Outlines of GLY 2030 topics for review
Note: you should also look at and review your in-class and homework exercises, as they are fair game for the test!!
ALSO: If you visit Lettuce Lake Park this week, it could help you on the test (though it will not hurt you if you don't!)
Ground Rules of Environmental Geology
1) Population increase is the #1 environmental problem on the Earth.
a) Exponential Population Growth
2) The Earth is a closed system, and all Earth processes interact.
a) Systems
i) Open
ii) Closed: The Earth is de facto closed.
b) Parts of the Earth System
i) Atmosphere
ii) Hydrosphere
iii) Lithosphere
a) Crust: Oceanic and Continental: 6-50 km thick.
b) Mantle: both non-convecting ('lithospheric') and
convecting (aesthenosphere): 2240 km thick.
c) Core: Liquid outer, and solid inner core, both composed
of Fe-Ni.
iv) Biosphere
c) Natural cycles: every process results in a feedback process
i) positive feedback: "vicious cycle": runs to completion
ii) negative feedback: produces a Steady State.
a) Our interference can disrupt steady state systems. Ex.:
pumping water from an aquifer.
3) The Earth is our only suitable habitat, and its resources are limited.
[See Ground Rule 1]
4) Processes modifying the environment today operated in the past and through much of geologic time. But the magnitude and rates of these proceses can be modified by us.
a) Uniformitarianism
5) Some processes are hazardous to us.
a) Hurricanes
b) Tornadoes
c) Earthquakes
d) Volcanoes
e) Flooding
f) Etc.
i) Discretion is the better part of valor, i.e., we must get out of the
way!
6) The use of land and water should balance economic and environmental considerations.
7) The effects of land use are cumulative. (i.e., the children will have to clean up their parent's messes, or live with the consequences.)
a) The end of the Maya.
8) A fundamental component of every person's environment is its geology. To understand the environment, one must be conversant in the earth sciences.
[Blatant self-promotion, partly]
General Principles of Geology
A) The Earth is a Differentiated Planet: a big, layered ball. Layers, from the top:
1) Crust. 2 kinds:
a) Oceanic. Beneath the seas. Mafic in composition (see below). 6 km.
b) Continental. Felsic in composition. 10-50 km.
2) Mantle. Consists primarily of Mg-rich minerals (Ultramafic - it's a real
term!)
a) Lithopsheric mantle: cooler mantle, plated to bottom of crust.
0-125 km thick.
b) Aesthenosphere: hot, convecting, plastic mantle, where melting
occurs. 2240 km - lithosphere thick.
3) Core. Comprised of Fe-Ni.
a) Outer core: molten
b) Inner core: solid
B) The crust and lithospheric mantle move together, rafted atop the roiling aesthenosphere - the Plate Tectonics view of how the earth works.
1) We call this crust-lithospheric mantle package a Lithospheric Plate.
2) Boundaries between plates are defined by how the plates interact.
a) If the plates pull apart: Divergence: aesthenosphere wells up to fill
the void, and melts, making new ocean crust.
i) This process forms the Ocean Ridges, a 25,000 mile long
mountain range on the seafloor.
b) If plates collide, then one slides beneath the other - called
Subduction: a Convergent Plate Boundary.
i) Subduction zones can be identified by deep ocean trenches at
the plate boundary, and by arcuate chains of volcanoes - Volcanic
Arcs - on the overriding plate.
ii) Most earthquakes, and all of the catastrophic volcanic events,
are caused by subduction.
c) If plates slide by each other: a Transform or Strike-slip plate
boundary.
i) The San Andreas Fault is such a boundary - makes
earthquakes!
C) So, this is how the earth works. But, what is it made of? ROCKS!
1) What are Rocks made of? MINERALS!
2) A mineral: A naturally occurring Solid with a definite, but not fixed,
chemical composition, and a unique crystalline structure.
a) All minerals are Solids!
i) water isn't a mineral: glacial ice is.
b) All minerals form crystals.
ii) many are not inclined to form perfect crystals, but all have an
ordered crystalline structure.
