ERTH 209 - Midterm 2

Weathering

The destructive process by which rocks are disintegrated by exposure to atmospheric agents (wind, water) at the Earth’s surface

Weathering is…

the first stage in a vast sediment redistribution process

Erosion

Set of processes which loosen and transport soil and rock downhill

Physical Weathering

Physical processes which disintegrate a rock without chemical change

Examples of Physical Weathering

1. Zones of weakness

2. Frost wedging

3. Mineral crystallization

4. Sheet and exfoliation, spheroidal

5. Biological activity

Zones of weakness

Joints (cracks) form along preexisting planes of weakness

Frost wedging

Water expands as it freezes in joints; this wedges the rock apart; it occurs in the northern climates or mountainous regions

Mineral crystallization

Caused by growing mineral crystals; usually form from evaporating solution; often in arid climates

Sheeting and exfoliation

For igneous rocks that form deep in crust; experience pressure release during uplift; causes fracturing parallel to the rock surface or in concentric layers; rocks split apart

Spheroidal Weathering

Peeling of small rock bodies into onion-like layerss; (“small scale exfoliation”)

Biological activity

Roots; burrowing organisms; human activity can affect weathering

Chemical Weathering

• Chemical reactions involving minerals thereby altering the original composition of the rocks

• Carbon dioxide often reacts with minerals and helps to disintegrate them in both silicate rocks and carbonate rocks

• Rainwater is naturally weakly acidic

• Carbon Dioxide + water = carbonic acid

Chemical Weathering in Carbonate Rocks

• Chemical weathering is seen in cracks and fissures

• Rain can dissolve carbonate rocks completely over time

• Dissolved constituents run off into surface and subsurface water

• e.g marble

Chemical Weathering in Iron-rich rocks

• Iron in rocks reacts with oxygen in water and atmosphere to form hematite, more commonly known as rust

Chemical Weathering in Silicate Rocks

• Less resistant minerals, such as feldspar, partially dissolve in water

• What doesn’t dissolve is changed into a clay mineral

• Minerals with low solubility, such as quartz, are not changed

Factors Controlling Weathering Rate

1. Parent rock

2. Climate

3. Soil

4. Time

Parent Rock

Rocks like carbonates weather more easily than silicates

Climate

Chemical weathering is favoured by tropical climates

Physical weathering is favoured by cold climates

Soil

Soil traps water and acidifies it further, enhancing weathering

Time

The longer the exposure to the elements, the greater the progress of weathering

Surface Area

• Physical weathering increases surface area

• Chemical weathering rates are directly proportional to surface area

• Therefore, physical weathering increases chemical weathering rates

Regolith

Fragmented & consolidated material (predominantly derived from weathering) overlying bedrock

Soil

Unconsolidated material capable of supporting vegetation

Dry-climate soil profile

A-horizon = zone of intensive leaching

B-horizon = lower mineral soil horizons, often has enrichment of clays and precipitates derived from A horizon

C-horizon = layer of partially weathered bedrock at the base of a soil profile

Soil Forming Factors

• Climate

• Parent material

• Topography

• Organisms

• Time

Parent Material

Fertile soils often on limestone and basalt, acid soils develop on sandstones

Topography

Soils thickness decreases “upslope”

Organisms

Burrowing animals and microorganisms are important factors in soil formation

Time

The required time for soil formation depending on the above factors may vary from few decades to >10,000 years

Humans as agents of erosion

• It takes ~30 years to create an inch of topsoil

• Agricultural practices can either slow down or accelerate topsoil erosion

Wind Erosion

• Dust storms in the 1930’s are a good example of how effective wind erosion can be on unprotected fields

Alberta Soil Conservation

• Leaving stubble when harvesting protects soil from wind and also conserves water

• Shelter belts also protect fields from wind erosion

Mass Movement

A downhill movement of masses of soil or rock under the force of gravity

3 main factors that influence mass movement

1. The nature of the materials

2. The steepness and stability of slopes

3. The amount of water in the materials

Unconsolidated materials

Loose and unsemented material

e.g. sand, silt, soil, rock fragments

Consolidated materials

Compacted and cemented material

e.g. jointed rock, deformed rock, massive rock

Slope (Angle of Repose)

