!GLG 101-Illustrated Vocabulary-Chapter 5
!Weathering and Soils
copyright 2003-Roger Weller
!abrasion
*the wearing away of a material by objects rubbing against each other.
!angular grain
*an unweathered grain, often with an irregular shape and sharp edges
!bauxite
*the ore of aluminum; hydrated alumina. Often has a pisolitic structure (made of small
rounded aggregates).
*[Bauxite]
!boulder
*a particle size larger than a cobble; no upper limit is established as to how large it can
be.
*[Mt. Whitney-Schlieren-4]
*[Mt. Whitney-Schlieren-5]
!caliche
*soil cemented by lime (calcite); very common in the Southwest. Almost resembles
concrete.
!carbonic acid
*carbonated water; a weak acid made of carbon dioxide and water. This acid can
eventually cause the decomposition of most silicate minerals.
!chemical weathering
*a mineral is changed into a different mineral through the process of a chemical reaction;
the new mineral is usually less resistant to erosion.
*[Arizona-Patagonia Mountains-Pyrite Converted to Sulfur and Iron Oxides]
!c-horizon
*the third layer of soil that consists of disintegrating rocks.
!clay
*can either refer to the tiniest particle size (submicroscopic particles) or it can apply to a
class of silicate minerals that only produce crystals in the submicroscopic range (clay
minerals).
*[Kaolinite]
!cobble
*a particle size between a pebble and a boulder. A cobble is often described as "bricksized".
!devitrification of obsidian
*like all glasses in which the constituent atoms are arranged at random, obsidian breaks
down as it slowly crystallizes over thousands or even millions of years.
!differential weathering
*due to differences in hardness, toughness, types of cementation, and chemical
composition, some rock types break down (weather) faster than other materials.
*[The slope beneath the Escabrosa Limestone cliff is the Martin Limestone]
!dissolution
*the process of dissolving a material; the material goes into solution.
!erosion
*is the process of carrying away materials in contrast to weathering which causes the
breakdown of materials.
!exfoliation
*a weathering process, common in semiarid regions, in which the weathered outer
portion of a granite boulder peels off in slabs parallel to the surface of the boulder. This
weathering process is often compared to the layers of an onion.
*[Chiricahuas-Exfoliation]
*[Mt.Whitney-Alabama Hills-Exfoliation-Close Up]
!fissility
*the property of a rock, such as shale, of splitting into thin layers
!friability
*the measure of how easy it is to crumble a material.
!frost wedging
*Water gets into a crack, freezes, and expands; the tremendous pressures exerted by the
expansion of water into ice can cause the rock to split further.
!grus
*a pile of small fragments from a decomposed granite. This loose material contains
single grains of quartz, mica, and weathered feldspar as well as small clumps of these
minerals.
*[Huachuca Mountains-Decomposed Granite-Grus]
!hardpan
*a cemented soil, such as caliche.
!hematite
*a reddish brown to dark, sub-metallic mineral consisting of iron oxide
*[Hematite-Streak]
*[Hematite-Botryoidal]
*[Hematite-Specular]
!humus
*dark organic matter found in soil.
!hydrolysis
*a chemical reaction in which water is added to the chemical formula of a mineral: the
addition of water to the mineral anhydrite chan
!hydrothermal alteration
*hot mineralized water coming from an igneous intrusion changes the composition and
structure of the minerals in the surrounding country rock.
*[HYDROTHERMAL ALTERATION]
*[HYDROTHERMAL ALTERATION-CLOSE UP]
*[California-Lassen Peak-Hydrothermal Alteration of Volcanic Ash]
!inselberg
*a steep-sided hill which is a residual remnant in the erosional process that leveled most
of the surrounding terrain.
!kaolinite
*a common clay mineral. Also the kao in Kaopectate; if you consume clay, it will have
an effect on your digestive process.
*[Kaolinite]
!leaching
*a chemical process in which water moving through permeable rock materials dissolves
some minerals and then carries these minerals to another area.