` c) All minerals are chemical compounds!
i) Some are simple: Quartz: SiO2, Calcite: CaCO3
ii) Others are not: Garnet: A3B2(SiO4)3 - A = Mg, Fe, Mn, Ca; B =
Al, Fe, Cr
iii) All have a definite chemical structure, if not a constant
composition.
d) Most minerals are built from only 9 of the 90 natural elements:
i) Oxygen, Silicon, Aluminum, Magnesium, Iron, Calcium,
Sodium, Potassium, and Titanium; or O, Si, Al, Mg, Fe, Ca, Na,
K, & Ti for short.
ii) Other elements are trace species in minerals made of the
above, or are concentrated by natural processes into their own,
unique minerals:
1) Lead: Galena; Mercury: Cinnabar; Copper: Chalcopyrite
- all in combination with Sulfur.
2) Gold, Platinum, Silver, and sometimes copper combine
with no other element, and form monoelemental
minerals.
e) Classes of Minerals: based on chemical similarities.
i) Silicates: all contain SiOx; the most common minerals. 2
classes:
1) Mafics: (Fe, Mg) SiOx + ...
2) Felsics: (Na, K) AlSiOx + ...
ii) Carbonates: contain CO3's. Very important in Florida.
iii) Sulfides: Metals + S.
iv) Oxides: Metals + O.
v) Sulfates: SO4
vi) Phosphates: PO3
vii) Borates: BO3
viii) Nitrates: NO3
ix) Halides: Cl, F, Br, I
1) groups v thru ix above can be lumped together and called the Evaporites, because most form during the
evaporation of bodies of water (Exception: some
phosphates are bone derived, as in Bone Valley, FL.)
D) Minerals combine to form Rocks.
1) A rock: A naturally occurring solid made of 1 or more minerals.
2) 3 varieties of rock, separated by the way they are formed:
i) Igneous Rocks: formed when molten rock material, called a Magma,
crystallizes. 2 subclasses:
a) Volcanic rocks: The magma erupts - then it's called a Lava -
and the lava rapidly solidifies in air or water.
1) often contain Vesicles: gas bubbles that formed during
cooling.
2) May quench to a glass, forming Obsidian.
b) Plutonic Rocks: Magma intruded deep in the crust, and cools
slowly, making a coarse grained rock, like Granite.
c) We make Igneous rocks in 3 places: Ocean ridges, Subduction
zones, and "Hotspot" settings like Hawaii. Generally, at the
edges of plates.
ii) Sedimentary rocks: these form when other rocks weather and break
up, and the bits are carried downhill into Sedimentary Basins: low
spots, like oceans or lakes.
a) the sediments become rocks via Compaction, Cementation, or
Precipitation.
b) As with limestones in FL, sediments can include materials
produced in the Biosphere, like shells.
c) Sedimentary rock names are descriptive: sandstone,
mudstone, siltstone, Rock Salt, Gyprock (rock made of Gypsum).
d) Sedimentary rocks typically form in the middles of plates,
where the calm, deep basins are.
iii) Metamorphic Rocks: rocks which started out igneous or
sedimentary, and have undergone change.
a) these form when some other kind of rock undergoes different
pressure-temperature conditions.
1) How do you change pressure and temperature? You
bury the rock!
2) Rock burial occurs best at Subduction zones, where
sediments pile up in trenches, and may even be dragged
down; or where continents rafted on 2 different plates
collide, making mountains.
3) The three different rock types together comprise a natural cycle - The Rock
Cycle, though I would say that what they really do is very accurately reflect the
Plate Tectonic cycle of the Earth's crust.
Earthquakes:
Where? Wherever crustal plates rub against one another. Usu. plate boundaries, esp. subduction zones and transform margins. When plates rub, it happens in fits and starts. Stress builds up at the contact. The product of stress is some sort of strain - the rocks either deform or break. Earthquakes are the physical expression of the release of stress.
Earthquakes occur along Faults.
Fault: a fracture in rock along which motion has occurred. . Plate boundaries are faults.
Ocean ridges undergo Extensional or Normal Faulting.
At subduction zones Reverse, or Thrust faulting: one plate is being thrust over the other.
