Geoscience Reference
In-Depth Information
7
Water present in the rocks below the ocean floor is
pushed under the surface during plate destruction.
At depths it reduces rock melting points, thus permitting
volcanism at island arcs and also manufacture of continen-
tal land masses over the last
phases, especially iron (from ferrous to ferric and back
again), and other elements essential to efficient cellular
metabolism. Lithospheric plates, lubricated and melted by
water fluxes, recycle all accumulated elemental deposits
from ocean water to sediment to atmosphere over
timescales of 10
6
-10
8
years. The absence of recycling over
such time periods would have meant that all atmospheric
and oceanic primary production and deposition of ele-
ments like carbon would simply have accumulated subse-
quently. Thus global cycling requires both flux and
reservoir; it is plate tectonics that supplies the necessary
renewal of the reservoir through the working of what we
term
Cybertectonic Earth
.
3Gy.
1.2.2 The planetary evolutionary consequences of
water and plate tectonic cycling: Cybertectonica
As we noted earlier, since about 2.5 Ga, the plant bio-
sphere has produced an oxygenated atmosphere that has
allowed the subsequent evolution of animal life and the
oxidative release of elements locked up in certain mineral
1.3
Earth systems snapshots
1.3.1
Dust storm from the 1930s “Dustbowl”
for sure, though a combination of causes is likely. Export
of dust across the Pacific to the western USA may in
future lead to intergovernmental cooperation to alleviate
environmental hazard.
Atmospheric winds pick up millions of tons of silt and clay
from the land surface annually. Atmospheric turbulence
initially suspends this finer sediment, leaving sand particles
to travel as denser bedload “carpets” close to ground sur-
face. Transfer to the middle atmosphere along frontal air
masses results in long distance transport, then deposition
from dry suspension to form sediment accumulations
called
loess
. The finest sediment, together with any pollu-
tants picked up en route, remains aloft for years, circum-
navigating the globe many times, eventually depositing
due to “rain-out.” Deposition in the oceans contributes
vitally to the input of elements, such as iron, necessary for
the efficient metabolism of phytoplankton. An increasing
frequency of dust storms in East Asia in recent years has
brought back memories of the infamous “Dust Bowl” of
the western USA in the 1930s (Fig. 1.6). The relative
importance of human environmental degradation versus
regional climate change in both cases is not known
1.3.2
River canyon cutting uplifting plateaus
Water collects in the upstream catchment to run down the
major river channel tributary, collecting sediment and
water from countless other tributaries and hillslopes as it
does so. The power of the water flow enables a certain
magnitude of sediment to be transported close to the bed
where it is able to erode bedrock by abrasion and hence to
form a valley. The abrasional process is rapid compared
with the lateral mass wasting of the valley walls that are
kept up by periodic layers of more resistant rock.
The River Colorado (Fig. 1.7) has thus been able to keep
pace with regional uplift of the entire Colorado Plateau
area caused by tectonic processes in the Earth's interior:
the end result is the spectacular Grand Canyon.
1.3.3
Desert flash flood from overland flow
The silt- and mud-laden
flash flood
in Arizona, USA
(Fig. 1.8) has developed tumultuous upstream-migrating
waves of turbulence. The flood started as thunderstorm
precipitation 24 h earlier. The dry, compacted earth and
rock outcrops in the upstream drainage catchment inter-
cepted the rainfall but low permeability of the rocky
surface soil and the absence of vegetation led to condi-
tions whereby water was unable to infiltrate the soil
sufficiently quickly to prevent development of overland
Fig. 1.6
Dust storm front with typical overhanging head composed
of lobes and clefts. Prowers Co, Colorado 1937.
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