Geoscience Reference
In-Depth Information
to Quaternary glaciation ~ with ice sheet and sea ice growth increasing albedo, reducing absorption and hence further
hemispherical cooling.
Tectonic influences go much further. Uplift controls the position, alignment and orography of high-altitude land surfaces
which disturb major components of atmospheric circulation at global scales. Rossby waves in mid-latitude zonal jet
streams of both hemispheres and the monsoon climates of the Indian Ocean region are triggered by western American
cordilleras and the Tibetan plateau respectively (
Figure 10.18
).
Most Himalaya/Tibet uplift occurred during the past 5
Ma since the early Pliocene, making the resultant monsoon a very recent addition to Earth's climate. Continental-
scale topography also channels or obstructs meridional air flow. The north-south alignment of North American
mountain ranges and lowland basins permit regular seasonal and aseasonal outbreaks of north-moving moist tropical
air from the Gulf of Mexico and south-moving cold Arctic air - reducing zonal climate indices. In direct contrast, the
east-west alignment of the Pyrenees - Alpine ranges blocks such meridional exchanges in Europe - reinforcing zonal
climate indices, with infrequent exceptions.
As well as disturbing global circulation mechanically, orography also substantially alters its moisture and related latent
energy transfers and precipitation regimes. Orographic airflow uplift greatly enhances precipitation on windward slopes
with corresponding rain shadow and aridity on leeward slopes. With active, subduction zone tectonic uplift
concentrated near continental margins, this greatly enhances continental aridity. Altitude alone ensures higher
proportions of snowfall. This also emphasizes the effects of substituting free atmosphere conditions with high-altitude
land surfaces in creating further, localized disturbance of a range of energy, moisture balance and biogeochemical
conditions.
The most recent advances in Earth systems science, tectonics and global climate change concern what can be termed
tectonobiogeochemical processing! Given the key role of atmospheric carbon dioxide in controlling greenhouse (+
CO
2
) and icehouse (- CO
2
) conditions and current global warming (+ CO
2
), carbon cycling involving Earth's two largest
reservoirs - the geosphere (66 10
9
Gt) and deep oceans (38
10
3
Gt) - comes under scrutiny. Here we outline
tectonic inputs of carbon dioxide to the atmosphere through volcanic activity and consider its sequestrationor removal
in the box, p. 294. The spreading rateor BLAG hypothesis(identified by its authors' initials) asserts that CO
2
emissions
from mid-ocean ridges (MOR) and subduction zone volcanoes increase during periods of faster sea-floor spreading
and compensating subduction. Whilst this relationship between spreading rates and global temperature appears to
work well during the greater part of the Cenozoic, global cooling despite faster spreading rates during the most recent
15 Myr suggests the climate-tectonic system is more complicated. Volcanic activity also has very short-term climate
impacts through the eruption of SO
2
, which combines with atmospheric H
2
O to form H
2
SO
4
sulphate aerosols capable
of blocking some incoming short-wave radiation.
additionally, some of Earth's least common elements - is
the descendant of Archaean crust. Its outer terrestrial
fractionates formed an upper layer of volcanic and
intrusive granitic, low melting-temperature products on
a metamorphosed granitic base. Only 15 per cent survives,
which emphasizes the role of erosional, sedimentary and
metamorphic processes in reworking primary continental
crust without removal from the continental system; and
offscrape, accretion and B-subduction recycling at
destructive plate boundaries and remelt of continental
lithosphere over hot spots.
Continental crust is more extensive than is suggested
by the ratio of land to sea area. It accounts for 39 per cent
of all crust, or 0·6 per cent of Earth's volume, whereas
Continent formation, evolution and architecture
We have seen that tectonic activity exchanges material
between ocean and continental crust, consigning some
terrestrial erosion products to the subduction melting
pot and accreting ocean sediments and crust (as ophiolite)
on to continental margins. Yet their mean ages indicate
that continental crust (1·1 Ga) is largely conserved,
whereas oceanic crust (55 Ma) is largely recycled. It is
thought now that new continental crust has been added
slowly, by the accretion of oceanic plate at a rate of
approximately 1·3 km
3
yr
-1
over the past 1 Ga. Some
50-70 per cent of primary continental crust was formed
by
c
. 2·5 Ga ago, during the Archaean. Continental
lithosphere - a 'penultimate silicate froth' containing,
