earth's interior
- how earthquake seismology is useful for determining the internal structure of the earth
wavefronts and ray pahts
- Moho: what it is (crust-mantle boundary) and how it was discovered
- the core-mantle boundary (Gutenberg seismic discont.) - P and S wave shadow zones,
- the mantle: low velocity zone (LVZ) - the asthenosphere
- the lithosphere
- the 410 and 670 km seismic discontinuities - the upper and lower mantle
- major subdivisions of the earth from core to surface and the materials that make them up
- inner core, outer core, mantle, mantle asthenosphere, mantle lithosphere + crust = lithosphere
- the average thickness and the predominant igneous rock types (felsic, mafic, etc.) found in ocean crust
and in continental crust
- evidence for interior composition:
crust: we walk on it;
mantle: xenoliths carried up in volcanoes, seismic velocity
core: must be high density material common in the solar system,
and account for seismic velocity and fluctuating magnetic field
be able to draw profiles of the Earth's interior and describe the major layers and boundaries
historical development of continental drift and plate tectonics
- continental drift:
Wegener and DuToit's paleoclimate indicators, truncated geologic features, far-flung fossils, and fit of the continents
- paleomagnetism: paleomagnetic evidence for continental motions
magnetism recorded in rocks can tell direction and distance to the pole at time rock formed
- seafloor spreading
evidence: marine magnetic anomalies
contributions of Harry Hess, Vine & Matthews (1963), Pittman & Heirtzler (1966)
plate tectonics
diveregent plate boundaries
midocean ridges
plates spread apart
new crust forms
source of magma: decompression (partial) melting of upwhelling mantle...
ridge stands high b/c hot; lithoshpere cools & contracts as it spreads away
continental rifts (e.g., East African Rift)
may continue to form new midocean ridge
or become a failed rift
convergent plate boundaries
ocean-ocean subduction zone (e.g., Aleutians, Mariannas, Philipines, Japan)
continent-ocean subduction zone (e.g., Andes)
deep ocean trench
volcanic arc (continental or island arc)
magma produced by partial melting of mantle above subducting crust...
continent-continent collision (e.g., Himalayas)
orogenic belt
transform plate boundaries
oceanic transforms: ridge offsets
transform faults
fracture zones
continental tranforms: (e.g., San Andreas, North Anatolian Fault in Turkey)
testing/further evidence for plate tectonics
Sykes (1967) first motion studies of earthquakes to determine fault movement on plate boundaries
midocean ridges: normal faults - extension - diveregence
near trenches: thrust & reverse faults - compression - convergence
ridge offsets: strike-slip in proper sense for seafloor spreading as opposed to "transcurrent fault" idea
Wadati-Benioff zones: plane of EQs descending from trench, down as deep as ~670 km
shows the location of subducting plate
problem: shallow EQs caused by brittle faulting; what about deep focus EQs? crust should be ductile!
hypothesis: EQs between 410 and 670 km result from sudden en masse phase changes
e.g., olivine -> spinel -> perovskite (transforming into higher density crystal structure)
be able to draw profiles and maps of midocean ridges, subduction zones, transform and fracture zones
what drives plate motions?
first hypothesis: mantle convection drags the plates
but some plates too large (Pacific plate) and the upper mantle too thin to allow a single convection cell
to drive the whole Pacific plate; seismic tomography shows multiple convection cells across the width
of the Pacific plate; some could help drive the plate, others would oppose it
best bet: gravity
slab pull: the weight of the plate as it sinks into the mantle pulls the plate
ridge "push": at midocean ridges the lithosphere lies on an inclined plane of elevated asthenosphere
the plate slides down the slippery slope of the asthenosphere
problem: slab pull and ridge push doesn't completely account for the motion of all plates
other forces:
mantle convection may help to drive continents with deep "keels" (thick lithosphere beneath mountains)
trench suction resulting from slab rollback may also help to pull continents
mantle convection
is the mantle composed of two separate upper and lower convection systems separated by the 670 km discontinuity?
the 670 km discontinuity marks the boundary between less dense (olivine->spinel) phase ultramafic silicates from
more dense (perovskite) phase ultramafic silicates
shouldn't these remain density stratified like oil and water?
no Benioff zone earthquakes occur below around 660-670 km
the old interpretation was that subducting plates may descend this far but no deeper
but, midocean ridge basalts and hotspot basalts (like Hawaii) have distinctly different chemical signatures
suggesting they are produced from different mantle reservoirs
midocean ridge basalts from the upper mantle, and hotspot basalts from plumes arising from lower mantle?
recent seismic tomography (like ultrasound scans for Earth's interior) results are beginning to show:
- midocean ridge upwhelling source in the upper mantle
- plumes below some hotspots arise from deep in lower mantle
while some rise from within upper mantle
- some subducting slabs sink all the way to the bottom of the lower mantle
while some stop at the 670 km discontinuity
therefore, there appears to be a combination of separate upper/lower mantle convection
with some plumes and some slabs penetrating the boundary
groundwater
- water reservoirs: the relative volumes of the ocean water, glacial ice, groundwater, surface water resources
- the hydrologic cycle: precipitation = runoff + infiltration + evapo-transpiration
- porosity, permeability
- typical permeable materials that make good aquifers: sand, gravel, sandstone, limestone
- impermeable aquiclude materials: clay, shale, unfractured igneous and metamorphic rocks
- zone of aeration, zone of saturation, water table, aquicludes, cone of depression, drawdown
- confined aquifers, pressure (potentiometric) surface, artesian and flowing artesian wells
- groundwater flows from where water table (or pressure surface) is high to where it is low
- water wells, how they work
- town water supplies, water towers
- land subsidence from over-pumping
- landfills (garbage dumps) and our groundwater supply
- sanitary landfills
- non-point sources of groundwater pollution: lawn (and golf course and agricultural) chemicals
- saltwater intrusion
- groundwater - surface water interaction: gaining and losing streams (discharge and recharge)
- perennial, ephemeral, and intermittent streams
be able to draw profiles of the groundwater system
streams
- stream characteristics: velocity, cross-sectional area, discharge
- stream hydrographs (plotting discharge vs. time following a rain event)
why does it take time for the stream discharge to increase following a rainstorm?
why does the stream's discharge not decrease to zero eventually after a period of no rain?
- stream drainage networks (just recognize that streams are organized in networks)
- stream transport - bed load, suspended load, dissolved load
- relationship between stream velocity and size of particles that can be transported
- meandering streams: deptha and velocity across a bend -> point bars, cut banks, oxbow bends and lakes
- youthful stream valleys (steep-sided, V-shaped)
and mature stream valleys (gentler valley slopes with flood plain)
- stream deposits: point bar sands, flood-deposited muds
- floods and sediment transport (most transport occurs during floods)
- floodplains, valley walls, natural levees
be able to draw profile and map views of streams and stream valleys
coastal processes
- coastal features molded by waves, tides, and gradual sea level rise
- size of waves determined by wind speed, duration, and fetch
- crest, trough, wavelength (L)
- orbital motion of water as wave passes, decreases to zero at depth of L/2
- what happens to a wave as it approaches shore (when water depth < L/2)
- breakers, swash, backwash
- beach profile: shoreface, berm, dune
- winter/summer profiles
- longshore drift and longshore currents
- coastal erosion & sea level rise
- thermal expansion of the oceans, and incresed melting of glaciers cause by global warming
- effects of groins and seawalls
- beach renourishment