Comprehensive Essentials of Physical and Historical Geology

Weathering

The process that breaks down rocks and minerals at the Earth's surface through physical, chemical, and biological mechanisms. This process leads to the formation of soil and sediment

What process leads to the formation of soil and sediment?

Weathering

Erosion

The physical picking up and removal of rock particles by mobile agents.

Mechanical Weathering

Disintegrates rock into smaller pieces without altering chemical composition.

Pressure Release

Weathering process that involves the unloading of overlying rock, enabling expansion and fracturing, and creating exfoliation domes.

Frost Action

The weathering process that involves the volumetric expansion of freezing water causes frost wedging and frost heaving

What are other mechanisms of weathering?

Salt crystallization pressure, plant roots wedging open fractures, burrowing animals, and thermal expansion/contraction cycles.

Chemical Weathering

Decomposes rock through reactions with water and atmospheric gases

Processes of chemical weathering?

• Oxidation

• Acid Hydrolysis & Dissolution

• Feldspar Alteration

Oxidation

Atmospheric oxygen reacts with iron-bearing minerals, producing characteristic iron-oxide stains.

Acid Hydrolysis & Dissolution

Chemical weathering process in which natural or human-made acids react with rock to completely dissolve soluble minerals (like calcite).

Which natural acid forms when atmospheric CO2 dissolves in rain water

Which is considered a human-caused (anthropogenic) source of acids that contribute to chemical weathering?

Acid hydrolysis and dissolution is highly effective at completely dissolving what type of soluble mineral?

Felspar Alteration

When slightly acidic waters alter feldspars into clay minerals and dissolved ions.

Minerals with more of what type of bonding resist weathering more effectively?

Silicate

Differential Weathering

Rocks weather at differing rates according to mineral durability

What rock weathers and erodes faster that quartz-rich sandstone

Clay-rich shale

Weathering and Global Carbon Cycling

Atmospheric CO2 combines with water to form carbonic acid, which weathers continental silicate rocks.

Weathering products precipitate as what in aquatic basins

Limestone

Weathering products subduct into what part of the Earth via plate tectonics?

The mantle

Weathering products return to the atmosphere via what process?

Volcanic degassing

What does soil consist of? (Soil Horizons and Profile)

• Weathered, unconsolidated sediment

• Clay minerals

• Organic matter

• Water

• Quartz that rests on bedrock

Soil Profile in Order

1. O Horizon: Uppermost layer dominated by organic material

2. A Horizon: Dark layer rich in organic matter with high biological activity

3. E Horizon: Zone of leaching; percolating water evacuates fine-grained material

4. B Horizon: Zone of accumulation; accumulates clays and iron oxides leached downward; forms hardpan in wet climates

5. C Horizon: Partially weathered parent rock.

What are the five main factors controlling soil formation? (Controls on Soil Formation & Erosion)

• Parent material

• Topography

• Biological activity

• Climate

• Time

Which factor is considered to be the primary control on soil formation?

Cimate

How do topography and parent material affect soil properties?

Topography: Slopes have thin soil (erosion); gentle uplands have thick soil

Parent Material: Can be residual (formed in place) or transported (loess - wind-blown)

What causes soil erosion, what layers are lost, and what are the consequences?

Causes: Water and wind

Layers Lost: Topsoil (O and A horizons)

Consequences: Loss of arable resources (farming land) and aquatic sedimentation (clogging water).

What is the sediment grain size classification from largest to smallest?

1. Boulder

2. Cobble

3. Pebble

4. Sand

5. Silt

6. Clay

High-energy water (e.g., flash floods, fast mountain rivers) erodes what type of particles?

Large particles (boulders, cobble)

Low-energy water (quiet lakes, lagoons, deep ocean) erodes what type of particles?

Small particles (silt, clay)

The size of the grains determine what?

The resulting rock name.

Sand creates what rock?

Sandstone

Clay or silt creates what type of rock?

Shale/mudstone

A mix of rounded pebbles and cobbles creates what type of rock?

Conglomerate

What is the order of the rock cycle?

1. Transportation

2. Deposition

3. Preservation

4. Lithification

Transportation

The movement of weathered rock fragments (sediments) through water, wind, or ice.

What is the result of transportation?

Rocks get rounded, their grain size gets reduced with distance, and they are sorted by velocity.

Deposition

The settling and coming to rest of transported material.

Environment of deposition

Where deposition occurs. (Typically will be in water)

Places where deposition typically occurs

• Deep-sea floor

• Beach desert dunes

• River channels

• Bottom of lakes

Preservation

Sediments get buried with additional sediments in order to become a sedimentary rock/protects deposits from erosion.

Lithification

Converting loose sediment into sedimentary rock through compaction and cementation.

What are the major rock categories?

• Clastic (Detrital)

• Chemical

• Organic

Clastic (Detrital) Rock

Rock made of cemented fragments.

What are some examples of a clastic rock?

