Introduction to Geology: Processes and Phenomena
Metamorphism
Metamorphism is the process of mineralogical and textural changes of rocks in their solid state in response to physical and chemical conditions which differ from those under which they originally formed.
Pressure
Hydrostatic pressure: confining pressure in water, proportional to the weight of the overlying water.
Lithostatic pressure: confining pressure in solids, proportional to the weight of the overlying rocks.
Directed pressure (same as differential stress): forces are stronger in one direction than in another, causing rock texture to become distorted.
Other Factors
Foliation: the repetitive layering in a metamorphic rock, caused by shearing or differential pressure.
Geothermal gradient: the rate of increase in temperature per unit depth in the Earth.
Sources of heat in metamorphism: migrating magmas, deformation processes (primarily along subduction zones).
Types of Metamorphism
Contact Metamorphism: produced when a body of magma alters the surrounding country rock. Factors controlling contact metamorphism are initial temperature, size of the intrusion, presence and chemistry of fluids.
Regional Metamorphism: produces most metamorphic rocks. It results from extreme heat and pressure generated by the processes at convergent plate boundaries (subduction zones). It can also occur at divergent boundaries, but at shallower depths. Covers large geographic areas, and usually shows a gradation of deformation in folded mountain belts.
Stress and Strain
Stress: force applied over an area.
Strain: change in size, shape, or volume in response to stress.
Types of Stress
- Compression: rock layers are shortened in the direction of stress.
- Tensional: lengthens rocks or pulls them apart, often resulting in faulting.
- Shear: forces act parallel to one another in opposite directions.
Types of Strain
- Elastic: returns to original shape after stress is removed.
- Ductile: retains new shape after stress is removed.
- Brittle: deformed object breaks under stress.
Folding and Faulting
Folding of rocks generally occurs under deep burial conditions (high temperature and pressure) when rocks behave like plastic (ductile) materials.
Faults (fractures) generally occur at shallower depths where rocks are brittle (break). The exception is in subducting slabs (convergent boundary) where brittle behavior (faulting) can occur at depths up to 700 km (results in deep earthquakes).
Types of Faults
- Normal faults (and horst and graben terrain) form under conditions of extensional (tensional) stress (such as at divergent boundaries).
- Reverse and/or thrust faults result from compressional stress (such as at convergent boundaries).
- Strike-Slip faults (side-to-side motion) result from shear stress (such as at transform fault zones).
Plate Tectonics
Continent-continent convergence created the Himalayas and Andes Mountains (compressional forces).
A graben is the result of a block of land being downthrown, producing a valley with a distinct scarp on each side. Graben often occur side-by-side with horsts. Horst and graben structures are indicative of tensional forces and crustal stretching.
Examples of Plate Boundaries
- Divergent (mid-ocean ridges): source of new oceanic crust and lithosphere, pillow basalts, hydrothermal vents, shallow earthquakes.
- Convergent (subduction zones): consume crust, deep earthquakes, trenches, volcanoes associated in arcs or linear ranges such as Andes Mountains, or continent-continent collision zones such as the Alps or Himalayas.
- Transform (strike-slip zones): plates slide past each other, sometimes called strike-slip faults. The San Andreas Fault in California is a good example. These zones can also create mountains and sag ponds.
Japan, the Aleutian Islands, and the Andes Mountains are along a convergent plate boundary.
The African Rift Valley is an example of the beginning of a divergent plate boundary.
Note that the Himalayan Mountains are a continent-continent collision that started as an oceanic crust to continental crust collision.
Earthquakes
Earthquake: trembling or shaking of the ground caused by the sudden release of energy resulting from the breaking of rocks beneath the Earth’s surface. The energy release is sent out as waves called seismic waves.
Seismic Waves
P (primary) waves are also known as push waves or compressional waves. P waves travel the fastest of all waves and through solids, liquids, and gases.
S (secondary) waves are known as shake or shear waves. They can ONLY travel through solids and are slower than P waves.
Earthquake Terminology
Focus: point of origin of the energy release, usually less than 100 km deep.
Epicenter: the spot on the surface of the Earth directly above the focus.
Seismology: study of earthquakes.
Seismograph: the machine (seismometer) that records the seismic waves.
Seismogram: the record of the seismic waves.
Minimum number of seismic stations needed to locate an earthquake epicenter: 3
Waves that cause the most damage to buildings and displacement: S waves
Earthquake Occurrence
Earthquakes occur: 95% occur at plate boundaries, and the most powerful are transform earthquakes.
Two potentially dangerous intraplate (within the plate) earthquake zones in the US are New Madrid, Missouri and Charleston, South Carolina, which actually occur within the North American plate (not at a plate boundary).
The San Andreas Fault consists of transform plate boundaries.
Effects of Earthquakes
- Liquefaction
- Fire
- Landslides
- Permanent displacement
- Tsunamis
Reducing Earthquake Damage
Building codes are important for reducing damage from earthquakes.
