formation of continental crust
why is there no original (4.5 b.y.) crust remaining on the Earth? (constant resurfacing: volcanism, impacts)
to generate the first true crust:
Earth must have been cool enough to generate partial melts of the ultramafic mantle parent rocks
since the mantle is ultramafic and continents comprise mafic, intermediate, and felsic rocks
oldest rocks (4 - 3.8 b.y., Greenland) formed over subduction zones
(modern day subduction zones derive magma by metamorphic dewatering of subducting slabs and
partial melting of the overlying mantle b/c water lowers the melting point of mantle rocks)
paired metamorphic belts of greenstones and granitic gneiss common in Archean terranes may be subduction related
volcanic arcs and backarc basins
oldest crust on Earth: 4 b.y. old (in Greenland)
oldest minerals: 4.3 - 4.4 b.y. old zircons
formation of ancestral North America: Laurentia
much crust formed in the Archean, by 2.5 b.y. ago
but was principally only the size of "microcontinents" or island arcs
many microcontinents (island arcs) that later became part of ancestral North America
merged into minicontinents by 2.5 b.y. (Kenoran/Algoman orogenies)
minicontinents accreted together into the ancestral core of North America (Laurentia) between 1.9 and 1.7 b.y.
Hudsonian Orogeny
Grenville Orogeny ~1 b.y. ago was the result of a continental collision with another large continent
all that remains is a metamorphic belt, what was originally deeply buried in the core of the Grenville mountains
Grenville Orogeny believed to represent the amalgamation of a supercontinent called Rodinia including most
or all of the existing major landmasses
Rodinia broke up in the late Proterozoic, 700-600 m.y. ago
evidence is late Proterozoic - early Paleozoic passive margin subsidence
earliest signs of life on Earth
3.8 b.y. "chemical fossils"
3.5 b.y. filamentous bacteria
3.2 b.y. stromatolites
oldest life was prokaryotic (no nucleus or membrane bound organelles): bacteria
more advanced life is eukaryotic (has nucleus & membrane bound organelles): everything else
eukaryotes probably evolved via "endosymbiosis" of two or more bacteria
commented on origin of life
lab experiments recreating early Earth atmosphere have produced
amino acids, carbohydrates, fat lipids, protein, etc.
amino acids (building blocks of protein, and ultimately DNA) even found in meteorites
Archeabacteria as more primitive than Eubacteria (including cyanobacteria)
midocean ridge hydrothermal vent systems as a likely/possible sit for the first life
Eukaryotes (organisms with a nucleus and membrane-bound organelles)
2 - 1.7 b.y. acritarchs (single cell true algae = eukaryotes)
1.1 b.y. eukaryote diversity increases - sexual reproduction!?
Multicellular animals (metazoans) arose ~600 m.y. ago, following the snowball Earth period
"Ediacaran" soft body animals included probable jellyfish, seafans, segmented worms, and others
also unusual forms apparently unrelated to Phanerozoic animals
Proterozoic ice ages:
Gowganda Formation (2.2 b.y.) and the oldest known glaciation
Snowball Earth glaciations of the later Proterozoic (~700 - 600 m.y.)
evidence supports glaciers to within 30 degrees of equator, perhaps Earth froze over
freeze-fry theory for what may have caused glacial advances and retreats at this time
Cambrian Revolution 543 m.y. - evolution of hard parts
"small shellies" - Early Cambrian
all modern animal phyla, (except Bryozoans) became established in the Cambrian
bizarre animals also arose but died out
The Rise of Vertebrates
Agnathans (Late Cambrian-Recent) jawless fish with cartilage skeletons
Placoderms (Silurian-Devonian) jawed fish, heavy bony armor, cartilage skeletons
Acanthodians (Silurian-Devonian) fast swimmers, jawed fish, cartilage skeletons
Sharks & Rays (Devonian-Recent) cartilage skeletons, good swimmers, great teeth
Bony Fish (Devonian-Recent) bony skeletons
Ray Fins gave rise to 99% of modern bony fishes
Lobe Fins mostly died out, except 1 branch that evolved into the tetrapods...
