Class Notes Semester II - 1999/2000
Geology 196: Oceans on Planets February 2, 2000 -Earth Themes:
-The Earth's hydrosphere: The hydrologic cycle:
-Where is the water and how does it move? -What are the reservoirs and systems and what is their stability?
-Systems and processes: Oceans, the cryosphere, groundwater.
-Processes, morphology, and recognition.
Earth as a System: The Hydrologic Cycle:
-How much water is there? 1.36 x 109 km3. -Where does water reside?
-In what state does it reside?
-What are residence times and the rates of movement?
-Powered by Sun.
-Atmosphere is link between oceans and continents.
-What happens to precipitation?
-Plants and transpiration.
-Water Balance: Volume of water that passes through each part of the cycle annually.
-Compare amount in atmosphere to annual throughput!
-Is the hydrologic cycle balanced?
-Continents: Precipitation exceeds evaporation. -Oceans: Evaporation exceeds precipitation. -Atmosphere: Amount remains the same.
-Oceans: Volumes constant, sea level about same. -So, hydrological cycle presently balanced.
-How might reservoirs, balance change in Earth history?
-How might these differ on other planets?
-Soil infiltration capacity:
1) Intensity and duration of rainfall.
2) Prior wetted condition of soil.
3) Soil texture.
4) Slope of land.
5) Nature of vegetative cover.
-Sheets -> rills -> streams.
-Streamflow: laminar and turbulent.
-Gradient: Slope of a steam channel.
-Channel characteristics: Cross-sectional shape. -Discharge: Amount of water flowing past a certain point in a given unit of time (cubic meters per second).
D = channel width x channel depth x velocity.
-Longitudinal profile: Head to mouth.
-Constantly decreasing slope (concave upward profile).
-Discharge increases toward the mouth.
-More additions from streams.
-Additions from groundwater.
-Inverse relationship between gradient and discharge:
-e.g., higher velocity near mouth because of
1) greater discharge,
2) larger channel, and
3) smoother bed.
-Base level and graded streams:
-Base level: The lowest elevation to which a stream can erode its channel.
-Ultimate base level.
-Local or temporary base levels (lakes, rock layers).
-Graded Stream: Has correct slope and other characteristics to maintain velocity required to transport debris supplied (not eroding or depositing, just carrying).
-Only about 0.6 % of the entire hydrosphere. -But very important part of the fresh water budget! -Very important as an erosional agent.
-Dissolving action of groundwater.
-Sustains streams during dry periods.
-Rate of exchange is 280 years.
Distribution of underground water:
-Runoff, evaporation, transpiration, infiltration. -Paths vary greatly in space and time!
-Belt of soil moisture.
-Zone of saturation (groundwater).
-All open spaces are completly filled with water.
-Upper limit is known as water table.
-Zone of aeration:
-Capillary fringe (water held by surface tension).
-Belt of soil moisture.
-The Water Table: Upper limit of the zone of saturation:
-Usually a subdued replica of the surface topography. -Water table reaches surface at swamps, lakes. -Why is water table irregular?
-Groundwater travels slowly.
-Evens out in dry periods.
-Variations in rainfall and permeability.
-Relationship between water table and a stream:
-Effluent stream (humid areas).
-Influent streams (desert areas).
Factors influencing storage and movement of groundwater:
-Porosity: percentage of total volume of rock or sediment that consists of pore spaces.
-Pores, joints, faults, solution cavities, vesicles. -Sediments: 10-50% porosity.
-Permeability, aquitards and aquifers:
-Permeability: Ability to transmit a fluid. -Specific yield:
-Groundwater that will drain under gravity.
-Retained as film on particle/rock surfaces.
-Aquifers: Permeable rock strata or sediment that transmit groundwater freely.
- Aquitards: Impermeable layers that hinder or prevent water movement.
Movement of Groundwater:
-Exceedingly slow: few cm/day.
-Gravity driven: Migrates from high parts of water table to low. -Pressure driven: Paths can be complex because of pressure gradients. -Hydraulic gradient: Water table slope.
-Head: Vertical difference between recharge and discharge points. -Springs: Wherever water table intersects the surface.
-Cone of depression.
-Nonflowing artesian wells.
-Flowing artesian wells.
Glaciers and Glaciation:
A part of the hydrologic cycle:
-Today cover 10% of Earth's surface.
-Up to 30% in the recent geological past.
-Thousands of meters thick.
-High elevations or high latitudes: can freeze.
-Becomes part of glaciers.
-Can be stored as glacial ice up to thousands of years.
-Valley (Alpine glaciers):
-Greenland: 1.7 x 106 km2; 1500 m thick average, up to 3000 m. -South Pole: 13.9 x 106 km2; up to 4300 m thick. -Ice shelves: Antarctica, 1.4 x 106 km2; Ross Ice Shelf ~ size of Texas.
-Ice sheets: Iceland.
Formation of glacial ice: Snow to ice conversion.
Movement of a glacier:
-Ice behaves as brittle solid until pressure on it is equivalent to about the weight of 50 meters.
-Plastic flow: involves movement within ice. -Basal slip: Meltwater, hydraulic jack, and lubricant.
What are sources of meltwater beneath glaciers?
Rates of glacial movement: Centimeters to meters per day.
-Surges: 20-100 times normal rate. 54 m/day!
Budget of a glacier: