The Earth Environment
uneven distribution of solar radiation striking Earth's
surface
heat distributed by atmospheric and oceanic circulation
large scale atmospheric convection modified by coriolis effect
general atmospheric circulation, prevailing winds
the dominant low pressure (rising air) zones and high pressure (sinking air) zones
climate belts simplified: equatorial, tropical deserts, mid-latitude
temperate belt, polar
ocean surface currents (clockwise flow in northern hemisphere, counterclockwise in southern hemisphere)
bottom water currents, cause of sinking water, important locales of deep water formation
ocean conveyor belt and climate
how could the conveyor belt slow? (global warming, increased melting in Greenland, decreased ocean salinity)
how could that possibly affect regional climates (e.g., NW Europe)?
Sedimentary Environments
equatorial: (hot, wet) aerially extensive coal
deserts: sand dunes -> large scale cross-bedded sandstone,
frosted sand grains
glacial deposits: poorly sorted glacial sediments, terminal moraines
stream deposition
point bar sand (sandstones) and overbank muds (shales); current ripples, terrestrial fossils, etc.
coastal deposition
beach -> offshore muds -> farther offshore reefs (if warm & clear)
beach sand and gravel -> sandstone (oscilatory ripples) & conglomerate
nearshore sand -> sandstone
offshore silt and clay (mud) -> shales
in warm water beyond where most mud is deposited coral reefs may grow
-> limestone
marine sandstones and shales contain fossils of marine (ocean)
life
Stratigraphy and Correlation
formations: group, formation, member,
bed
changes of sedimentation resulting related to sea level
lateral differences in sedimentation due to position relative to sea level (on land, beach, offshore)
sea level rise yields fining of sedimentation, sea level fall
yields coarsening of sedimentation
transgression-regression
correlation
based on lithology (rock characteristics) and fossils
geophysical methods: e.g., magnetostratigraphy
unconformities: nonconformities, angular unconformities,
disconformity
Geologic Time
sequence of geologic events
Steno's Principles (1669): original horizontality, superposition, lateral continuity
crosscutting relations, baked contacts, fossils, unconformities
- know how to interpret a geologic cross section of the sort we did in class
development of the geologic timescale
Smith, Cuvier, and fossil succession
Sedgewick, Murchison, and the Cambrian and Silurian (plus the
Ordovician)
finishing up: Devonian (from Devonshire), Carboniferous (coal
measures), etc.
absolute time - how old is the Earth?
Bishop Ussher and the generations since Adam - ~6000 yr
Werner (18th ct) and Catastrophism (rocks formed by precipitation from primeval sea [as in Genesis?]
Hutton, Lyell (18th & 19th ct.) and uniformitarianism - OLD (millions of years?)
Darwin: many hundreds of millions to account for evolution
British geologists (latter 19th ct.) and time required to deposit all sedimentary layers - ~100 m.y.
Joly (1899) - time required to make the oceans salty - ~100 m.y.
Lord Kelvin (latter 19th ct.) and the cooling Earth - 20 - 40 m.y. to
100 m.y.
Becquerel, Curie, and the discovery of radioactivity (1896)
Rutherford: radioactive decay in the Earth invalidates Kelvin's assumption
Rutherford and Soddy (1902), radioactive decay proportional to amount
of unstable isotope;
parent/daughter ratio might be useful for dating natural
materials
B.B. Boltwood (1907), "first man to date a rock," after estimating
decay rate of uranium into lead;
samples ranged in age from 410 m.y. to 2.2 b.y.
radiometric dating
unstable parent atoms decay to stable daughter atoms at
known rate (half-life)
3 kinds of nuclear decay: beta emmission, beta capture, alpha emmission
measuring ratio of parent to daughter yields the age of
crystallization
- know how to determine a radiometric age like we did in class
oldest preserved rocks that formed on Earth are about 4 b.y.
old
oldest preserved minerals (zircon crystals) are 4.3 to 4.4 b.y.
old
oldest rocks formed in the solar system (meteorites, moon rocks) are
4.5 to 4.6 b.y. old