ-
ated with tectonic uplift increases erosion and removes
enough carbon dioxide from the atmosphere to control
climate has its dissenters. Ollier (2004a) identified what
he termed 'three misconceptions' in the relationships
between erosion, weathering, and carbon dioxide. First,
weathering and erosion are not necessarily concurrent
processes - erosion, especially erosion in mountainous
regions, may occur with little chemical alteration of rock
or mineral fragments. Second, in most situations, hydrol-
ysis and not carbonation is the chief weathering process -
weathering produces clays and not carbonates. Further-
more, evidence suggests that chemical weathering rates
have declined since the mid- or early Tertiary, before
which time deep weathering profiles formed in broad
plains. Today, deep weathering profiles form only in