Slope angle of unconsolidated material at gravitational equilibrium

Angle of slope depends on

• Particle size

• Angularity

• Sorting

• Wet or dry

Amount of water depends on:

1. Porosity of materials

2. Amount of exposure to rain and other water

Role of water

• The surface tension of water causes cohesion of grains if only a film is present

• Small amounts of water increase cohesion

• If water content is high enough to fill pores, grains will “float”

• Any increase in water pressure will push grains apart, leading to flowage or unconsolidated materials

• Large amount of water decrease cohesion (may cause liquefaction)

Quick CLay (Leda Clay/Champlain Sea Clay)

Unique form of highly sensitive marine clay, with the tendency to change from a relatively stiff condition to a liquid mass when it is distributed

• although solid, has very high water content, up to 80%

• surface tension holds water-coated flakes of clay together

• when structure is broken by shock, reverts to a fluid state

Earth Creep

Creep is the slowest form of mass movement. Unconsolidated material slowly drifts downhill by gravity

Frost wedging contributes to…

rocks falls by weakening joints

Rockfalls result from

Steep slopes

Rubble (TALUS)

Collects at the base at angle of repose

Rock Avalanches

Are flows that are faster and travel farther than rock slides or falls

Triggers for Mass Movement

• Unfavourable Geological Structures

• Heavy rainfall or snow melt

• Earthquakes

• Unfavourable Artificial Structures

How can we protect against mass movement debris?

• Revegetating a slope results in the growth of roots that can hold a slope together

• An avalanche shed diverts debris or snow over a roadway

• A retaining wall can trap falling rock

• Bolting or screening a cliff face can hold loose rocks in place

Sedimentary Rocks

The result of the weathering of solid rock is the production of sediment

Sediments are transported by water, wind, and ice. They are deposited and lithified into sedimentary rocks

Sedimentary rocks are important because…

They contain a record of the history of the Earth’s surface

How are sediments formed?

Weathering —> Transport —> Deposition —> Lithification

Transport of loose sediment

• By wind, water, and glacial ice

• Coarsest particles closest to the source area

• Finest particles travel farthest

• The greater the distance of transport, the smoother or more rounded grains become

Types of Sediments

1. Clastic Sediments

2. Biochemical Sediments

3. Chemical Sediments

Clastic Sediments

Derived from the weathering of rocks, transported by wind, water, or ice, and deposited in layers on the Earth’s surface

Biochemical Sediments

Precipitated directly by the activities of organisms

Chemical Sediments

Precipitated directly from water

Lithification

The conversion of sediment into rock

Compaction

Reduction in volume of sediments resulting from weight of newly deposited sediments above

Cementation

A process by which precipitates bind together the grains of a sediment, converting it into sedimentary rock

The combination of cementation and compaction during burial causes…

Hardening or “lithification” of rock

Carbonates

1) limestone (made of calcite) 2) dolostone (made of dolomite)

Limestone (CaCO3)

Consolidated product of carbonate sand, mud, and shells

The primary source of calcite in limestone is most commonly marine organisms

Carbonate environments

Shallow, warm, clear water

Carbonate reefs

A reef complex on a carbonate shelf or platform

• are an important setting for deposition of LIMESTONES

Corals need…

Clear, warm, shallow water

Chalk

Consists of shells and billions of microscopic calcareous organisms

Chert (aka flint)

Small particles of silica, often from shells of microscopic organisms

So chert is NOT a carbonate, it is made of silica

Coal

Consists of the compressed, altered remains of vegetation

Lithification proceeds via: peat, lignite, coal, anthracite

Coal has different ranks based on its formation

Hard coals (anthracite), which have been subjected to higher temperatures and pressure, burn more cleanly than soft coals (lignite)

Chemical rocks: Evaporites

• Precipitated under hot, dry conditions

• Evaporation increases the salinity of the water

• Primary halite, southern part of the Dead Sea (Israel)

