 Rocks are continually recycled on the Earth’s surface and in Earth’s interior, on a geological time scale.  Rock is often exposed to an environment different than the one in which it was formed.

 Formation  Forms at great depth  Under high temperature and  Pressure  Exposure – if moved to the surface  Colder temperatures  Less pressure  Chemicals  Physical contact

 The granite becomes unstable in this new environment under the new conditions.  The granite will change to attain an equilibrium with its new conditions.  The change is called weathering, which includes the physical breakdown and chemical alteration of the parent rock.

 Results in the decomposition of rock and its conversion to gravel, sand, clay and/or dissolved material.  There is little or no movement of the weathering products as these accumulate at the location of decomposition.

Breaking a rock apart creates more surface area, which results in faster weathering Weathered rocks take on a spheroid shape as corners offer more surface where more mechanisms for change can occur.

 Two methods  Mechanical or Physical weathering  Chemical weathering

 This reduces solid rock to fragments or grains but does not change the chemistry of the original rock or the fragments.  Fragments are just smaller than the original rock.

 Pressure-release (exfoliation)  Frost-wedging  Salt pressure  Organic activity  Thermal expansion and contraction

 Many igneous and metamorphic rocks are formed at great depth under very high pressure.  Over time these rock may be uplifted to the Earth’s surface which decreases the pressure on the rock.

 The rocks expand upon the release of the pressure but because the rock is cool and brittle it fractures as it expands and layer break or peel off.  This peeling is termed exfoliation.

 This action occurs in areas where both freezing and thawing can occur.  When water freezes, it expands by nine percent.  If water (as a liquid) accumulates in cracks or fractures of rocks, and then freezes, its expansion pushes the rock apart.  Upon melting of the ice, the weakened rock may actually break into fragments.

 If soil collects in a crack in a rock a seed may fall there and sprout.  The roots of the plant can work their way into the crack, expand with growth, and may eventually widen the crack.  Burrowing animals, such as earthworms, can break apart soft rocks such as gypsum or limestone.

 Earth’s surface are continually exposed to changes in temperature.  Rocks expand when heated and contract when cooled.  During rapid temperature changes, this expansion and contraction may fracture the rock.

 When groundwater containing some salt, seeps into pores and cracks in rocks is when this takes place.  If the water evaporates (as pure water), the salt crystallizes.  The growing crystals exert pressure on the rock.  This pressure may be enough to loosen grains and widen cracks in the rock. Very common along coastlines where sea spray brings saltwater onto rocks on land

 Here the weathering involves a change in chemistry when air and water chemically react with the rock to alter its composition and mineral content.  The final product of chemical weathering differ both physically and chemically from the original rock

 Oxidation  Acid dissolution  Solution weathering  hydrolysis

 With the oxidation of most of these elements in nature sulphur is involved.  When sulphur is released and reacts with water it form sulphuric acid, a strong agent of chemical weathering.

 This is a chemical reaction that occurs between rock and an acid.  When an acid mixes with water it breaks down (dissociates) releasing hydrogen ions (H+). These ions are very small and have a positive charge.  When the hydrogen ions come in contact with a rock, the hydrogen ions are substituted for ions in the minerals that make up the rock. This changes the structure of the mineral and causes it to break down.

 The stronger the acid the more rapidly the rock will break down. There are very few naturally occurring strong acids on Earth.  The most prevalent cause of acid dissolution is from rainwater.  Rainwater is slightly acidic. This is because atmospheric carbon dioxide dissolves in raindrops and react to form weak carbonic acid. Acid Rain

 In this process it is similar to Acid Dissolution except that in the chemical reaction the mineral is dissolved, leaving no solids, and turned into a liquid.  The minerals are not changed in terms of the chemical make up they merely change to a liquid state.  Rocks like limestone and gypsum, caves are formed when groundwater dissolve the minerals.

 Here the mineral of a rock bonds with the water.  Water reacts with the mineral to form a different mineral.  This type of weathering is the most complex of all chemical weathering processes.  Still it produces the most common sediment on Earth, clays.  These clays, once deposited and lithified, become shale.