Geologic Time
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Deposition of the Barkley Sandstone
Deposition of the Grin Sandstone
Deposition of the Lemin Ashtone
Deposition of the Cave Limestone
Deposition of the Lewin Mudstone
Folding
Possible unconformity dividing upper
and lower sequences (only inferred)
Deposition of the Jep Shale
Deposition of the Peaty Shale
Deposition of the Coal Measure
Deposition of the Basin Conglomerate
Deposition of the Deekin Sandstone
Deposition of the Shallow Shale
Deposition of the Park Claystone
Faulting lower sequence into blocks
Thrust faulting of upper sequence over
lower sequence
Intrusion of Basalt
Continuation of thrust faulting
A little quantitative exercise:
• A radioactive isotope A has a half life of T1/2 = 10,000
years and decays into a stable daughter isotope B. At
t = 0, the rock cools just below its blocking temperature
and begins to trap the daughter isotope B, which would
otherwise be lost.
• On the provided sheet of graph paper carefully plot the
relative abundance of A and B from t = 0 to t = 5 T1/2.
• The analysis of an igneous rock specimen yields 12,500
isotopes of A and 87,500 isotopes of B. How old is the
rock?
A brief history of oceans, continents and
Connecticut
• oceans : mostly basalt
• continents : mostly granite
If Earth started out as one molten blob – how do
we separate crust into continents and oceans ?
Magmatism of Subduction Zones
Which magma types are produced ?
(wet) partial melting
in subduction zones
can produce
•andesitic
•basaltic
•rhyolithic magma
Mt. Stromboli, Italy
image: USGS volcano hazards program
Mt. Spurr, AK
image: ETH
Clark AFB, Philippines, image: USGS volcano hazards program
http://www.elams.org/Places/Yosemite%202001/Half%20Dome%20with%20Royal%20Arches.jpg
Early Continents
• small island arcs (e.g., Japan)
• have core of granitic rocks (light, won’t be
subducted)
• are pushed across surface of earth by plate
tectonics
• combine with other small (micro)continents
• form accreted terranes
A Closer Look at Subduction Zones
• most of the oceanic slab goes down
• some sediment and oceanic rock will be
caught in subduction zone to for accretionary
prism
• small microcontinents will not be subducted
• accreted terranes
Early Terranes of North America
More Recent
Additions
From: Bell, 1985
The Wilson Cycle
• continents are hard to destroy → float on top
of crust
• they are bound to crash into each other
• form super continent (Pangea)
• what drives plate movement?
• what makes continents break up?
The Elusive Mechanism:
What Drives Plate Tectonics ?
• Ridge-Push forces
from: NOAA's National Geophysical Data Center
Why do Ridges Stand High?
• temperature of rocks ?
• density of rocks ?
• what’s below the lithosphere ?
• what are the physical properties of the
asthenosphere?
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hot rocks are less dense
buoyant
float high on asthenosphere
become denser as they cool
• oceanic lithosphere slides down the ridges
Temperature
Distribution in
Subducted Slab
Which Processes are Important?
• how would you investigate importance of slab
pull and ridge push forces?
• where does ridge push occur?
• where does slab pull occur
• what is easier to move: a small or a large
plate?
• which plates should move the fastest?
from: Cox and Hart, 1986
What Makes Continents Break up?
• Continents are thick, good thermal insulators
• How will temperature change under thick
continental crust?
• How will buoyancy change?
• How might this affect stability of continent?
The Wilson Cycle
• continents are pushed together by plate
tectonics
• can form super continent
• thick layer of insulating rocks
• base of continent heats up
• softens rocks, makes them buoyant
• continents can drift apart
from: Bell, 1985