Transcript Chapter 8: Major Elements

Continental Drift
Alfred Wegener (1912)
First serious proponent
Alfred manning the weather station,
Greenland - 1913
Fit of the Continents
A more modern view than Wegener’s uses 1000 or 2000 m
isobath as estimate of edge of continental crust
Fit of Structural Elements
Pennsylvanian (300 Ma) Glaciation
Glacial striations in
bedrock, South Australia
Pennsylvanian (300 Ma) Glaciation
Arrows indicate ice
movement directions
Using present continental locations
Pennsylvanian (300 Ma) Glaciation
Arrows indicate ice
movement directions
Using pre-drift continental locations
Fossil Evidence
Glossopteris: an
ancient seed fern
(200 Ma)
Distribution of
Glossopteris
fossils
Fossil Evidence
Distribution of
Mesosaurus fossils
Mesosaurus couldn’t
swim in open ocean
Paleomagnetism
The Earth as a dipole
Magnetic declination and inclination
Paleomagnetism
Magnetization of volcanic rocks and sediments
Paleomagnetism
“Polar Wandering” curves
2) The Earth’s Interior
Plate Tectonics: a breakthrough
Brian Mason (Scripps) led a group that studied a 2-D area spanning
the Mid-Atlantic Ridge in detail
Miles from ridge axis
An explanation of the
curious magnetic anomaly
pattern
The process at mid-ocean ridges
The process at mid-ocean ridges
Black Smokers
Seismicity
Earthquakes occur due to motion along faults
Dip-slip Faults
Normal Fault
Reverse Fault (thrust)
View is cross-section
Seismicity
Earthquakes occur due to motion along faults
Strike-slip Faults
Right-Lateral
Left-Lateral
Map View
Seismicity: global distribution of earthquakes
Earthquake foci in the vicinity
of the Japan trench
Seismicity
First motion studies
Bomb
Seismicity
First motion studies
Earthquake
Seismicity
First motion studies tell us that earthquakes:
 At ridges  normal faults (extension)
 At trenches  thrust faults (compression)
 At fracture zones  strike-slip faults
Seismicity: global distribution of earthquakes
The Deep-Sea Drilling
Program
Sediment ages directly on crust
Age of the ocean crust
Hot Spots
Hot Spots
Hot Spots
The Modern Plates
Three types of plate boundaries
Three types of plate boundaries
Divergent boundary
Divergent boundary
Where on Earth is continental rifting occurring today?
Transform boundary
Note opposite sense of motion (first motion studies)
San Andreas
Transform
Transform boundary
Convergent
boundary
Three sub-types
 Ocean-Continent
 Ocean-Ocean
 Continent-Continent
Can you name an
example of each?
Convergent boundary
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Ocean-Continent:
Andes, Cascades
Ocean-Ocean:
Aleutians, Japan
Continent-Continent:
Himalaya, Alps
Convergent boundary
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Trench and subduction zone
Earthquakes
Linear chain of andesitic volcanoes (granites below)
Creation of mountain ranges (also linear chains)
 Andean type - continental arc
 Himalayan type - collisional (a terminal type)
“Andean-type”
orogenesis
Continental crust
thickens by addition
of magma from the
subduction zone
Compression due to
plate convergence
“Himalayan-type”
orogenesis
Begins as Andean-type
“Himalayan-type”
orogenesis
How do you locate the suture zone today?
How can you determine the “polarity” of subduction?
Slivers of oceanic crust and upper mantle (ophiolites)
The “suture zone” is marked by the mélange and
become incorporated into the “mélange” in the
particularly by the occurrence of ultramafic rocks
accretionary wedge of deformed sediments
composing the mantle portion of the ocean lithosphere
Chain of ultramafic bodies in Vermont indicating a
suture zone of the Ordovician Taconic Orogeny. The
ultramafics mark a closed oceanic basin between
North American rocks and an accreted island arc
terrane. From Chidester, (1968) in Zen et al., Studies
in Appalachian Geology, Northern and Maritime.
Wiley Interscience.
Appalachia
n History
Can “accrete”
island arc
terranes as well
as continents
Plate Motions
Plate Tectonics in the Pacific Northwest
The Wilson
Cycle
The Breakup of Pangea
The History
of an Ocean
Basin
Igneous Processes
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Decompression partial melting at divergent zones
Igneous Processes
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Decompression partial melting at divergent zones
Partial melts: low-T fraction is always more Si-Al-NaK-rich and Fe-Mg-poor than source rock.
Leaves behind Mg-Fe-rich refractory residue
Igneous Processes

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Hot spots and mantle plumes
Subduction zones:
 Conveyor of basalt to melt  andesite
 Water lowers melting point of mantle wedge
Igneous Processes

Subduction zones: water lowers melting point