invasion of the land
first land plants & animals (spores & burrows) Late Ordovician
ferns, annelid worms, arthropods (wingless insects, etc.)
ferns need water fertilization
gymnosperms (seed ferns, conifers, cycads, ginkoes) use wind fertilization
gymnosperms became dominant with late Paleozoic climatic drying
Tetrapods evolved from one line of lobe fins in the Late Devonian
modern amphibians (frogs, toads, salamanders) are descended from these first tetrapods
Amniotes: tetrapods that developed the amniotic (water-tight) egg
evolved Carboniferous, became dominant in Permian as climate became drier
Synapsid amniotes were dominant
Sauropsid amniotes eventually gave rise to modern reptiles
after Permo-Triassic Extinction (greatest known)
Synapsids dwindled and only one group survived, but those gave rise to the mammals in the Late Triassic
Sauropsids thrived; the most successful Sauropsids of the Mesozoic were the dinosaurs (evolved Late Triassic)
Groups of Organisms Mentioned (to be familiar with - not to memorize)
prokaryotes
Kingdom Archaebacteria (primitive extremophiles)
Kingdom Eubacteria (including cyanobacteria)
eukaryotes
Kingdom Protista (mostly single celled eukaryotes plus multicelled algae = seaweed)
algae (photosynthetic protists)
protozoans (nonphotosynthetic protists: amoebas, parameocia, etc.)
Kingdom Animalia
Phylum Porifiera (sponges)
Phylum Cnidaria (corals, sea fans, jelly fish, anemones)
Phylum Annelida (segmented worms)
Phylum Arthropoda (jointed appendages: trilobites, insects, crustaceans, arachnids)
Phylum Brachiopoda ("lamp shells" non-mollusc bivalves)
Phylum Bryozoa ("moss animals" coral like colonial animals, but the zooids are closely related to Brachiopods)
Phylum Mollusca (bivalves, snails, squid & octopus)
Phylum Echinodermata (crinoids, sea urchins, sand dollars, starfish)
Phylum Chordata (including vertebrates)
jawless fish (agnathans)
acanthodians
placoderms
sharks
bony fish
ray-fins
lobe fins
tetrapods
amniotes
synapsids
mammals
sauropsids
dinosaurs: saurishian and ornithishian
extinct marine reptiles
pterosaurs
modern reptiles (lizards, snakes, turtles, crocodilians)
Kingdom Plantae (vascular plants)
spore-bearing plants (ferns and more primitive plants)
gymnosperms (naked see plants: cycads, ginkoes, conifers)
angiosperms (flowering plants: oaks, maples, grasses, etc.)
Kingdom Fungi (mushrooms, etc.)
periodic flooding of the continents
major, unconformity-bound sedimentary sequences on continents including much marine strata
result of large scale, long period (slow) transgression-regression cycles
apparently caused by changing rates of seafloor spreading
From One Supercontinent to Another: Rodinia to Pangea
Grenville Orogeny (1 b.y.)
Laruentia collides with another large continent
final stage in formation of Rodinia
passive margin sequence eastern North America (began 700-600 m.y. ago)
breakup of Rodinia
Taconic Orogeny (Ordovician)
collision of island arc with Laurentia
eastern Laurentia now active margin
Acadian Orogeny - Caledonian Orogeny (Devonian)
Laruentia and Baltica (Northern Europe) collide
form supercontinent Laurussia (Euramerica)
Alleghenian (Appalachian) Orogeny (Penn-Permian)
collision of Laurussia with Gondwana
also, Ouchita Orogeny, Hercynian Orogeny,
Uralian Orogeny (Permian) - collision of Siberia/northern Asia with Europe
all of this resulted in the formation of Pangea
geologic clues to continental breakup
passive margin sequence
geologic clues to collisions/orogenies
clastic wedge
metamorphic belts
evolution of plants
spore-bearing (dominant Ord-Perm)
rely on fertilization in water
gymnosperms (dominant Perm-Cret)
rely on airborne fertilization
angiosperms (dominant Cret-Rec)
pollination by insects
seed spread by animals that eat the fruit
Paleophytic, Mesophytic, Cenophytic eras of plant life
Permo-Triassic Mass Extinction
greateast known extinction; sudden event
sea level fell 100 meters during last 2 m.y. prior to extinctions (less ecological space for marine species)
black shales indicate deep sea anoxia indicating breakdown in deep ocean circulation
Siberian Traps: greatest Phanerozoic continental flood basalt eruptions just prior to extinctions
could volcanic ash have darkened the skies killing the primary producers and making it cold?