Bedding plane

Pause in deposition, change in environmental conditions

Principles of original horizontality

Most water-laid sediments are deposited in horizontal or nearly horizontal layers

Principle of superposition

Oldest layers are at the bottom and younger layers are deposited on top

Symmetrical ripples

(beach); waves, equal slopes

Asymmetrical ripples

(dune); gentler slope, steeper slope, wind or water

Ripple marks

Wind or water currents

1. symmetrical

2. asymmetrical

Mud cracks

Fine sediment dries and shrinks

Sedimentary Environments

Deep Ocean

Shallow Ocean

Transitional

Terrestrial

Continental Shelves

• Margins of continents after rifting

• Deposition of clastic sediment from continental rivers

• If arid and warm enough, may have carbonate shelf on continental margin

Abyssal Plains

• In the open ocean, a blanket of sediment covers the igneous rocks of the oceanic crust

• Sediment made of silica-based and carbonate shells of small organisms as well as wind-derived clay from continents

Rift Valleys

• Rifting creates a topographic gradient

• Erosion of highlands into newly formed valley results in layers of sediment deposited in the rift valley

Backarc Basin

Uplift of mountain belt causes enhanced erosion of highlands and deposition of sediments

Metamorphic rocks have changed mineralogically and/or texturally through…

Heat and/or Pressure

Fluids are also commonly involved

Metamorphic rocks provide a window into past…

temperatures, pressures and fluid composition

Unlike igneous and sedimentary processes…

We can’t see metamorphic rocks forming naturally

Metamorphism occurs…

• at pressures (P) and temperatures (T) much higher than those on the Earth’s surface

• But below P and T conditions that cause melting. Metamorphic rocks form in the middle and lower crust

IMPORTANT info about metamorphic rocks

Metamorphic rocks…

• remain in the solid state

• changes depend on the protolith (starting rock)

• processes are slow

Categories of Metamorphic Rocks

1. Foliated

2. Non-foliated

Metamorphic Textures (foliated)

Slate —> Phyllite —> Schist —> Gneiss —> Migmatite

Low grade Intermediate grace High grade

Slate

• Protolith: shale

• Elongated and platy minerals aligned and compressed

• Breaks smoothly along foliation planes

• Very fine grained

Pyllite

• Micas start growing perpendicular to stress, shiny surface due to micas

• Fine grained

Schist

• Micas growing parallel

• Coarser then phyllite, will break along cleavage planes

• Often folded (i.e. higher pressures)

Gneiss

• Minerals segregated into light (quartz and feldspar) and dark (biotite and amphibole) bands

• Can be tightly folded

• Coarser than schist, won’t break along foliation planes

Migmatite

• Partial melting in the core of mountain belt

• Mixture of igneous (granite) and metamorphic (gneiss) rock

• Transitional rock

Non-Foliated Metamorphic Rocks

If rocks composed of granular minerals such as calcite or quartz are subjected to stress result = equigranular texture

• No preferred orientation/alignment

There are no elongated or platy minerals such as amphiboles or micas to define a foliation in these rocks

Non-Foliated Metamorphic Rocks

• Marble (CaCO3)

• Quartzite (mainly quartz)

Quartzite

• Non-foliated, recrystallized sandstone

• Composed almost purely of quartz (SiO2)

• Some sedimentary structures (bedding, cross-bedding) survive the recrystallization

• Contact or regional metamorphism

Marble

• Non-foliated recrystallized limestone

• Composed of calcite (caCO3)

• Impurities result in secondary minerals, streaks and bands

• These impurities are valuable in commercial marble quarries

• Contact or regional metamorphism

Heat (cause of metamorphism)

• speeds up chemical reactions (input of energy)

• Mobilizes fluids and gases

Contact Metamorphism

Pressure (cause of metamorphism)

• reduces space

• drives reactions to denser forms of minerals

• confining pressure reduces space, drives reactions to denser forms of minerals = FOLIATION

Compressive; differential stress

Differential stress

Causes alignment of elongated or platy minerals and growth of new minerals in same directions (foliation)