Geology Reference
In-Depth Information
ROCK AND WATER CYCLES
basin water runs like this. Precipitation entering the
system is stored on the soil or rock surface, or is inter-
cepted by vegetation and stored there, or falls directly
into a stream channel. From the vegetation it runs down
branches and trunks (stemflow), or drips off leaves and
branches (leaf and stem drip), or it is evaporated. From
the soil or rock surface, it flows over the surface (overland
flow), infiltrates the soil or rock, or evaporates. Once in
the rock or soil, water may move laterally down hillsides
(throughflow, pipeflow, interflow) to feed rivers, or it may
move downwards to recharge groundwater storage, or it
may evaporate. Groundwater may rise by capillary action
to top up the rock and soil water stores, or it may flow
into a stream (baseflow), or may exchange water with
deep storage.
The Earth's surface - the
toposphere
- sits at the inter-
faces of the solid lithosphere, the gaseous atmosphere,
and the watery hydrosphere. It is also the dwelling-place
of many living things. Gases, liquids, and solids are
exchanged between these spheres in three grand cycles,
two of which - the
water
or
hydrological cycle
and
the
rock cycle
- are crucial to understanding landform
evolution. The third grand cycle - the
biogeochemical
cycle
- is the circulation of chemical elements (carbon,
oxygen, sodium, calcium, and so on) through the upper
mantle, crust, and ecosphere, but is less significant to
landform development, although some biogeochemical
cycles regulate the composition of the atmosphere, which
in turn can affect weathering.
Rock cycle
Water cycle
The
rock cycle
is the repeated creation and destruc-
tion of crustal material - rocks and minerals (Box 2.1).
Volcanoes, folding, faulting, and uplift all bring igneous
and other rocks, water, and gases to the base of the atmo-
sphere and hydrosphere. Once exposed to the air and
meteoric water, these rocks begin to decompose and dis-
integrate by the action of weathering. Gravity, wind, and
water transport the weathering products to the oceans.
Deposition occurs on the ocean floor. Burial of the loose
sediments leads to compaction, cementation, and recrys-
tallization, and so to the formation of sedimentary rocks.
Deep burial may convert sedimentary rocks into meta-
morphic rocks. Other deep-seated processes may produce
granite. If uplifted, intruded or extruded, and exposed at
the land surface, the loose sediments, consolidated sedi-
ments, metamorphic rocks, and granite may join in the
next round of the rock cycle.
Volcanic action, folding, faulting, and uplift may all
impart potential energy to the toposphere, creating the
'raw relief ' on which geomorphic agents may act to
fashion the marvellously multifarious array of landforms
found on the Earth's surface - the physical toposphere.
Geomorphic or exogenic agents are wind, water, waves,
and ice, which act from outside or above the toposphere;
these contrast with endogenic (tectonic and volcanic)
agents, which act upon the toposphere from inside the
planet.
The
hydrosphere
- the surface and near-surface waters
of the Earth - is made of
meteoric water
. The water cycle
is the circulation of meteoric water through the hydro-
sphere, atmosphere, and upper parts of the crust. It is
linked to the circulation of deep-seated
juvenile water
associated with magma production and the rock cycle.
Juvenile water ascends from deep rock layers through
volcanoes, where it issues into the meteoric zone for the
first time. On the other hand, meteoric water held in
hydrous minerals and pore spaces in sediments, known
as connate water, may be removed from the meteoric
cycle at subduction sites, where it is carried deep inside
the Earth.
The land phase of the water cycle is of special inter-
est to geomorphologists. It sees water transferred from
the atmosphere to the land and then from the land back
to the atmosphere and to the sea. It includes a
surface
drainage system
and a
subsurface drainage system
.
Water flowing within these drainage systems tends to be
organized within
drainage basins
, which are also called
watersheds
in the USA and
catchments
in the UK.
The basin water system may be viewed as a set of water
stores that receive inputs from the atmosphere and deep
inflow from deep groundwater storage, that lose outputs
through evaporation and streamflow and deep outflow,
and that are linked by internal flows. In summary, the
