Discussion 5

John Aubert  American River College Weathering and Mass Movement Physical Geography Lecture Notes Page 1 of 3 [email protected]

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Weathering and Mass Movement

I. Landform Development – A Complex Process: It was opriginally thought that landscapes

initially went through one major uplift, and were subsequently wasted over time to a flat surface (theory attributed to William Morris Davis, 1850-1934). We now understand that the variables that shape landforms are constantly changing. Landforms respond to these changes in a dynamic fashion. This is referred to as the dynamic equilibrium model.

II. Weathering: Earth’s surface rocks are influenced by the weathering processes. Literally, they are

exposed to the weather. The immediate surface is usually soil (1-2m thick), which is the most highly weathered material. Next, the weathered bedrock just below is referred to as regolith (literally a mixed pile of dirt, gravel, rocks, etc). Below that is solid bedrock. Fractures (or joints) in the bedrock are what allow for the exposure of the rock to the external elements

Profile of a Weathered Surface (started as solid bedrock)

A. Physical Weathering Processes: Physical weathering is the mechanical breakdown

(e.g. crushing, breaking) of solid rock into smaller pieces. It is important to note that physical and chemical weathering often operate simultaneously; we separate them for convenience.

1. Frost Action (also called Frost Wedging) – see text

2. Salt-Crystal Growth (also called Crystallization) – see text

3. Hydration – some consider this chemical weathering, see text

4. Pressure Release Jointing – see text

5. Root Wedging– see photo in text

6. Thermal Expansion– Rock expands as it heats and contracts as it cools;

this causes cracking.

John Aubert  American River College Weathering and Mass Movement Physical Geography Lecture Notes Page 2 of 3 [email protected]

TIP: Scan these notes before this topic is covered in lecture! TIP: Review your notes as soon as possible after every lecture!

B. Chemical Weathering Processes: Chemical weathering involves the chemical

reactions of certain rock minerals with water and/or gasses leading to the weakening of the rock. Quartz is the most resistant mineral when it comes to chemical weathering (i.e. it hardly weathers at all).

1. Oxidation: (see text)

2. Hydrolysis: (see text)

3. Carbonation and Solution: (see text)

III. Mass Movement: The downslope movement of material propelled by gravity. Any material at

rest on a slope has potential energy (stored energy), while any material in motion has kinetic energy (energy actually in the process of being released).

A. Factors Leading to Mass Movement

1. Oversteepening of Slope: Slope refers to a tilted land surface. Above a

certain degree of steepness (which depends on what the slope is made of), the slope is said to be oversteepened and is prone to mass movement.

a) Natural Causes of Oversteepening

(i) Mountain Building

(ii) Reverse Faulting

(iii) Glacial Erosion

(iv) Fluvial Erosion (Rivers and Streams)

(v) Coastal Erosion (Waves)

b) Human Causes of Oversteepening

(i) Roadbuilding

(ii) Construction

2. Loose/Weak Slope Materials: Loose (or unconsolidated) materials become

oversteepened at lower angles, while hard bedrock can maintain stability at higher angles. Weathering (discussed previously) causes rock to become loose and weak! As such, highly weathered materials are prone to mass movement.

3. Saturation of Slope Materials

a) Water lubricates

b) Water adds weight

4. Earthquakes and Heavy Construction: Sudden shaking or vibration can

initiate movement.

Road Cut

(note oversteepening)

Original Slope

John Aubert  American River College Weathering and Mass Movement Physical Geography Lecture Notes Page 3 of 3 [email protected]

TIP: Scan these notes before this topic is covered in lecture! TIP: Review your notes as soon as possible after every lecture!

B. Classes of Mass Movement: Mass movements can be categorized into diffeent classes,

primarily based on the speed of movement and the water content. See text for details.

1. Falls and Avalanches – See text

2. Slide (or Landslide) – See text

3. Flows – See Text

4. Creep (also called Soil Creep) – See Text

C. Analysis of a Mass Movement Event (in class activity)

Sample Questions: Questions similar to these will be on your exam. As you study you should

anticipate how I might use these questions to create new questions on the same concepts.

1 What is the driving force behind mass movement? A. radiation B. friction C. gravity D. gasoline E. erosion

2

Which type of weathering involves minerals in a chemical reaction with oxygen?

A. root wedging B. oxidation C. frost action D. carbonation E. hydrolysis

3

Which type of weathering is a process that dissolves limestone and marble?

A. hydration B. carbonation C. frost action D. crystallization E. pressure release jointing

4

POSSIBLE ESSAY QUESTION: For mass movement to occur (such as rockfalls, landslides, etc), slopes must be oversteepened (in other

words, too steep). List the causes of oversteepening and list and explain the three other critical factors which can cause and/or worsen

mass movement events.

GOOD NEWS!!! The practice questions above - plus many, many more – can be found in the online practice quizzes

discussed in your syllabus and in class. You can take each quiz multiple times, and each time you will get some new questions. Once submitted, the quizzes are graded automatically, with the correct answers provided immediately. This is a great way to prepare for the exams!!!

SERIOUSLY, TAKE THE ONLINE PRACTICE QUIZZES!!!