• Shale

• Siltstone

• Claystone

• Mudstone

Description of Shale

Fine-grained, silt and clay-sized, and fissile (splitting into thin sheets)

Description of Siltstone

Does not really split into sheets and is slightly coarser grained

Description of Claystone

Consists predominantly of clay and is non-fissile

Description of Mudstone

Massive and blocky

Chemical Rock

Formed by direct mineral precipitation from solution.

What are some examples of a chemical rock?

• Carbonates

• Chert

• Evaporites

Description of Carbonates

Contain CO3 as part of their composition, and limestone falls into this category, which is composed mostly of calcite.

Description of Chert

Hard, compact, fine-grained rock, formed almost entirely of silica , and can occur as layers or as lumpy nodules within other sedimentary rocks, especially limestones

Description of Evaporites

Form from evaporating saline waters (lake,ocean)

What are two examples of evaporites

Rock salt and rock gypsum

Organic Rock

Formed from biological debris

What are some examples of a organic rock?

• Coal

• Oil and natural gas (plankton)

Description of Coal

Formed by the compaction of plant material

Description of Oil and natural gas (plankton)

Derive from organic matter in marine sediment and can accumulate in porous overlying rocks

What are the sedimentary structures

• Bedding

• Cross-Bedding

• Graded Bedding

• Surface Markings and Traces

Bedding

• Series of visible layers within a rock which is most common sedimentary structure 

Cross-bedding

• Series of thin, inclined layers within a horizontal bed of rock which is also common in sandstones 

Cross-bedding shows there was deposition in what?

Ripples, bars, dunes, deltas

Graded-bedding

Progressive change in grain size from bottom to top of a bed

Surface Markings and Traces

Ripple Marks, desiccation, mud cracks, and fossil traces

Ripple marks

Small ridges formed on the surface of a sediment layer by moving win or water

Mud cracks

Polygonal cracks formed in drying mud

Fossils

Traces of plants or animals preserved in rock

Metamorphism (Metamorphic Rocks and Processes)

Solid-state changes to rocks within the Earth’s interior are produced by increased heat, pressure, or the action of hot reactive fluids. Old minerals are unstable under new conditions and continue on to recrystallize into stable ones.

Metamorphic Rocks

Rocks produced from pre-existing (parent rocks)

Cratons

Old stable cores of continents that metamorphic rocks are commonly found in.

What are the metamorphic agents?

Temperature and Pressure

Description of Temperature

Heat comes from the Earth’s deep interior. All minerals are stable over a finite temperature range. If this range is exceeded, new minerals form as a result. If the temperature is high enough, melting will occur.

Description of Pressure

Confining pressure is applied equally in all directions. Pressure is proportional to the depth within the Earth, and high-pressure minerals are more compact/dense.

Differential Stress (Element of pressure)

Created by forces that aren’t equal in all directions

Compressive Stress (Element of pressure)

Causes flattening perpendicular to stress

Shearing (Element of pressure)

Causes flattening by sliding parallel to stress

Foliation (Element of pressure)

Planar rock texture of aligned minerals produced by differential stress

Foliated Rocks

Progressive burial and heating convert shale through increasing grade:

• $\text{Slate} \rightarrow \text{Phyllite} \rightarrow \text{Schist} \rightarrow \text{Gneiss}$.

Non-Foliated Rocks

Named by their mineral composition.

Three examples of non-foliated rocks

• Marble

• Quartzite

• Hornfels

Marble

Coarse grained rock composed of interlocking calcite crystals 

Quartzite

 Produced when grains of quartz sandstone are welded together

Hornfels

Fine-grained rock typically composed of microscopically visible micas formed from clay particles in shale 

Contact Metamorphism

Produce non-foliated rocks as high temperature plays a huge factor within the process.

Where does contact metamorphism happen?

Happens close to magma bodies intruding cooler country rock and occurs in the narrow zone (~1-100 m wide) known as the contact aureole.

Rocks that go through contact metamorphism may be what?

Rocks may be fine or coarse-grained.

Hydrothermal Metamorphism

Rocks precipitated from or altered by hot water

Hydrothermal metamorphism commonly happens where?

Common at divergent plate boundaries

How does Hydrothermal Processes form ore deposits (veins)

Water passes through rocks and precipitates new minerals on the walls of cracks and in pore spaces

Factors that control Metamorphic Rock Characteristics

• Composition of the Parent Rock

• Fluid

• Time

Regional metamorphism

The process that results in rocks with foliated textures as high pressure is a dominant factor. Happens over a wide temperature range

Regional metamorphism can mostly been seen where

In intensely deformed mountain ranges.

Shock metamorphism

Produced by rapid application of extreme pressure (meteor impacts)

Shocked rocks are found around and beneath where?

Impact craters

Relative Time

The sequence in which events took place from oldest to youngest (Steno’s pinciples)

Numerical age

 Actual age given in years or other unit of time - 4.6 billion

James Hutton

18th Century Father of Geology who realized geologic features could be explained through present-day processes and what it would require vast amounts of time (Uniformitarianism)

Principles Used to Determine Relative Age

• Contacts

• Formations