Transcript Kump_Ch07_TH - Camosun College

Circulation in the Solid Earth
& Plate Tectonics
Geos 110 Lectures: Earth System Science
Chapter 7: Kump et al 3rd ed.
Dr. Tark Hamilton, Camosun College
Wegener’s
Pangea
• Late Paleozoic
(Permian –
Early Jurassic)
• Supercontinent
closer to S. Pole
• Permian
glaciations
• Jurassic
supercontinent
desert
Geological & Geophysical Evidence for
Pangea (Continental Drift & Plate Tectonics)
• Widespread evidence for S. Hemisphere glaciation:
(S.America, Africa, Antarctica, Australia &
Antarctica) = Gondwanaland
• Fossil ties between separated continents:
Glossopteris – giant seed ferns + Mesosaurus ~ croc
• Geological ties: belts of correlative Precambrian
through Paleozoic geology: plutons, sediment basins
• Fit of Continents – especially at 2000 m isobath
• Paleomagnetism, inclinations, apparent polar
wander
• Now recognized to be caused by seafloor spreading
Geophysical Evidence for Earth’s
Interior Structure & Composition
• Earthquakes occur in Earth’s outermost 700 km of
cold, brittle, rigid, lithosphere
• For the largest earthquakes, the earth “rings like a
bell” and vibrates for days.
• Elastic waves reflect from and refract through
internal layers with different rigidity & density
• Speed of sound in rock varies as a function of
temperature, pressure and composition
• This is useful to provide internal seismic
tomography
Seismic Evidence for Earth’s Interior
• Big earthquakes occur on pre-existing faults
• Elastic wave vibrations spread out, bounce & bend
• Arrival times indicate distances, locations & physical properties
Seismic P-Waves: Volume Compresses
• Elastc waves have different modes of vibration, travel & velocities
• Body Waves travel through by restorably deforming it or making it
vibrate. 2 kinds: Primary – P waves and Secondary – S waves
• Compressional waves (push-pull), the fastest body wave arrives 1st
• Speed increases with depth because rocks become stiffer at higher
pressure than they become denser denser M = K + 4µ/3
Seismic Shear S-Waves (shape deforms)
• Elastc waves have different modes of vibration, travel & velocities
• Body Waves travel through by restorably deforming it or making it
vibrate. 2 kinds: Primary – P waves and Secondary – S waves
• Secondary (Shear) waves side to side or up and down, 0.7 speed of P
• Speed increases at a slower rate with depth than P does
• Liquids have no shear strength or S waves, outer core shadow zone
Seismic Tomographic Imaging of Earth
• Transect
under Nam
• To 2700 km
depth = base
of Mantle
• Dark (blue) =
fast speeds or
colder
• Light (red) =
slow speeds
or warmer
• Image shows
the last of the
subducted
Farallon
Plate under
Texas
Horizontal Motion Seismograph
• Anchored to bedrock or pier
• The mass stands still while the
Earth moves around it
• Magnet and pick up coil
• Lateral motion versus time is
a wave
• Now this is done digitally
except for temporary field
instruments
Seismic Damage: 1989 Loma Prieta Quake
• October 17, 1989 RM = 6.9, San
Andreas Fault biggest since 1906
• Collapsed 2 level highway CF880
Cypress viaduct
• 68 dead, >4,000 injured
• $7,000,000,000 property damages
• To be a damaging earthquake it needs
to be large >/= 6.4 and shallow
• Each number on the richter scale is X
31.66 more energy release and x 10
higher amplitude of ground motion
Seismograms: 1989 Loma Prieta Quake
• Thick fill or unconsolidated sediment amplifies ground motion due to
surface waves: local geology & proximity both affect amplitude
• More ground motion, more & infrastructure building damage
Earth’s Composition
• Carbonaceous chondrites = most primitive solar
system material: silicates, oxygen, rock forming
elements, Carbon, water (oldest type of meteorite)
• The Crust + Hydrosphere + Atmosphere is
enriched in light elements
• Mantle & Core are depleted in light elements and
enriched in heavier and more refractory (high
melting point) elements
• While layering formed early in a few hundred Ma,
the inner core and crust appear to have been
growing throughout Earth history
Seismology & Earth’s Interior
• Concentric layers by mechanics & composition:
Lithosphere: solid, strong, cold, brittle & Quakes
• UM-Partial Melt~5, LM-Plastic Solid, OC-Liquid, IC-Solid
Seismology & Earth’s Interior
• Lithosphere = Crust plus brittle upper Mantle ~100
km thick
– Mostly Intermediate density Igneous rocks (cooled from
melts), lesser Metamorphic (strained, recrystallized in
solid state) & Sedimentary in uppermost few km
• Asthenosphere- weak upper mantle below the
Moho is ~95% solid ultramafic (olivine) with
partial melting and low strength.
