Compiled by Arthur Reed Print Date: January 8, 2015

Chapter OutlinesNOTE: This is intended to help students ‘organize’ their understanding of each topic.

It is not a comprehensive study guide for quizzes or midterms, i.e. study your text!

Weathering, Soil, Mass Wasting

I. Earth's external processes include:A. Weathering— the process of Earth material (formerly deeply buried rock/mineral) changing to

become more stable in its new surface environment…or simply, the breakdown over time of rock atEarth’s surface to form sediment. It is the result of interactions of air, water, and temperature onexposed rock surfaces.

1. Generally, the further a mineral is from its environment (conditions) of origin, the moreunstable it is and therefore more susceptible to weathering

2. Weathering results in Earth materials becoming small enough to be eroded, transported, andto settle in low areas to eventually become sedimentary rock

3. Driving forces for weathering include:a. Tectonic forces…pushing deeply buried rock up forming mountains and thereby exposing

them to Earth’s surface environmentb. The Sun… indirectly responsible for wind, rain, glaciers, various climates, and vegetation

B. Mass wasting—the mass transfer of rock material downslope under the influence of gravityC. Erosion—the movement of particles followed by the transportation of material by a mobile agent,

usually water, wind, or ice (glaciers)II. Weathering - Two kinds of weathering:

1. Mechanical weathering – breaking of rocks into smaller piecesProcesses include: frost wedging, unloading, biological activity, salt crystal growth

2. Chemical weathering - chemical changes in rocks/minerals as Earth materials react at thesurface with Earth’s surface environment, principally carbon dioxide, oxygen, and water. Itinvolves decomposition of minerals, and often formation of new minerals. The mostimportant reactions occur by oxidation or hydrolysis (both CO2 and oxygen are highlyreactive components of the atmosphere):a. Solution weathering (affects calcite/limestone)

i. Acidity is key, CO2 in the atmosphere makes all rain acidiciii. CaCO3 (calcite/limestone) dissolves to ionsiii. Generalized reaction: H2CO3 + CaCO3 the ions Ca++ and HCO3

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b. Oxidation (includes rust)i. Oxygen from the atmosphere combines with iron to form iron oxide.ii. Generalized reaction: 4Fe + 3O2 2Fe2O3

c. Hydrolysis (affects silicate minerals)i. Reaction between water (acidic from CO2 in the atmosphere or ground) and mineralsii. Feldspars (and other silicate minerals) dissolve to clay and other productsiii. Sample reaction: 2KAlSi3O8 + 2H2CO3 + 9H2O Al2Si2O5(OH)4 + 4H4SiO4 + 2K+ + 2HCO3

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3. Mechanical and chemical weathering usually occur together, with their effects beinginterrelated

Example: Granite weathers to clay, quartz sand, and dissolved ions. (Mineral stabilityusually follows Bowen’s reaction series, but in reverse)

4. Rates of weatheringa. Surface area; more surface area = faster weatheringb. Climate; warmer and wetter = faster weatheringc. Parent materials; Bowen’s Reaction Series gives the general trend, but in reversed. Presence of plants and animals; more plants = faster weatheringe. Topography; steeper gradient = faster weathering due to quick removal of weathered

materialf. Composition – variations cause differential weathering and occasional dramatic

landscapes

Example ofmechanicalweathering

The RockCycle

Result of chemicalweathering

Biological activity ofplant roots causing

mechanical weathering

Mechanical weathering only breaksmaterial into smaller fragments

Compiled by Arthur Reed Print Date: January 8, 2015

III. When rock weathers in-place and is not transported away, a soil profile will form.A. It normally consists of (these classifications often vary!):

O - horizon – the organic material at and near the surfaceA - horizon – surface soilE - horizon – leaching downward from the A to the B.B - horizon – Subsoil: this layer accumulates iron, clay,

aluminum and organic compoundsC - horizon – fragments of bedrock partially decomposed

unweathered parent materialB. Factors affecting soil type are: climate, parent material, time, and slopeC. Soil erosion

1. Part of the recycling of Earth materials2. Natural rates of erosion depend on: soil characteristics, climate,

slope, and type of vegetation3. Soil erosion and sedimentation can cause

a. Reservoirs to fill with sedimentb. Concentration of contamination by pesticides and fertilizers