!liesagang figure
*water getting into a joint leaches out some minerals and deposits others in a curved
semi-concentric pattern.
*[Liesagang1]
!limonite
*a hydrated , yellow-brown iron oxide.
*[Limonite-Streak]
!mechanical weathering
*the form of weathering whereby a larger rock is broken into smaller fragments without a
change in chemistry.
*[California-Coast-Wave Action]
*[Shale, Talus Slope-Western Colorado]
!pedalfer
*a soil formed in warm humid regions, rich in iron oxides (red) and clay (gooey).
!pedocal
*a soil found in semiarid regions, rich in calcite.
!physical weathering
*the breakdown of rocks into smaller pieces without any change in chemistry; also called
mechanical weathering.
!regolith
*the loose material on top of bedrock: soil, gravel, boulders, etc.
!rock flour
*rock that has been crushed and ground to a fine powder by glacial action.
!root wedging
*roots grow to a thin crack in a rock; as the root grows it exerts pressure and expands the
crack.
*[Root Wedging-1]
*[Root Wedging-2]
!rounding
*the physical weathering process of converting angular grains to a rounded shape,
primarily by abrasion.
*[Rounded Stream Pebbles and Cobbles-Colorado]
*[Rounded Beach Pebbles-California]
!salinization of soils
*the buildup of salt in the soil thereby decreasing the useful of the soil for agriculture; a
growing problem in the western part of the United States.
!salt buildup
*if there is insufficient drainage of agricultural watering of crops, salt (which is produced
by the breakdown of soil minerals) is not washed out of the system. Consequently, the
concentration of salt slowly builds up uintil plants cannot live in the soil.
!salt wedging
*is a physical weathering process similar to frost wedging but instead is caused by the
growth of salt crystals within cracks. The force of the growing salt crystal is sufficient to
widen cracks in rocks.
!sap rock
*decomposed, crumbly granite; basically a "dead" rock.
*[Huachuca Mountains-Decomposed Granite-1]
!saprolite
*same as sap rock: "sapro" means "dead".
!sheeting
*sets of joints in bedrock that are produced by unloading; the joints produced by this
process are usually parallel to the surface of the Earth.
*[Yosemite-Sheeting]
!silt
*a particle size larger than clay particles but smaller than sand grains. This material feels
gritty but you cannot see the individual particles with the naked eye.
!soil
*the finely broken down rock material on the earth's surface.
*[Idaho-Craters of the Moon-Dirt]
!solution
*a chemical dissolved in water.
!spheroidal weathering
*chemical weathering within fractures in rock first causes a breakdown of the corners of
the block and then edges of the block are also destroyed, thus producing a rounded
boulder from an angular block.
*[Arizona-Chiricahuas-Spheroidal Weathering]
!tearpants structure
*limestone in a semiarid environment weathers into a very sharp, raspy surface.
*[Limestone-Tearpants Structure]
!thermal expansion
*occurs when most materials are heated. Similarly, most materials contract in size when
cooled. Repeated heat-cold cyles loosen grains within rocks and may cause the rock to
eventually crumble.
!ultraviolet light
*light just outside of the normal visible spectrum, just past violet. This type of light is
responsible for sunburn on bright, sunny days. Ultraviolet light can cause chemical
changes in minerals; many colored minerals such as amethyst, rose quartz, and pink topaz
can fade when exposed to ultraviolet light.
!unloading
*when the pressure is reduced of a rock that formed under pressure, the rock expands and
breaks. Granite intrusions formed at depth crack and split when the overlying bedrock is
removed through erosion.
!wave action
*a mechanical weathering force capable of breaking and rounding rock fragments
through constant abrasion.
*[California-Coast-Wave Action]
!weathering
*the physical and chemical process that cause the breakdown of rocks and minerals.
!weathering rind
*the outer part of a rock when exposed to weathering process often changes color; this is
why a rock must be broken in order to see a fresh surface.
*[Obsidian, Black-1]