Chapter 5- Weathering and Soils

Physical Geology Illustrated Vocabulary     
Cochise College                                                     
Geology Home Page  
Roger Weller, geology instructor
wellerr@cochise.edu    

 
oxidized basalt, Hawaii

Chapter 5
copyright 2015-Roger Weller
last edited: 12/7/15
links:
Chemical Weathering of Igneous Minerals-Lecture
NEW

Chemical Weathering and Sedimentary Minerals

weathering videos
weathering  (10)

 

abrasion
-is the wearing away of a material by objects rubbing against each other. 

A horizon
-is also known as either top soil or the zone of leaching.
Typically it is dark colored because of
the humus (organic materials).  Decomposed materials are washed out of this layer, a process                                                      
known as leaching.
-soil layers-Opal's Pals

angular grain
-is an unweathered grain, often with an irregular shape and sharp edges. 
-angular sand grain-quartz
-angular grains-Opal's Pals

-semi-angular quartz sand grains

bauxite

-is the ore of aluminum, hydrated alumina.  Bauxite often has a pisolitic structure (made of small
rounded aggregates).

-bauxite

B horizon
--is the second layer of soil.  It is also known as the zone of accumulation because materials
washed down from the top layer, clay and iron oxides, accumulate in this layer.

-soil layers-Opal's Pals                                                                                                                                                                

 

boulder

-is a particle size larger than a cobble; there is no upper limit established as to how large a
boulder can be.

-Mt. Whitney-boulder-1 
-boulder-Opal's Pals

 

caliche

-is a soil cemented by lime (calcite); caliche is very common in the Southwest.  Caliche closely
resembles concrete.
-caliche:
  1  2
-caliche-Opal's Pals

 

carbonic acid

-consists carbonated water; carbonic acid is a weak acid made of carbon dioxide and water. 
Carbonic acid can eventually cause the decomposition of most silicate minerals.

 

chemical weathering

-occurs when a mineral is changed into a different mineral through the process of a chemical
reaction; the new mineral is usually less resistant to further weathering.
Chemical Weathering of Igneous Minerals-Lecture NEW
-pyrite weathering to sulfur and iron oxides-A 
-pyrite weathering to sulfur and iron oxides-B 
-pyrite weathering to sulfur and iron oxides-C 
-chemical weathering lecture


chemical weathering of feldspar
-Chemical weathering of feldspar changes feldspar into clay
-weathered feldspar within an igneous rock (2 views)

 

C-horizon

-is the third layer of soil that consists of disintegrating rocks.
-soil layers-Opal's Pals

 

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 

-is the particle size between a pebble and a boulder.  A cobble is often described as "brick-sized".

 

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.
-pitchstone is obsidian undergoing devitrification.

 

differential weathering

*due to differences in hardness, toughness, types of cementation, and chemical composition, some
rock types break down (weather) faster than other materials.

-differential weathering between chert and limestone

 

dissolution

-is 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

-is 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 

 

fissility

-is the property of a rock, such as shale, of splitting into thin layers.

-Frost wedging of folded siltstone, Santa Catalina Mountains, Arizona 

 

friability

-is the measure of how easy it is to crumble a material.
-friabilirty-Opal's Pals

 

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.
-Frost wedging of thin-bedded siltstone 
-Soda cans split by ice expansion

 

grus

-is 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.

-grus 

 

hardpan

-is a cemented soil, such as caliche.
-caliche-Opal's Pals

 

hematite

-is a reddish brown to dark, sub-metallic mineral consisting of iron oxide

-hematite, streak 

-hematite, botryoidal 

-hematite, specular 
-basalt weathering to hematite, Maui, Hawaii
-hematite-Opal's Pals

 

humus

-consists of dark organic matter found in soil.

 

hydrolysis

-is a chemical reaction in which water is added to the chemical formula of a mineral: the addition
of water to the mineral anhydrite changes its chemical composition and the material becomes
gypsum.
-anhydrite
-gypsum
 

hydrothermal alteration

-occurs when 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 
-hydrothermal alteration and spheroidal weathering of basalt in Maui:  1  2  3

 

inselberg

-is a steep-sided hill which is a residual remnant in the erosional process that leveled most of the
surrounding terrain.

 

kaolinite

-is a common clay mineral. Kaolinite is the kao in Kaopectate; if you consume clay, it will have
an effect on your digestive process.

