Environmental Engineering Reference
In-Depth Information
Chapter
13
Denudation, weathering and mass wasting
Rocks are stable in the environment in which they form and inherently unstable in any
other. As endogenetic processes elevate continental crust and expose new land surfaces to
the atmosphere, geomorphic (exogenetic) processes commence their attack and
denudation
begins. Denudation describes the overall degradation and levelling of
continental land mass and is achieved by three sets of processes - weathering, mass
wasting and erosion. The first two occur in sequence and are found everywhere, although
their form and rates vary. Erosion removes rock debris in turn but it can also bypass these
stages and remove substantial volumes of fresh, unweathered rock, especially during
glaciation. This chapter focuses on why and how weathering occurs and its products
become redistributed downslope by gravity. Weathering has been described as the
static
attack
of meteorological elements, and the role of gravity on slopes as
passive
, to
distinguish it from the
dynamic
role of flowing water, ice and wind. This has some merit
but weathering may continue in transit, all mass wasting involves movement (sometimes
over long distances and at high velocity) and gravity also empowers rivers, glaciers and
coastal tides. Erosional and depositional processes in these four distinct domains are
examined in subsequent chapters, after an overview of denudation.
DENUDATION
Tectonic uplift mobilizes potential energy in the landscape; the higher the land surface,
the greater the potential energy. Sea level forms a general base level for the terrestrial
environment, although we know that terrigenous sediment fluxes continue across the
continental shelf and slope to the ocean floor. Sea-level change independent of tectonic
activity further disturbs denudation rates and, through ice sheet coupling, is a regular
feature of the Quaternary Earth. Valley floors, lakes, etc., act as temporary, local bases
for adjacent slopes. Denudation also requires the impact of solar-powered systems at
Earth's surface, determining hydrothermal and biological conditions of weathering,
applying force through wind circulation and raindrops and raising water through the
hydrological cycle to provide the kinetic energy of rivers and glaciers.
DENUDATION RATES
The assessment of denudation rates is complicated and takes various forms. We can start
with tectonics and follow the sediment cascade. Since granitic plutons and minerals such
as diamond are formed at depths between 10 km and 150 km, their surface exposure in
the core of orogens testifies to severe denudation since emplacement. Granitic rocks 8ยท8
km high on Mount Everest required the removal of 30-35 km of overburden, and a