Strike-slip faulting atTransform boundaries: where plates rub side-by-side
Seismic Waves, vibrations that travel through the Earth from the site of fault motion.
P waves: Compressional waves, the first, or Primary waves to be detected at recording stations after an earthquake. P waves can travel through both solid and liquid media.
S waves: Shear waves, which have both a vertical and forward component to their motion. These are the Secondary waves picked up at seismic recording stations after an earthquake. S wave velocities are about half that of P waves, and S waves can't travel through liquids. Both P and S are Body waves, that travel through rock.
Love Waves and Rayleigh Waves: These are Surface Waves, which travel relatively slowly along the surface of the earth from a quake, and do most of the damage.
The vibrational character of the ground beneath buildings can serve both to attenuate or accentuate earthquake vibrations: note in the '89 Loma Prieta quake a lack of damage in old, hilly San Fran, but devastation on the bay edges and in the Marina District, where everything is built on fill.
Focus of a quake: origin of vibrations
Epicenter: the point on the surface directly above the focus, and this is what the news agencies report.
Earthquake measurement:
Energy Release, or Magnitude measurement: Richter Scale, and Moment Magnitude (MM) scale.)
The Richter magnitude of an earthquake is a measure of the largest shear wave amplitude (or size) recorded on seismographs. Logarithmic, so magnitude 6 is 10 times greater than magnitude 5. The MM scale provides better calibrations for quakes with Richter magnitudes 7 and above, as seismograph response is nonlinear at such levels.
Modified Mercalli Intensity Scale: observed earthquake intensity. May not correlate w/ magnitude. Useful for engineers and planners who want to identify areas of a region that are particularly vulnerable to earthquakes.
Identifying Active Fault zones:
Physical features of the land may identify fault motion: offset streams, linear ridges, and fault scarps. Differences in soil age on either side of a suspected fault can help define when the fault last moved. Soil dating has been used in Calif. to place constraints on earthquake recurrence.
Earthquake risk assessment:
First step: where do they occur, how often, how bad?
Risk maps based on the maximum intensity of ground shaking that is likely to occur in a given area. Detailed maps exist for southern Calif.
Earthquake Effects:
Ground rupture and displacement along the fault: what the whole event was about anyway.
Ground motion: 'waves on land,' sudden accelerations of the ground surface, which can shear off trees, crumble buildings, bridges, and roads, and knock folks down. Crevasses can open and close, chunks of the subsurface can be turned to the surface, all such things.
Liquefaction: shaking may break up the coherence of wet sedimentary strata, causing them to behave like a liquid. The result can be vast landslides on almost flat slopes, the foundering of large structures, and the buoyant rise of buried tanks. In the 1964 Alaskan earthquake much of the damage to the city of Anchorage was due to the liquefaction of a clay layer well beneath the city. A low angle landslide ensued, destroying all that sat atop the clay.
Landslides
Fires:
Tsunamis: seismic sea waves. Earthquake-related ground disruptions can produce very large (4-5 km amplitudes), fast moving (800 km/hr) ocean waves. Occur in clusters, In the deep ocean, these waves are seldom >1 m high, but as they hit the shallows, they slow down abruptly, and grow to as much as 15 m (45 feet!) in height.
Changes in land elevations: Usu. occurs in great earthquakes. 1964 Alaskan quake uplifted a large section of the inner Gulf of Alaska by several meters, and caused regional subsidence on Kodiak island and the Kenai peninsula.
Hazard Reduction with Earthquakes:
Effects of motion along faults: seismic shaking and ground rupture.
Materials respond differently to these effects, and vary in their ability to transmit and amplify seismic waves
Crystalline, massive igneous and metamorphic rocks: excellent transmitters, poor amplifiers.
Alluvium, fill, silt and mud: intense amplification of waves, but very poor transmission. The worst earthquake damage occurs as a result of construction on alluvium or fill. The 1989 Loma Prieta quake in San Fransisco produced the most damage in the Marina District (fill from the 1906 quake), the Mission district (alluvium), and highways along the bay in Oakland (fill and alluvium). 1985 Mexico City quake caused catastrophic damage although the focus of the quake was several hundred km SW, in large part because Mexico City sits on alluvium in an old lakebed. The alluvium magnified the attenuated waves from the quake to extreme degrees.