Mesozoic life after the Permo-Triassic mass extinction
sauropsid reptiles dominant in Mesozoic
turtles & crocodilians
marine reptiles (ichthyosaurs, plesiosaurs)
flying reptiles (pterosaurs)
the first mammals evolved (late Triassic) from the dwindling synapsid lineage
dinosaurs (hole in the hip-socket) evolved late Triassic
(saurischians="lizard hipped" & ornithischians "bird-hipped")
birds evolved in the Jurassic from some small, saurischian (lizard-hipped), raptor
Archaeopteryx: the oldest bird fossils
eventually lizards & snakes
ammonites (squidlike cephalopod molluscs with chambered shells)
Cretaceous-Tertiary (K/T) mass extinction
killed dinosaurs, ammonites and many others
iridium anomaly, glass spherules, shocked quartz
Chichilub crater off Yucatan
tsunami deposits in Caribbean and Gulf Coast
charcoal in western US
- all above point to meteor impact (not extraordinary volcanism) as cause of extinction
"nuclear winter" scenario
Paleocene-Ecocen evolutionary radiation of mammals
22 orders of mammals rapidly evolved after dinosaurs (except birds) became extinct
Pangea Breakup
initial stages of Pangea rifting in the late Triassic and early Jurassic (e.g., Newark Basin)
mantle can't cool with supercontinent overhead
lithosphere gradually gets heated and weakened
mantle eventually gets so hot that large plumes rise from deep in mantle to base of weakened lithosphere
Pangea rifting
Newark Rift Basin: formation and structure
Jurassic restricted seaway, evaporite (salt) deposition, and petroleum traps
mid-Jurassic thru Cretaceous high heat flow from mantle
rapid seafloor spreading
onset of many large mantle plumes
breakup sequence of Pangea and Gondwana
opening of Central Atlantic between Laurasia and Gondwana (mid Jurassic, ~165 m.y.)
Gondwana "unzips" clockwise from around Antarctica
South America+Africa (155 m.y.), then India (125 m.y.), then Australia (100 m.y.), then New Zealand (85 m.y.)
South Atlantic opens between South America & Africa (125 m.y.)
North Atlantic opens between North America (finally born) and Europe (Cenozoic)
Alpine Himalayan Mountain Belt
breakup of Gondwana resulted in the collision of India, Arabia, and Africa with southern Eurasia
southeast Asia is being squeezed out from between India and Asia
Messinian Salinity Crisis:
convergence of Africa & Europe led to drying-up of the Mediterranean around 5 m.y. ago
The Western Cordillera of North America:
a result of the breakup of Pangea and westward movement of North America
passive margin since breakup of Rodinia (Late Proterozoic)
Antler Orogeny (Devonian): now an active margin, subduction, volcanic arcs
several arcs accrete with deformation events during late Paleozoic and Early Mesozoic
mid-Jurassic thru Cretaceous: voluminous volcanic arc activity - Sierra Nevada and other western batholiths
Late Cretaceous Sevier Orogeny
Latest Cretaceous thru Eocene Laramide Orogeny
North America approached and then "ran over" a midocean ridge
subducted progressively younger, hotter oceanic lithosphere
subduction angle went from steep to shallow to flat
San Andreas Fault (system) connects the remaining segments of the still subducting midocean ridge
Basin & Range extension (normal faults) occuring since subduction of the ridge
from the Cretaceous greenhouse to the Cenozoic icehouse
Cretaceous greenhouse climate
10-15 degrees (C) warmer than today
hi sea level, flood basalt eruptions, hi seafloor spread rate
no magnetic reversals (118 - 84 m.y.)
