Introduction to Soil Forming Factors

Soil Forming Factors

Learn about how the following factors all work together to form and develop soil.

Earth Deposits: A Basis for Creating Landforms and Soil
About Minerals
Parent Material: The Story Of Rocks and Soil
About Weathering

Earth Deposits: A Basis for Creating Landforms and Soil

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

              Loess deposit
          (Loess deposit in Illinois. Image courtesy of the Illinois State Museum)
Landforms are constantly developing and changing as weathering forces continually erode rocks and transport agents (volcanoes, wind, water, ice, and waves) continually deposit the eroded rocks and sediment in to different depositional environments,on Earth's surface.

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.

 


Lacustrine Deposits

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.

Three major volcanic landforms are created as a result of volcanic activity.

  1. 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.
  2. 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.
  3. 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!


About Minerals

A mineral is defined as being a naturally occuring element or compound that is formed by inorganic processes and contains a crystalline structure. Pedologists are primarily concerned with minerals in soil because minerals form the basic framework of soil. Minerals originally form when once-heated Earth material magma (molten rock) cools and forms solid igneous rock. During the cooling process of magma, ions (an atom, a group of atoms or compound that is electrically charged when the loss or gain of electrons occurs) become bonded together, due to electrical attraction. The attracted, bonded ions remain fixed in position and produce solid crystalline minerals within igneous rock.The Earth's crust formed and continues to form in this manner.

Earth's crust contains a combination of naturally occurring elements, of which eight elements are predominant: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), Sodium (Na), potassium (K), and magnesium (Mg). As you can imagine, combinations of these elements along with the other naturally occuring elements that form Earth's crust produce a wide variety of minerals.

Igneous rocks contain original minerals that form as magma cools but sedimentary rocks are formed by secondary minerals that grow and join sediment particles together and become cemented. Metamorphic rocks were once igneous rocks and sedimentary rocks that become chemically altered to form different minerals. Minerals that combine to create inorganic parent material can be released from their attractive bond, during chemical weathering, and become deposited as soil. Deposits that come from parent material are either residual or transported. Residual deposits result when a rock is weathered in situ (in place). In contrast, transported deposits get moved by transport agents, often long distances.


 

Original Minerals

 

    NAME

CHEMICAL FORMULA

Quartz

SiO2

Microcline

KAlSi3O8

Orthoclase

KAlSi3O8

Na-Plagioclase

NaAlSi3O8

Ca-Plagioclase

CaAlSi3O8

Muscovite

KAlSi3O10

Biotite

KAl(Mg-Fe)3Si3O10 (OH)2

Horneblende

Ca2Al2Mg2Fe3, Si6O (OH)2

Augite

Ca2(Al-Fe)4(Mg-Fe)4Si6O 24

       

      ( Extracted from The Geography of Soils, by Donald Steila. Prentice Hall, Inc., New Jersey )

       

       


       

 

Secondary Minerals

 

NAME

CHEMICAL FORMULA

Calcite

CaCo3

Dolomite

CaMg(CO3)2

Gypsum

CaSO4-2H20

Apatite

Ca5(PO4)3 - (Cl, F)

Limonite

Fe2-O3-3H20

Hematite

Fe2O3

Gibbsite

Al2-O3-3H2O

Clay Minerals

Al silicates

( Extracted from The Geography of Soils, by Donald Steila. Prentice Hall, Inc., New Jersey )

Click here to learn more about minerals and their classifications


Written by: Christy Spector


The Story of Rocks and Soil

Although many of us don't think about the ground beneath us or the soil that we walk on each day, the truth is soil is a very important resource. Processes take place over thousands of years to create a small amount of soil material. Unfortunately the most valuable soil is often used for building purposes or is unprotected and erodes away. To protect this vital natural resource and to sustain the world's growing housing and food requirements it is important to learn about soil, how soil forms, and natural reactions that occur in soil to sustain healthy plant growth and purify water. Soil is important to the livelihood of plants, animals, and humans. However, soil quality and quantity can be and is adversely affected by human activity and misuse of soil.

Certain soils are best used for growing crops that humans and animals consume, and for building airports, cities, and roads. Other types of soil have limitations that prevent them from being built upon and must be left alone. Often these soils provide habitats for living creatures both in the soil and atop the soil. One example of soils that have use limitations are those that hold lakes, rivers, streams, and wetlands. Humans don't normally establish their homes in these places, but fish and waterfowl find homes here, as do the wildlife that live around these bodies of water.

Natural processes that occur on the surface of Earth as well as alterations made to earth material over long periods of time form thousands of different soil types. In the United States alone there are over 50,000 different soils! Specific factors are involved in forming soil and these factors vary worldwide, creating varied soil combinations and soil properties worldwide:

The Five Soil Forming Factors

1. Parent material: The primary material from which the soil is formed. Soil parent material could be bedrock, organic material, an old soil surface, or a deposit from water, wind, glaciers, volcanoes, or material moving down a slope.