Earthquake prediction:
Predictions run from long term probabilities (which we do reasonably well) to short term warnings. Much effort has been expended on short term prediction, with limited success.
Foreshocks and microearthquakes: many quakes are heralded by precursory foreshock activity.
Rates of uplift or subsidence: changes in such rates, or the abrupt occurrence of such changes, has been precursory to quakes, esp. one in Niigata, Japan.
Seismic gaps: places along a fault where recent seismicity is low are candidates for quakes. Regularly used in Calif. along the San Andreas system to estimate quake probability. Doesn't tell you the day of the week, just the likelihood in the long run. In combination with other factors, it's strong evidence.
Changes in electrical resistivity: Tests have shown that the resistivity of rocks along faults change before quakes, due to changes in water content, coherence etc
Changes in radon in groundwaters: Radon levels may increase significantly in deep wells before quakes, perhaps due to the grinding up of rock at depth. This may be the tool with the most potential.
Anomalous animal behaviors.
Volcanic Hazards: see exercises!!
Landslides, Sinkholes, and Mass Wasting Related Phenomena.
landslides :natural processes mankind has managed to exacerbate
Slopes: different in wet and dry climates,
West, slopes are rugged with a steep, rocky freeface. At their bases: either a talus slope (material emplaced via rockfalls) or an Alluvial Fan (emplaced via water).
East: entire slope is covered by soil and vegetation,
Creep: Loose material moving on a slope, very slowly. You can recognize creep by looking for bent trees on a hillside.
Rarely much material slides at once: landslide.
Water acts as both a lubricant and a transport medium.
Factors affecting Landslide frequency:
Steepness of Slopes
Type of Material: Platy material like shales slides best.
Climate and Vegetation:abundant water increases likelihood, heavy vegetation w/ deep roots decreases chance.
Time: Wet seasons worse than dry, extreme changes in Temp (as in Spring, Fall) increase chance.
Causes of landslides:
Internal or External causes: the inherent weakness, or the triggering event.
Human use of land can aggravate landslide hazards, and produce more frequent slides.
Nature and chance: Nevado Del Ruiz volcano in south America erupted in the late 1980's, melted glaciers , resulting in mudslides that obliterated several major towns in the valleys below. Over 15,000 people died.
Human induced landslides accompany road building and other like construction projects on steep mountain slopes.
Ex: In the SE U.S., the Pigeon River Gorge segment of Interstate 40, cutting through the Smoky Mountains between Tennessee and North Carolina.
Faulting and bedding in Smoky Mountains rocks makes area very prone to Wedge Failure slides. The roadcuts along I-40 exacerbated that situation by removing chunks of the underlying strata, which cut resistance further. The result: >100 slides since early '70's. Est. prevention cost: $10 billion.
Landslide Prevention and control:
Involves recognizing the hazard where it occurs, and taking corrective measures when the hazard is recognized too late to avoid. Requires geological determination of bedding orientation, and whether there are weak, slide-prone interbeds.
Engineering solutions: concrete retaining walls, heavy chain link screens across rock faces to prevent tumbling stones, "stairstep" roadcuts to attenuate downslope events, even steel and concrete straps
Subsidence and Sinkholes:
A Karst problem: rainwater dissolves limestone country rock, Eventually, fractures expand to become caves, which collapse when the roof gets too weak to support overburden. All round lakes in FL are sinkholes
We aggravate the problem of sinkholes 2 ways:
1) we unwittingly build on them, increasing the overburden, and
2) and most importantly, we drain groundwater for our own uses, emptying the shallow caves. The water itself provides significant overburden support. In its absence, the roof collapses and you have a sinkhole.
May 1981: Winter Park, FL: it ate a house, several cars, businesses. Cost: $2 million.
There are sinkholes on our very own campus!
Most of the sinkholes I have seen since moving here are like the ones here at USF: not tremendously deep or large. In the cypress swamps you can sometimes spot them as particularly low spots.
Other subsidence hazards: Usu. have to do with mining activities. Coal mines, Salt mines, in TX, LA, WV.