Residual soils develop on plains and lowlands with moderate to gentle slopes and consist of loose, heterogeneous material left behind from weathering.
This material may include particles of parent rock, clay minerals, metal oxides and organic matter. This loose material is collectively called regolith , whereas the term soil is reserved for the topmost layer which contains organic matter.
The A-horizon is the topmost layer and is usually a meter or two thick. The upper portion of the A-horizon is often rich in organic matter, called humus , and may also contain inorganic material like insoluble clays and quartz. The A-horizon may take thousands of years to develop depending on the climate and acitivity of plants and animals.
This is the layer that supports crops and other types of vegetation. These precipitated minerals often accumulate in small pods, lenses and coatings. Organic matter is sparse in the B-horizon. The lowest layer constitutes the C-horizon and is comprised of cracked and variably weathered bedrock mixed with clays. Soils can vary significantly in color and composition. The particular type of soil that is produced in a region depends on the available materials, climate and also time.
The high temperatures, heavy rainfall and humidity of tropical regions have driven chemical weathering to the extreme. As a result, feldspars and other silicates have been completely altered while silica and calcite is extensively leached from the soil. The upper zone of laterite consists of insoluble precipitated iron and other oxides along with some quartz. At best, only a very thin layer of organic matter resides at the top of the soil to support the jungle vegetation.
When the jungle vegetation is cleared, the humus oxidizes quickly and soon disappears. For this reason, laterite can only be farmed extensively for a few years after clearing and afterwards must be abandoned. Feldspar, one of the most abundant rock-forming minerals, chemically reacts with water and water-soluble compounds to form clay.
Water contains many weak acids such as carbonic acid. This weak, but abundant, acid is formed when carbon dioxide gas from the atmosphere mixes with rainwater.
Sulfur dioxide and nitrogen gases create other types of acid rain that act as chemical weathering agents. Some sources of sulfur dioxide are power plants that burn coal; as well as volcanoes and coastal marshes. Hydration is the absorption of water into the mineral structure. A good example of hydration is the absorption of water by anhydrite, resulting in the formation of gypsum.
Hydration expands volume and also results in rock deformation. Figure 6. Dehydration is the removal of water from rock or mineral structures. A good example of dehydration is the removal of water from limonite, resulting in the formation of hematite.
Figure 7. The images above show the dehydration reaction of limonite Fe 2 O 3. H 2 O on the left to hematite Fe 2 O 3 on the right. They will best know the preferred format. When you reach out to them, you will need the page title, URL, and the date you accessed the resource. If a media asset is downloadable, a download button appears in the corner of the media viewer. If no button appears, you cannot download or save the media.
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The rock cycle is a web of processes that outlines how each of the three major rock types—igneous, metamorphic, and sedimentary—form and break down based on the different applications of heat and pressure over time. For example, sedimentary rock shale becomes slate when heat and pressure are added. The more heat and pressure you add, the further the rock metamorphoses until it becomes gneiss. If it is heated further, the rock will melt completely and reform as an igneous rock.
Empower your students to learn about the rock cycle with this collection of resources. An element is a substance that cannot be broken down into a simpler format.
They are distinguished by a unique atomic number. The elements are organized by their atomic number in the periodic table, which highlights elements with similar properties. Water is an example of a compound, a mixture of two or more elements, and is created when two hydrogen atoms bond to an oxygen atom. Use these resources to examine the properties and uses of elements and compounds. Weathering is the process of the weakening and breakdown of rocks, metals, and manmade objects. There are two main types of weathering: chemical and physical.
An example of chemical weathering is acid rain. Caused mostly by the burning of fossil fuels, acid rain is a form of precipitation with high levels of sulfuric acid, which can cause erosion in the materials in which it comes in contact. An example of physical weathering is wind blowing across the desert playas. This process causes rocks to form a specific pyramid-like shape and they are called ventifacts. Select from these resources to teach about the process of weathering in your classroom.
Sedimentary rocks are one of three main types of rocks, along with igneous and metamorphic. Metamorphic rocks start as one type of rock and—with pressure, heat, and time—gradually change into a new type of rock.
Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. Weathering describes the breaking down or dissolving of rock s and mineral s on the surface of the Earth. Water, ice, acids, salts, plants, animals, and changes in temperature are all agents of weathering. Once a rock has been broken down, a process called erosion transports the bits of rock and mineral away.
No rock on Earth is hard enough to resist the forces of weathering and erosion. Together, these processes carved landmark s such as the Grand Canyon, in the U. This massive canyon is kilometers miles long, as much as 29 kilometers 18 miles wide, and 1, meters 1 mile deep.
Weathering and erosion constantly change the rocky landscape of Earth. Weathering wears away exposed surfaces over time. The length of exposure often contributes to how vulnerable a rock is to weathering. Rocks, such as lava s, that are quickly buried beneath other rocks are less vulnerable to weathering and erosion than rocks that are exposed to agents such as wind and water. As it smoothes rough, sharp rock surfaces, weathering is often the first step in the production of soil s.
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