Weathering
is the process in which rocks are broken into smaller pieces. Small pieces may
be moved away from the original rock when erosion occurs. There are three main
types of weathering: physical, chemical and biological.
Physical Weathering
Physical
forces can break rock into smaller pieces. This may be caused by expansion and
contraction of rock as it heats and cools. If water seeps into a crack or pores
in a rock, it expands when frozen and can break apart the rock.
This volcanic bomb was broken apart by frost wedging in
Alaska. (MFitz, Katmai National Park and Preserve 2010, Public domain)
Wind and
waves shape rocks at seashores and in deserts into dramatic shapes. The process
occurs more quickly as the wind or water carries sand and other sediments. The
sediment pieces act as tiny hammers, bashing away at the rock and creating more
sediments (broken rock material). Over time, running water can carve dramatic
landscapes.
The bluffs at Torrey Pines State Natural Reserve are
shaped by wind and waves.
Chemical
Weathering
The most
common causes of chemical weathering are acidic water and oxygen. Acidic water moves
through cracks in limestone or marble, gradually dissolving rock and creating extensive
cave systems like Jenolan Caves.
Jenolan Caves were carved out by chemical weathering of
limestone due to acidic water. (CSIRO Science Image)
Oxidation is
the reaction of oxygen with minerals to create new substances such as the iron
oxide which can be found on the surface of many of Australia’s rocks. You may
be able to spot oxidation and other signs of chemical weathering on the surface
of broken rocks. Weathering causes colour to change.
This sandstone has a weathered outside that is much
darker in colour than the inside.
Webpage:
- How do limestone caves form? (Jenolan Caves)
Biological
Weathering
Living
things are responsible for biological weathering. This may have a chemical
component, like the weathering caused by plants. Plants release a weak acid that
dissolves minerals in the rock. The minerals can be used by the plant for
nutrients and the acid makes little holes for rootlets to grow into.
Lichens and moss release weak acids that gradually
dissolve minerals in the rock.
Animals and
plants also cause physical weathering of rock. Tree roots grow into cracks in
rock and pry the rock apart. Some animals dig holes in rocks by scraping away
the grains. This may be to create protective holes or as a side effect of
feeding behaviour. Animals also cause physical weathering by tracking. Rock is
worn away on tracks that are often used like stone stairways in popular parks.
Chitons have a rasping tongue with teeth on it called a
radula that scrapes algae off rocks, but also scrapes away some of the rock.
Sandstone steps in the Blue Mountains have been worn down
by thousands of hikers.
The most
extreme example of biological weathering is a type of shipworm described in
2019. This ‘worm’ is actually a clam that eats limestone.
Video:
- These Clams Eat Rock! (LiveScience)
- Discovery of a new species/genus – Lithoredo abatanica (Northeastern University)
Explore
weathering at home
BBC Bitesize Science provides a written summary of weathering and the rock cycle.
Observe the
effect of oxidation.
Watch videos
about chemical, physical and biological weathering.