Soil Forming Factors: Deposits

Physical and chemical weathering processes erode parent material into mineral particles and dissolve minerals in solution, but weathering does not transport or deposit weathered minerals. Volcanoes, wind, water, ice, and waves transport and deposit weathered minerals, they are influenced by gravity, and upon deposition create landforms from which soil formation begins. Minerals weathered from parent material determine the mineral content in sand, silt, and clay.

Sand: The major mineral in sand is called quartz and it is composed of silica and oxygen (SiO₂). Sand grains vary in size depending on how long they are exposed to weathering. Quartz is very resistant to weathering; therefore, sand grains are larger in diameter compared to silt and clay particles:

very coarse sand: 2.00 - 1.00 mm diameter
coarse sand: 1.00 - .50 mm diameter
medium sand: 0.50 - 0.25 mm diameter
fine sand: 0.25 - 0.10 mm diameter
very fine sand: 0.10 - 0.05 mm

Because sand is larger in diameter than other soil separates, sand grains provide larger spaces in which water and air can more easily move through soil. Sandy soils are limited in nutrients because the nutrients leach out from the large pore spaces between sand grains, so crops are generally not grown in sandy soils.

Silt: Silt contains silicate minerals like sand but the diameter of silt particles is smaller, 0.05 - 0.002 mm, and so the pore space between silt particles are smaller. Therefore, silt has the ability to hold water between particles and retains nutrients for plant use. Silt is an ideal soil for growing crops.

Clay: Silicates, mica, iron, and aluminum hydrous-oxide minerals are found in clay. The silicate clay group is primarily located in the mid-latitudes, while the iron and aluminum clays are found in the tropic zones. Clay particles are 0.002 mm in diameter or smaller, so the pore spaces between clay particles are very small. Thus, water and air movement through clays particles is significantly decreased. When clay becomes wet it swells, sticks together (cohesion), and feels "sticky". As wet clay dries it shrinks and cracks. Clay also becomes dense, hard, and brittle making it difficult for plant roots to grow through. Clay soils containing laterite and smectite properties are not desirable to grow crops in or to build on.

Depositional Environments and Landforms

Alluvial Deposits- rock debris that has been eroded into fine sediments that are subsequently transported by a mountain stream or river to the valley floor, as the gradient of the mountain decreases. Sediment is carried by either ephemeral (intermitant) water flow that occurs in arid climates or perennial stream water flow that occurs in humid climates, and is subsequently distributed into fan shaped landforms called alluvial fans.

Alluvial fans are known to be either wet or dry, depending on if the fans are located in humid or arid climates. Alluvial soils are finely layered and are very deep. Closer to river banks and on natural levees alluvial soils are more sandy, however alluvial soils are more clayey to peaty when closer to swampy areas. Alluvial deposits such as the Mississippi River Delta and China's vast alluvial plains have rich top soil and are known for being very fertile, crop growing regions.

Colluvial Deposits- materials that move downslope by force of gravity and/or erosion and collect at the base of mountains or foothills, with little or no sorting. Talus cones are a type of colluvial deposit. Soils from colluvial deposition are generally deep and fragipans (hard clay soil) are common.

Eolian Deposits- eolian deserts form in arid regions of the world where dry air masses create wind systems that transport and deposit loose sediments. Silt particles, called loess, are carried by wind even longer distances than sand and collect around the fringe of deserts. Large areas of the desert environment that receive more than 125 square kilometers of eolian sand are called sand seas or ergs, such as Erg Chech in Algeria. The largest desert in the world, the Sahara Desert, is 7 million square kilometers and contains several ergs. Smaller areas are called dune fields. Wind force and variable wind directions transport and deposit sand and in the process create different types of dunes. Some dunes are shaped by the wind into ridges, strings, domes, stars, or barchans (half-moon shaped dunes). Deserts primarily consist of wind-deposited sand which originated from sandstone that eroded over time.

Glacial Deposits- Glaciers are large and small ice masses that are found at high latitudes on Earth.Mountains located at all latitudes have small glaciers. During the Pleistocene, 10,000 years ago, glaciers extended into much lower latitudes and elevations than are currently located. As the climate changed and weather got warmer, glaciers began to melt and abrade bedrock lying below the glaciers. Varying rates of ice melt caused eroded sediment to "drop out" of retreating, melting glaciers. This "glacial till" formed deposits called moraines and drumlins. Glacial till consists of unstratified (unlayered) and unsorted glacial deposits, some the size of huge boulders.