model: superplume rises from CMB cools CMB, vigorous outer core convection, stable magnetic field - no reversals
Descent to Cenozoic Icehouse
oxygen isotope (and leaf shape) record of Cenozoic cooling
slowing of seafloor spreading (less CO2 outgassing)
increased mountain building (Rockies, Alpine/Himalayas) - increased chemical weathering reduces atmospheric CO2
opening of Drake Passage between Antarctica and South America and onset of glaciers in Antarctica (~35 m.y.)
closure of Isthmus of Panama and onset of northern hemisphere glaciation (2-4 m.y.)
The "Ice Ages"
northern hemisphere ice began to buildup by 3.5 - 3 m.y., fully glaciated by 2.5 m.y.
many advances and retreats of ice sheets since then modulated by variations of Earth's orbit
cooling also led to drying (e.g., expansion of Sahara)
Milankovitch Cycles
precession cycle (like spinning of top): ~23,000 yrs
tilt cycle (less tilt to more tilt & back): ~41,000 yrs
eccentricity (from more round to more elliptical): ~100,000 yrs
glaciers
snow - firn - ice
glaciers flow outward from zone of maximum accumulation
movement by internal ductile flow and sliding on bottom (where wet-based)
moraines: ridges of poorly sorted glacial til dumped at end of glacier
some North American glacial features
terminal moraines in the midwest
terminal moraines cross New Jersey, Staten Island, Long Island, to Massachusets
Finger Lakes of New York state were carved by glaciers flowing in former stream valleys
the Hudson fjord was also carved out by glaciers
the channeled scablands of Washington state
caused by catastrophic floods from ice dam failure
ice dam was holding back pro-glacial Lake Missoula
The Ice Ages ended with a warmup (~15,000 yrs ago)
followed by a return to glacial conditions: the Younger Dryas
sudden (decades) climate swings occurred
climate variations during the Holocene
medieval warm period, the "Little Ice Age"
hominid evolution
primates evolve by Paleocene
hominoids - true apes - Oligocene
hominids - by Pliocene
genus Australopithecus ("southern man") by ~4 m.y. ago
A. afarensis (Lucy's species)
A. africanus
Parantrhopus robustus (robust Australopithecine lineage)
genus Homo arose by ~2.4 m.y. ago
Homo habilis ("handy man") fashioned crude stone tools
Homo erectus spread throughout old world
Homo Neanderthalensis in Europe by 100,000 yrs ago
Homo Sapiens in subsaharan Africa by 125,000 yrs ago
north Africa & middle east by 100,000 - 90,000 yrs ago
Europe by ~35,000 years ago - coexisted with Neanderthals for several thousand years
colonized North & South America around 10,000 yrs ago when glaciers broke up
the Holocene epoch - the present and future climate
climate optimum was about 6,000 yrs ago
4 cool periods during past 3500 yrs (including "Little Ice Age")
rapid warming since mid 19th century - the industrial revolution
average temperature of past decade or so is warmer than anytime in past 1000 yrs
temperature did rise by statistically distinguishable amount during 20th century
rising CO2 from burning fossil fuels is responsible for at least part of the warming
sea level rose about 1 foot during 20th century at a rate of ~1-2 mm/yr
expected to be more and faster in 21st century
thermal expansion of the oceans
increasing rate of melting of Greenland ice cap & mountain glaciers
computer climate models far from perfect but..
best constrained models predict several degrees global average temperature rise in 21st century
greatest temperature rise expected in polar regions
Arctic surface temperatures rose ~0.5 degrees C per decade, 1981-2003
mores summertime melting around margins of Greenland ice cap past two decades
northern snow cover retreated about 2% per decade, past two decades
Arctic summer minimum sea ice coverage declining 2% - 3% per decade, 1978-2003
sea ice in Arctic Ocean has thinned since the 1950's
Beware the Positive Feedbacks
less snow & ice, less reflection, more absorption of sunlight, increased warming