-Kaolinite 

 

leaching

-is a chemical process in which water moving through permeable rock materials dissolves some
minerals and then carries these minerals to another area.
-minerals bleached by leaching process
-leaching-Opal's Pals

 

liesagang rings

-Water getting into a joint leaches out some minerals and deposits others in a curved
semi-concentric pattern.
-liesang rings

 

limonite

-is a hydrated , yellow-brown iron oxide.

-limonite, streak 
-basalt weathering to limonite, Maui, Hawaii

 

mechanical weathering

-is the form of weathering whereby a larger rock is broken into smaller fragments without a
change in chemistry.

-California coast-wave action 

-decomposing shale-frost action 

oxidation
-occurs when a mineral picks up extra oxygen in its structure a changes to a new mineral.
As an example, magnetite in basalt is altered to a red hematite.
-red soil in Maui, Hawaii

-old cars rusting in Nevada desert

 

pedalfer

-is a soil formed in warm humid regions, rich in iron oxides (red) and clay (gooey).

 

pedocal

-is a soil found in semiarid regions, rich in calcite.

 

physical weathering

-is the breakdown of rocks into smaller pieces without any change in chemistry; also called
mechanical weathering.

 

regolith

-is the loose material on top of bedrock: soil, gravel, boulders, etc.

 

rock flour

-consists of 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-A 

-root wedging-B 

rounded grain
-is a weathered grain, where the sharp edges of the grain have been worn off by erosion.
-sand from Bermuda-rounded shell fragments
-sand from Hawaii-partially rounded volcanic fragments 
-semi-rounded basalt sand grains
 

rounding

-is the physical weathering process of converting angular grains to a rounded shape, primarily by
abrasion.

-rounded beach pebbles-Maui, Hawaii

-rounded beach pebbles 
 

 

salinization of soils

-is 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 until 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.

sand
-is a grain size between granules and silt.
-beach sand, Kihei, Maui, Hawaii
-black sand, Hana, Maui, Hawaii
-magnified sand grains-black volcanic sand
-magnified sand grains-immature sand
-magnified sand grains-rounded shell fragments
-magnetite sand-  10x  30x
-fine grain quartz beach sand-  10x  30x  30xb
-basalt beach sand from Hawaii, big island-  10x  30x
 

sap rock

-consists of decomposed, crumbly granite; basically it is a "dead" rock.

-decomposed granite 

 

saprolite

-is the same as sap rock: "sapro" means "dead".
-saprolite-Opal's  Pals

 

sheeting

-refers to 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

-is 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

-is finely broken down rock material on the Earth's surface.
-soil layers, Kebler Pass, Colorado

-Idaho-Craters of the Moon-dirt 

-thin layer of new soil  
-soil from Maui, Hawaii

soil subsidence
-occurs when loosely compacted material settle, often after rain storms.
-tension fractures resulting fro soil compaction
 

solution

-consists of 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.
-spheroidal weathering:  1  2  3

 

talus

-consists of loose rock that has slid down a slope.

-Mt. Whitney-talus 

-talus 

-Chiricahuas-talus slope 

 

talus slope

-is an inclined surface made of talus.

-Mt. Whitney-talus slope 

-Yosemite-talus slope  

-Mono Craters-talus slope  

   

tearpants structure

-is where limestone in a semiarid environment weathers into a very sharp, raspy surface.

-tearpants structure 

 

thermal expansion

-occurs when most materials are heated.  Similarly, most materials contract in size when cooled. 
Repeated heat-cold cycles loosen grains within rocks and may cause the rock to eventually
crumble.

 

ultraviolet light

-refers to the frequencies of 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.
-minerals that fluoresce under ultraviolet light

 

unloading

-occurs 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.
-jointing in an granite intrusion

varve
-is a thin layer of silt or clay deposited on the bottom of a lake bed on an annual basis. 
Sediment varies slightly from winter to summer, producing small distinguishable layers.
-varves

 

wave action

-is  mechanical weathering force capable of breaking and rounding rock fragments through
constant abrasion.

-California coast-wave action 

 

weathering

-is the breakdown of rocks and minerals by chemical and physical processes..

 

weathering rind

-is 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.

-weathered obsidian 
-weathered basalt, Maui, Hawaii