2. climate: Weathering forces such as heat, rain, ice, snow, wind, sunshine, and other environmental forces, break down parent material and affect how fast or slow soil formation processes go.

3. Organisms: All plants and animals living in or on the soil (including micro-organisms and humans!). The amount of water and nutrients, plants need affects the way soil forms. The way humans use soils affects soil formation. Also, animals living in the soil affect decomposition of waste materials and how soil materials will be moved around in the soil profile. On the soil surface remains of dead plants and animals are worked by microorganisms and eventually become organic matter that is incorporated into the soil and enriches the soil.

4. Topography: The location of a soil on a landscape can affect how the climatic processes impact it. Soils at the bottom of a hill will get more water than soils on the slopes, and soils on the slopes that directly face the sun will be drier than soils on slopes that do not. Also, mineral accumulations, plant nutrients, type of vegetation, vegetation growth, erosion, and water drainage are dependent on topographic relief.

5. Time: All of the above factors assert themselves over time, often hundreds or thousands of years. Soil profiles continually change from weakly developed to well developed over time.

 


 
    Differences in soil forming factors from one location to another
    influence the process of soil formation
     

    processes of soil formation

  (Image courtesy of the United States Department of Agriculture, Soil Conservation Service)  

  Parent Materials

Soil forms from different parent materials; one such parent material is bedrock. As rocks become exposed at Earth's surface they erode and become chemically altered. The type of soil that forms depends on the type of rocks available, the minerals in rocks, and how minerals react to temperature, pressure, and erosive forces. Temperatures inside the Earth are very hot and melt rock (lithosphere) that moves by tectonic forces below Earth's surface. Melted rock flows away from the source of heat and eventually cools and hardens. During the cooling process, minerals crystallize and new rock types are formed. These types of rocks are called igneous rocks, the original parent material rocks formed on Earth.

Igneous rocks, under the right environmental conditions, can change into sedimentary and metamorphic rocks. Volcanoes produce igneous rocks such as granite, pumice, and obsidian.

Sedimentary rocks are formed when older rocks are broken apart by plant roots, ice wedges, and earth movements and become transported by glaciers, waves, currents, and wind. The transported particles then become bound together (cemented) as secondary minerals grow in the spaces between the loose particles and create a new, solid, sedimentary rock. Sandstone, limestone, and shale are types of sedimentary rocks that contain quartz sand, lime, and clay, respectively.

Metamorphic/Crystalline rocks form when pressure and temperature, below Earth's surface, are great enough to change the chemical composition of sedimentary and igneous rocks. Metamorphic rocks, such as quartzite, marble, and slate form under intense temperature and pressure but were originally quartz sandstone, limestone, and shale.

Other types of parent material that mineral soils form from are called Recent Cover Deposits and include alluvium, colluvium, eolian deposits, glacial deposits, lacustrine (lake) deposits, loess deposits , marine deposits, and volcanic ash deposits.

Soil Formation image extracted from "From the Surface Down-An Introduction to Soil Surveys for Agronomic Use", United States Department of Agriculture, Soil Conservation Service, 1994


About Weathering

As you drive or ride in a car, take a train or plane, ride a bike ride, or go for a nature walk you see the spectacular and varied landscapes on Earth's surface. As Earth's crust is built up by volcanic and tectonic forces (thrusting and deformation of Earth's crust), weathering forces simultaneously reduce landforms and release minerals from rocks. Natural weathering processes occur around us everyday, continually rearranging and building landforms on Earth's surface.

 


 

Chemical Weathering Processes

 

    Chemical weathering occurs as minerals in rocks are chemically altered, and subsequently decompose and decay. Increasing precipitation (rain) speeds up the chemical weathering of minerals in rocks, as seen on tombstones and monuments made of limestone and marble. In fact, water is an essential factor of chemical weathering. Increasing temperature also accelerates the chemical reaction that causes minerals to degrade. This is why humid, tropical climates have highly weathered landforms, soils, and buildings.

    bulletCarbonation and Solution: this weathering process occurs when precipitation (H20) combines with carbon dioxide (CO2) to form carbonic acid (H2CO3). When carbonic acid comes in contact with rocks that contain lime, soda, and potash, the minerals calcium, magenesium and potassium in these rocks chemically change into carbonates and dissolve in rain water. Karst topography, originally named after the Krs Plateau in Yugoslavia where it was first studied, is a result of this type of chemical weathering that possesses characteristic sinkholes, caves, and caverns.