Meltwaters flowing upon, under, within or at the margin of glaciers accumulate deposits known as outwash plains and kettles (depressions), kames (small, mound shaped accumulations of sand or gravel), and eskers (narrow, sinuous ridges of sediment).Where glaciers extend beyond the mouths of river valleys and enter the sea, their glaciomarine sediment load is dumped into the ocean.

As climates warm glaciers melt and retreat. Glaciofluvial (glacier stream water) sediment is transported downstream by way of glacial meltwater and is deposited in braided streams. Glaciolacustrine (glacier lake water) sediment is deposited in glacial lakes when damming of ice or moraines occurs, and fluctuations of meltwater flow create distinctive varve deposits. Fine glacial debris consisting of silt and clay becomes airborne where vegetation is not present to hold this sediment down, and often traveling hundreds of kilometers before landing and forming loess deposits. The Muir Glacier and Margerie Glacier in Glacier Bay, Alaska are actively retreating glaciers.

Lakes are different than marine environments in that sedimentation of lakes is ten times higher than in marine environments. Lakes are also smaller, are nearly closed systems, and tides in lakes are less pronounced. Therefore energy levels in lakes are lower, coarser sediment (sand and gravel) is deposited in shallow water areas of lakes, especially during summer, while finer-grained sediment (silt and clay) is deposited in deeper water areas of lakes, and more so during winter. Varves, alternating thin layers of light-colored coarser grained sediment and dark-colored finer grained sediment, are one type of lacustrine deposit and form in both glacial and nonglacial lakes.

Deposits in open lakes come mainly from rivers but may also be deposited by wind, ice-rafting, and volcanic rock erosion. Sedimentation in closed lake systems consists of evaporite minerals, carbonate muds, sands, and silts. Lacustrine deposits are often rich in organic shales which are important source rocks for petroleum. Well-known lacustrine shale deposits in the world include the Eocene Green River Formation in Wyoming,Utah and Colorado; the Jurassic Morrison Formation of the Colorado Plateau; Devonian sediments from the Old Red Sandstone of the Orcadian Basin in northeast Scotland; and the Triassic Keuper Marl of South Wales, just to name a few.

Loess Deposits- Loess is comprised primarily of silt grains, with less significant anounts of clay and sand. The mineral quartz is most dominant in loess with feldspars, carbonates, and clay minerals present in smaller amounts. For instance, in arid regions loess contains larger amounts of calcium carbonate; whereas, in humid regions clay minerals in loess are more prevalent. Desert regions of the world may be thought of as prime locations for loess deposition because of the availability of loose sediment, sparse vegetal cover, and moderate to strong winds. However, loess deposits are more commonly located in or near glacial regions.

Glacial outwash debris containing sand, silt, and clay is transported to floodplains by rivers that drained glacial meltwater. The glacial debris, primarily the silt and clay, becomes airborne via strong winds as vegetation is not present to hold sediment down. Loess can sometimes become suspended several kilometers high and hundreds of kilometers in distance, with tens to hundreds of tons of sediment being transported in a single "dust storm", as was the case in the 1935 dust storm over the midwest United States. Near Wichita, Kansas a dust storm had suspended about five million tons of sediment over a 78 square kilometer area and around 300 tons per square kilometer of dust was deposited from the same storm near Lincoln, Nebraska. Click here to see a picture of a Nile River dust storm.

Marine Deposits- physical processes mainly rework and distribute carbonate materials on marine shelf but can also help in the production of carbonates. Moderate water ciruculation on marine shelf brings nutrients from deeper water to shallow shelf region which aids in organic growth of ooids, fecal pellets that eventually become cemented together. Waves constantly move fine carbonate mud and coarser sediment to form sand or gravel covered tidal flats, beaches, dunes, marshes, lagoons, and swamps or transports these sediments seaward to form spits, tidal deltas and bars, and barrier islands. Waves pounding against coastal rocks also contribute rock particles and sediment to the coastal shelf and seaward. The Outer Banks of North Carolina are a chain of barrier islands that contain beaches, lagoons, and spits. Reefs can be characterized as either thick masses of living carbonate "rock", or structures produced by sediment-binding, live organisms. The Great Barrier Reef, Australia, is the largest coral reef in the world.