Tsunamis
Caused by submarine earthquakes and displacement of the sea floor. Move at ~800 km/hr across ocean basins. At sea, there is virtually no visible wave height.
Wave Behavior
Wave reflection returns some of the waves to the surface after bouncing off a rock boundary.
Wave refraction is the bending of waves as they pass from one material to another with different wave speeds.
Earth’s Interior
The Moho is the boundary between the Earth’s crust and mantle.
P wave shadow zone: 103-143 degrees from the epicenter due to refraction of waves at the core-mantle boundary.
S wave shadow zone: Less than 103 degrees from the epicenter due to the refraction of waves at the core-mantle boundary. S waves can’t travel through the liquid outer core.
The outer core is liquid, and the inner core is solid.
Magnetometers record the Earth’s magnetic field.
The Earth’s magnetic field is thought to be generated by convection (fluid flow) in the metallic outer core, and the inner core spins faster than the rest of the Earth.
Driving Force of Plate Tectonics
The driving force for plate tectonics is thought to be heat sourced in the Earth’s interior (primarily through radioactive decay), which results in very slow movement of the mantle in large convection cells.
Mass Wasting
Mass wasting is movement in which bedrock, rock debris, or soil moves downslope in bulk, or as a mass, because of the pull of gravity. Landslides are a general term for the slow-to-very rapid descent of rock or soil.
Regolith: Layer of all rocks and mineral fragments.
Debris: As applied to mass wasting processes, debris is any unconsolidated material at the Earth’s surface, such as soil and rock fragments (weathered or unweathered) of any size.
Types of Mass Wasting
- Flow: The debris is moving downslope as a viscous fluid. A mudflow is a flowing mixture of debris and water, usually moving down a channel.
- Slump: involves movement along a curved surface, the upper part moving downward while the lower part moves outward.
- Slide: A descending rock mass remaining relatively coherent, moving along one or more well-defined surfaces. A rock slide is the rapid sliding of a mass of bedrock along an inclined surface of weakness, such as a bedding plane or a major fracture. In contrast, a rock avalanche is a very rapidly moving, turbulent mass of broken-up bedrock.
- Fall: occurs when material free-falls or bounces down a cliff.
Controlling Factors in Mass Wasting
- Water: When debris is saturated with water (as from heavy rain or melting snow), it becomes heavier and is more likely to flow downslope.
- Local relief: Steeper relief always speeds mass wasting.
- Thickness of debris: Great thicknesses of debris favor downslope movement.
Other Mass Wasting Terms
Creep: is a very slow, continuous, downslope movement of soil or unconsolidated debris.
Earthflow: debris moves downslope as a viscous fluid.
Solifluction: is the flow of water-saturated debris over impermeable material.
Permafrost: is ground that remains frozen for many years.
Triggering of Mass Wasting
Several types of events can trigger mass wasting, including earthquake shocks, slope modification by erosion or man-made cuts, exceptional precipitation, and volcanic eruptions.
Earth’s Water
Sources of Earth’s Water
- Oceans: 97.2%
- Other: 2.8%
- Glaciers: 2.15%
- Groundwater: 0.62%
- Freshwater Lakes: 0.009%
- Saline Lakes/Inland Seas: 0.008%
- Stream Channels: 0.0001%
- Atmosphere: 0.001%
Erosion and Rivers
Dissolved Load: Ions from mineral weathering.
Divides: Uplands that separate drainage basins.
Valleys and Canyons: Stratigraphic variations often yield a stair-step profile. Strong rocks yield vertical cliffs, while weak rocks produce sloped walls. Geologic processes stack strong and weak rocks. Valleys store sediment when base level is reached. Renewed incision creates stream terraces. Terraces mark former floodplains.
V-Shaped Valley: Formed by the erosive power of a river.
Abrasion: Sediment grains in flow “sandblast” rocks. Bedrocks exposed in channels are often polished and smooth. Gravel surrounding turbulent eddies drills holes in the bedrock, creating bowl-shaped depressions called potholes. These processes can create unusual and intricate sculptures.
Sheetwash: Streamflow begins as a moving sheetwash, a thin surface layer of water that moves down steep slopes and erodes the substrate.
Low Discharge: Large clasts are stranded.
Important Geological Agent: Flowing water erodes, transports, and deposits sediments. It sculpts landscapes and moves mass from continents to ocean basins.
Breaking and Lifting: The force of water can break chunks of rock off the channel and lift rocks off the channel bottom.
Runoff: Water in motion over the land surface.
Erosional Processes: Streamflow does work. Energy imparted to streamflow is derived from gravity. Streams do work by converting potential to kinetic energy.
Curved Channels: Maximum velocity travels the outside curve. The outside curve is preferentially scoured and deepened. The deepest part of the channel is the thalweg. Flow around the curve follows a spiral path.
Processes: Evaporation, Transportation, Precipitation, Infiltration, Runoff.
Dam Construction: Reservoirs provide irrigation and hydroelectric power, as well as recreation areas. However, they can alter ecosystems, create barriers to fish migration, decrease nutrients downstream, and remove the delta sediment source.