IV. Creation of ore deposits by weatheringA. Process called secondary enrichment

1. Metals are concentrated into economic deposits2. Two mechanisms (the resulting soil sequence is often called a ‘laterite’)

a. Weathered material is removed from the decomposing rock, leaving the desired elementsbehind

b. Desired elements are carried (leached) to lower zones and depositedB. Examples

1. Bauxite, the principal ore of aluminum2. Many copper and silver deposits

V. Mass wastingA. The downslope movement of rock, regolith, and soil under the direct influence of gravity

(the controlling factor). Weathering weakens rock and produces a layer of rock debris calledregolith. Mass wasting provides a short distance transit system for regolith. Streams are thelong distance transit system for regolith

B. Important triggering factors:1. Saturation of the material with water destroys particle cohesion, and water adds weight2. Slopes become unstable if they exceed their angle of repose.3. Removal of anchoring vegetation4. Ground vibrations from earthquakes

D. Types of mass wasting processes. Generally,each type is defined by:

a. The material involved: debris, mud, earth, or rockb. The movement of the material: fall (free-fall of pieces), slide (material moves along a

well- defined surface), or flow (material moves as a viscous fluid)c. The rate of the movement: slow (regolith/creep can move < 1cm/year), to fast

(thundering avalanches of over 125 mph)2. Forms of mass wasting

Slump – rapid movement along a curved surfaceRockslide – rapid movement of bedrock blocks down a slopeDebris flow – rapid flow of debris with water, often confined to channelsEarthflow – rapid flow of water-saturated soil. Similar to liquefactionCreep – very slow movement of soil and regolith downhill

- causes fences and utility poles to tiltSolifluction – slow movement of saturated surface soil over buried permafrost

Photo of an O, A & Esoil profile

A well-developedsoil profile

The top dark brownlayer is an example ofsecondary enrichment

Addition of water‘mobilizes’ soil

particles

A devastatingdebris flow

Examples of mass wasting

Example of alaterite

Surface layer ’creeps’downslope

Compiled by Arthur Reed Print Date: January 8, 2015

What happens when granite is weathered?(from the site of Pamela J. W. Gore, Georgia Perimeter College)

First, unweathered granite contains these minerals:o Sodium Plagioclase feldsparo Potassium feldsparo Quartzo Lesser amounts of biotite, amphibole, or muscovite

The feldspars will undergo hydrolysis to form kaolinite (clay) and Na and K ions

The Na and K ions will be removed through leaching The biotite and/or amphibole will undergo hydrolysis to form clay, and oxidation to form iron

oxides. The quartz (and muscovite, if present) will remain as residual minerals because they are very

resistant to weathering. Weathered rock is called saprolite. Weathered rock fragments are one of the constituents of soil.

What happens after this?o Quartz grains may be eroded, becoming sediment. The quartz in granite is sand- sized; it

becomes quartz sand. The quartz sand will ultimately be transported to the sea (bed load),where it accumulates to form beaches.

o Clays will ultimately be eroded and washed out to sea. Clay is fine-grained and remainssuspended in the water column (suspended load); it may be deposited in quiet water.

o Dissolved ions will be transported by rivers to the sea (dissolved load), and will become partof the salts in the sea.

Yungay landslide

The 1970 Ancash earthquake was an undersea earthquake thatoccurred at 20:23:31 UTC (15:23:31 local time) on Sunday, May31, 1970, affecting the Peruvian regions of Ancash and LaLibertad, and that combined with a subsequent landslide, was themost catastrophic natural disaster ever recorded in the history ofPeru. The epicenter of the earthquake was located 30 km off thecoast of Casma and Chimbote on the Pacific Ocean, where theNazca Plate is being subducted by the South American Plate. It hadan intensity of 7.5 on the Richter scale and up to VIII on theMercalli scale. The earthquake lasted 45 seconds and destabilizedthe northern wall of Mount Huascarán, inducing a rock and snowavalanche and burying the towns of Yungay and Ranrahirca. Theavalanche started as a sliding mass of glacial ice and rock about

3,000 feet wide and one mile long. It swept about 11 miles to the village of Yungay at anaverage speed of more than 100 miles per hour. The fast-moving mass picked up glacialdeposits and by the time it reached Yungay, it is estimated to have consisted of about 80million cubic yards (61,000,000 m³) of water, mud, and rocks.