Plate Tectonics and the Geologic Record
What causes the Earth’s cycles of
uplift, erosion, and deposition?
Hutton’s angular unconformity at Siccar Point, Scotland
Plate Tectonics
rifting
lift
up
subduction
up
lif
t
subduction
Tectonic plates separate (diverge) and collide
(converge) as they move across the globe.
C
Plate
Margins
D
transform
convergent
C
D
divergent
Divergent Plate Boundaries
• Plates are moving away from each other.
• Magma is upwelling to form new oceanic crust.
• Mid-ocean ridges
• Continental rift zones (east Africa)
Rifting in east Africa
Eroding uplands
Rift lake
Normal fault
Rift valley
deposition
African rift volcanoes
Basalt
Rock formation in a growing rift basin
Rift Lake and shoreline
beige siltstone
and dark shale
Braided river / floodplain
red sandstone and siltstone
Alluvial fan
conglomerate
and arkose
Basalt lava flows
Bounding
fault
Geologic Evidence for
Continental Rifting
• Normal faulting - rift valley basins
• River and lake sediments
– Red beds, siltstones, black shales
– Fresh water and terrestrial fossils
• Basalt lava flows and subsurface intrusives
• Evaporite beds
Rifting in east Africa
Lake Assal - seawater-fed salt lake at
the northern end of the East African
Rift.
Passive margin
Passive Margin
•Tectonically
inactive
•Ocean crust and
continent are
connected on
single plate
Passive margin
•Continental
margin sinks
(subsides) as
ocean floor
cools.
Clastic sediments
Ancient rift sediments
Carbonate sediments
Evaporite deposits
Geologic Evidence for a Passive
Continental Margin
•
•
•
•
Thick accumulations of limestone.
Thin sandstones and shales.
Slow rate of deposition of detrital rocks.
No volcanoes, no metamorphism, no folding
of rock layers.
Interbedded limestone,
dolostone, and shale
Niagara Gorge, NY
Convergent Plate Boundaries
• Plates are colliding with each other.
• 1. Ocean crust - ocean crust
• 2. Ocean crust - continental crust
• 3. Continental crust - continental crust
Ocean - Ocean Convergence
Geologic Evidence for OceanOcean Collision
• All ocean floor is eventually subducted back into
the mantle - little record of the deep sea floor is
preserved in the rock record.
• Island arc my eventually be pushed against a
continent in an island arc to continent collision
Ocean - Continent Convergence
Foreland
Basin
foreland
basin
Andes Mountains - folded and uplifted sedimentary rock
Erupting Andean volcano, Ecuador
Geologic Evidence for OceanContinent Collision
•
•
•
•
Metamorphic rock
Folded sedimentary rock
Granite plutons (magma bodies)
Foreland basin filled with thick sequence of
eroded sediments (gravel, sand, mud)
• Deep sea sediments, volcanic islands, and
oceanic crust sutured onto the margin of the
continent.
Serpentinite - metamorphosed oceanic crust
San Francisco, CA
(Mesozoic)
Staten Island, NY
(Paleozoic)
Serpentinite bodies are remnant slivers of metamorphosed
oceanic crust caught up in ancient plate tectonic collisions.
Continent - Continent Convergence
Crust thickens greatly, produces high mountains
Foreland basin
Foreland basin
Metamorphism
Deposition of
detrital sediment
eroded from
mountains.
Geologic Evidence for ContinentContinent Collision
• Metamorphic rock
• Folded sedimentary rock
• Foreland basin filled with eroded sediments
– Thick accumulations of mud, silt, and sand
– Deposited over geologically short interval of time
(several millions of years).
Carboniferous sandstone and shale, Pound Gap, Kentucky
37
Foreland basin deposits