Presentation on theme: " 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."— Presentation transcript:
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
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.