    bulletHydrolysis: this chemical weathering process occurs when water (H20), usually in the form of precipitation, disrupts the chemical composition and size of a mineral and creates less stable minerals, thus less stable rocks, that weather more readily.

    bulletHydration: water (H20) combines with compounds in rocks, causing a chemical change in a mineral's structure, but more likely will physically alter a mineral's grain surface and edges. A good example of this is the mineral Anhydrite (CaSO4). Anhydrite chemically changes to Gypsum (CaSO4-2H20) when water is added. Gypsum is used in the construction industry, to build buildings and houses.

    bulletOxidation: this process occurs when oxygen combines with compound elements in rocks to form oxides. When an object is chemically altered in this manner it is weakened and appears as "oxidized" . A good example of this is a "rusting" sign post . The iron in the metal post is oxidizing. Increased temperatures and the presence of precipitation will accelerate the oxidation process.

    bulletSpheroidal Weathering: water penetrates through cracks in rocks and dissolves the cement that binds particles together and also erodes sharp edges and corners of rocks, making a rock appear spheroidal. Physical weathering processes, such as frost wedging, can then act upon the enlargened cracks in rocks.

 


 

Physical Weathering Processes

 

    Rocks that are broken and degrade by processes other than chemical alteration are physically or mechanically weathered. A rock broken in to smaller pieces exposes more surface area of the original rock. Increasing the exposed surface area of a rock will increase its weathering potential.

    bulletAnimals and Plants: Animals burrow into Earth's substrate and move rock fragments and sediment on Earth's surface, thereby aiding in the disintegration of rocks and rock fragments. Fungi and Lichens are acid-producing microorganisms that live on rocks and dissolve nutrients (phosphorus, calcium) within rocks. These microorganisms assist in the breakdown and weathering of rocks.

    bulletCrystallization: As water evaporates moisture from rocks located in arid climates mineral salts develop from mineral crystals. The crystals grow, spreading apart mineral grains in the process, and eventually break apart rocks.

    bulletTemperature Variation: minerals in rocks expand and contract in climates where temperature ranges are extreme, like in glacial regions of the world, or when exposed to extreme heat, like during a forest fire. Crystal structures of minerals become stressed during contraction and expansion and the mineral crystals separate. For instance, repeated cycles of freezing and thawing (known as Freeze-Thaw) of water in rock cracks further widens cracks and splits rocks apart. Frost-wedging forces portions of rock to split apart.

    bulletUnloading and Exfoliation: Cracks in rocks appear when pressure is released as overlying rocks or sediment are removed, thus allowing the expansion of the newly exposed rock. Exfoliation occurs as sheets or slabs of the cracked rock slip off and become further eroded. Domes form as the unloading and exfoliation weathering processes continue. Half Dome at Yosemite National Park , California is a result of unloading (pressure-release jointing) and exfoliation.

 


 

Although one weathering process can dominate in a given area, physical and chemical weathering processes occur simultaneously to break down rock parent material. Rocks that are formed under intense temperature and pressure and cool rapidly forms crystalline structures in minerals that are less stable when exposed to low temperatures and pressures at Earth's surface, so they will weather more rapidly.

Rocks that are formed under intense temperature and pressure, but cool more slowly and later in the volcanic magma cooling process, are more stable when exposed to the low temperatures and pressures at Earth's surface. Bonds holding atoms together determine mineral hardness. Rocks that have cooled more slowly have time to build stronger bonds, so they are more resistant to the forces of weathering.

Friedrich Mohs, an Austrian mineralogist, devised a scale of mineral hardness in 1812. He used ten minerals, listed below, as standards by which to determine the hardness of minerals and other objects. These ten minerals were arranged on a scale of increasing hardness. For instance, gypsum can scratch talc, and apatite can scratch fluorite, calcite, gypsum, and talc.Your fingernail has a general hardness of 2.5, so you can scratch gypsum and talc! Diamonds are the hardest mineral in existence and are used as cutting instruments.

Moh's Scale Of Mineral Hardness

Talc

Gypsum

Calcite

Flourite

Apatite

Orthoclase

Quartz

Topaz

Corundum

Diamond

1

2

3

4

5

6

7

8

9

10

(Information derived from Rocks and Minerals, an Eyewitness Book produced and published by Dorling Kindersley Limited, London, England. Also published by Alfred A. Knopf, Inc., New York. 1988.)

Since some minerals weather more rapidly than others and weathering processes vary in intensity and combination, weathering products contain different mineral combinations. Pedologists, or soil scientists, classify these weathered mineral products as soil separates. Soil separates range in size and are known as sand, silt, and clay 


Unless otherwise noted, information contained in this document was extracted from Geosystems- An Introduction to Physical Geography, by Robert W. Christopherson,MacMillian College Publishing Company, New York. Second Edition. 1994.