Geography Reference
In-Depth Information
Metamorphic rocks form through the partial melting and
reforming of existing sedimentary or igneous rocks often under
high pressure. Limestone and shale change to marble and slate
when metamorphosed, for example. These rocks tend to be more
resistant to weathering as they are harder than other rock types.
The rock cycle means that all rock types can convert into other
types. All rocks can be melted and cooled to form igneous rock.
All rock types can be weathered and eroded to form the layers of
sediment that can eventually become sedimentary rocks. Under
pressure and heat, igneous and sedimentary rocks can transform to
metamorphic rocks.
Weathering
Weathering is the breakdown of rocks by physical and chemical
processes. These processes often work together and biological
processes can include both of these mechanisms. Some types of
rock are more difficult to break down than other types of rock and
so weathering can result in interesting landforms such as headlands
or areas of protruding rock in an otherwise flat landscape (e.g.
Ayers Rock - now known by its Aborigine name Uluru - central
Australia) where one type of rock wears down more quickly than
another in the vicinity.
Physical weathering
Physical weathering transforms rock by breaking it into smaller
fragments through mechanisms such as freeze-thaw, salt weathering
and thermal cracking. However, physical biological action through
roots forcing openings in rock can be important locally. Freeze-
thaw is the process by which water freezes in small cracks and
expands by 9 per cent as it does so. This then forces cracks open
further, eventually splitting the rock. The process is more active
where temperature frequently fluctuates above and below 0°C.
Salt weathering is where salts within the environment form
crystals in small cracks under desert conditions. The relatively bare
rock surfaces in deserts along with large diurnal temperature ranges,
and the excess of evaporation over precipitation, can lead to salts
becoming concentrated in surface locations and cracking or flaking