Bioherms are mounds of dead organic material that have collected in rocks of different composition. Organisms are able to extract calcium carbonate (CaCO₃) from seawater to build protective shells or skeletons, although the availability of CaCO₃ in seawater is controlled by pH, temperature, and carbon dioxide content. When these organisms die their remains collect and form carbonate deposits known as bioherms. Carbonate formers in Earth's current oceans are not the same as those that formed carbonates in ancient oceans.

Other marine depositional environments include deltas, beaches, barrier bars, estuaries, lagoons, and tidal flats. Beach and barrier bar deposits are mostly contain fine to medium grained, well sorted sand as well as placer gold, platinum, and other minerals.

Estuarine deposits, like in the Chesapeake Bay and San Francisco Bay, consist of cross-bedded sands and mud, or a mixture of both sand and mud. Lagoonal deposits include evaporites, fine-grained sediments, and black shales. Delta deposits and tidal flat deposits, like the Mississippi River Delta, primarily contain muds in the upper zone, mud and sand in the middle zone, and sand in the lower zone.

Volcanic Deposits- Volcanoes produce magmas consisting of various mineral compositions that in turn create various rock types. The amount of gas in magma and the viscosity (thickness) of magma determine the volatility of a volcanic eruption and the types of landforms that are formed. Continents and oceanic environments contain highly fluid, basaltic magma whereas magma forming as island arcs at the margins of some continents consists of high silica lavas that are more viscous and crystallize into rhyolites, andesites, and dacites.

Lava in orogenic (mountain building) environments is most viscous (thick) and has a higher gas content so eruptions are more explosive and form an extrusive, solid volcanic material called tephra. Volcanic ash is found in the United States primarily in Hawaii, Washington, Oregon, and also in Japan, Indonesia, Central America, and other mountainous regions of the world. Most volcanic ash forms into very fertile soil that is used for growing crops.

Lava Plains and Plateaus - these types of volcanic landforms are created as a large volume of fluid lava flows over an expansive surface area. The resulting topography are extremely flat surfaces that aggrade with each successive lava flow that is mafic in composition. An example of a lava plain is located in the Columbia River Plain in Washington and Oregon. Oceanic plains and plateaus can also form, even more extensively, from this type of lava flow.
Cones or Shields - most volcanoes form into composite cones that have a distinctive appearance of layers of interbedded, blocky lava with tephra that is mostly ash or cinder. The peaks of composite cones have narrow, circular bases whose peaks rise several thousands of meters. Mount Ranier, in Washington is a composite volcano. Conversely, shield volcanoes are comprised of fluid basaltic magma with very little tephra and therefore have lower peaks than composite cone volcanoes. The chain of Hawaiian volcanoes are shield volcanoes.
Calderas- this type of volcanic landform is created once eruptions occur and subsequently the upper part of a volcano collapses inward. Volcanoes containing tephra sheets, such as the composite cones, are more prone to forming calderas once an eruption occurs. Crater Lake, Oregon and Yellowstone Plateau,Wyoming are calderas.

Landforms are not always formed by deposition alone. For instance, the spectacular Grand Canyon land formation has been cut and etched by the erosive forces of wind and water over millions of years. The Colorado River has deeply cut gorges into less resistant rock and created a canyon while more resistant, less weathered rocks, such as sandstones, give the Grand Canyon its statuesque appearance.

Also, human activity on the land can have a significant affect on erosion due to construction and agricultural practices, as is seen in this picture of eroded sediment entering and filling the San Francisco Bay, in California.

Can you think of other landforms that have been created by erosional and depositional forces?

Can you think of ways we can improve how land is used and how we can protect sediment from eroding from the land and entering into rivers, lakes, deltas, and bays?

Information contained in Earth Deposits: A Basis for Creating Landforms and Soil was derived from Principles of Sedimentology and Stratigraphy, by Sam Boggs, Jr., Prentice Hall, Englewood Cliffs, NJ. 1995.

Images in Earth Deposits: A Basis for Creating Landforms and Soil, were extracted from the Earthrise web site. Make sure to visit the Earthrise web site, to see NASA Space Shuttle images of landforms worldwide!