– Involved in convection & melt generation of basaltic
magmas at spreading ridges & under volcanoes
– Low velocity zone ~30-80 km is most active
Seismology & Earth’s Interior
• Lower Mantle = Mesosphere
– 660 km discontinuity to ~2900 km
– Most of Earths volume. Entirely hot, plastic, solid,
ultramafic (Mg-Fe rich silicates)
– Insulating, slowly convecting at ~15 cm year
• Outer Core – 100% Liquid Fe-Ni, 2900-5150 km
– Will not pass shear waves, shadow zone
– Convects rapidly generating internal magnetic field
• Inner Core – Solid, super high density Fe-Ni
– Heterogeneous, E-W, layered, ~1 km scale variation
– Growing as it cools, driving convection, spins > 1/day
Magnetic Dynamo & Earth’s Interior
• Outer Core – 100% Liquid Fe-Ni, 2900-5150 km
• Differences in heat, spin & composition drive convection
• Flowing conductor creates circular electromagnetic field
Plate Tectonics & Cycling the Solid Earth
• Ocean Basins are low -3.9 km deep & young <100 Ma
• Continents are +800 m asl & old ~4 Ga, high
Earth’s Geocentric Dipole Field
• Dipole: Tan (I) = 2 Tan ( λ), Surveying by compass
• Cannot tell longitude, can tell geomagnetic latitude
• Rocks get thermally or chemically magnetized
Detecting & Measuring Seafloor Spreading
• Mid-Ocean Ridges ~2km deep, hot & normally magnetized
• Magnetic Stripes are symmetric and a proxy crustal age
Detecting & Measuring Seafloor Spreading
• Mid-Ocean Ridges ~2km deep, hot & normally magnetized
• Magnetic Stripes are symmetric and a proxy crustal age
• Iceland a Hotspot on a Mid ocean ridge
Paleogeographic Reconstruction
• Paleogeographic Earth Reconstruction, Early
Cambrian (540Ma),
Dr. Ron Blakley (2010)
Paleogeographic Reconstruction
• Paleogeographic Earth Reconstruction, Early
Devonian (400Ma),
Dr. Ron Blakley (2010)
Paleogeographic Reconstruction
• Paleogeographic Earth Reconstruction, Early
Permian (280Ma), Dr. Ron Blakley (2010)
Paleogeographic Reconstruction
• Paleogeographic Earth Reconstruction, Tertiary –
Cretaceous Boundary (65 Ma), Dr. Ron Blakley (2010)
• Inset 90 Ma Late Cretaceous Highstand
Composition & Behavior of Earth’s Interior
• Plate tectonic motions depend on
materials strengths & rheology
(response to forces, flow)
• Continental & Ocean Crust and
Mantle are compositional layers
• Lithosphere & Asthenosphere are
rheological discinctions
• The Lithosphere is rigid, solid
and buoyant plates on a
convecting mostly solid but weak
dense, ductile asthenosphere
Plate Tectonics & Cycling the Solid Earth
• Ocean Crust & Continental Crust both on many plates
• 7 large plates & 3-4 smaller ones
Plate Tectonics & Cycling the Solid Earth
• Most Large Earthquakes > Rm 6 occur on plate
boundaries
• Little strain beneath or within plates
3 Types of Plate Boundaries
• Mid Ocean Ridges & Continental
Rifts (divergent)
• Subduction zones: ocean crust
sinks beneath a more buoyant
plate margin (convergent)
• Transform Faults connect the
other 2 types and segment the
ridge system allowing for lateral
motion and shear
3 Types of Motions & Faults
• Divergent  Normal Faults, crust in
tension atop a heat bulge in the upper
mantle.
– New lithosphere is made.
• Convergent  Thrust or Reverse
Faults and crustal thickening plus
uplift of overriding plate.
– Old ocean lithosphere is recycled
• Shear  Lateral motion to right or
left depending on offset of ridges or
subduction zones
Rifts Divide Plates: Make New Ocean Lithosphere
• Slow ridges are steep and rugged, fast are wide & low
• ~1 km3 magma per km ridge per year, quakes < 6.4 Rm
Trenches Collect Sediments where Old Ocean
Crust Subducts
• Megathrust earthquakes > Rm 9.4, volcanic arcs,
hydrothermal mineral deposits on upper plate
Transform Faults Cut Mid Ocean ridges or
Subduction Zones
• Megathrust earthquakes < Rm 8.8 on active segments
between ridge offsets (QCFault, San Andreas)
Continental Rifts in East Africa (since Cretaceous)
• Red Sea to Lake
Victoria
• Cradle of Human
Evolution
• Hot Springs
• Salt Deposits
• Atlantic looked like this
200 Ma ago
Deep Sea Hydrothermal Vents on Ridges & Arcs
• Chemosynthetic Life Web
based on S, Fe
• Larger animals eat
bacteria, tube worms,
clams, crabs
• Hydrothermal massive
sulphide deposits: Au, Ag,
Cu, Zn, Pb, Ba
• Axial Seamount Juan de
Fuca, 21 North EPR,
TAG, Galapagos,
Kermadec Arc, Caribbean
Plate Tectonics:
MOR’s, Trenches & Fracture Zones
• Most Large Earthquakes > Rm 6 occur on subducting
plate boundaries
• Ridge Length = Subduction Length & 3X active transform
3 Types of Convergent Plate Boundaries
• Ocean Subducts under Continent
(Andes, Cascades)
• Ocean Subducts under Ocean
Crust (Philippine, Indonesia,
Aleutians, Antilles, South Scotia)
• Continent-Continent
Convergence, no Subduction
(Himalayas-Urals-Appalachians)
Oceanic
Continental
7.19mislabelled in text
Transform Faults Cut Mid Ocean ridges or
Subduction Zones
• Sovanco FZ connects Juan de Fuca and Gorda
• Mendocino FZ connects Gorda & EPR Sea of Cortez
Active 2 Kinds of Continental Margins Passive
• Passive margin rifted at beginning of Wilson Cycle
• Active Margin subducting, MOR in between
Mantle Convection Drives Plate Motion
• Whole mantle convection driven from heat in
Outer Core, Hotspots require this at least
Mantle Convection Drives Plate Motion
• 2 Layer Mantle Convection, isolated compositional
layers, depleted upper mantle, more irregular
Possible Forces Affecting Plate Motion
• Gravity sliding from thermal bulge under MOR
• Negative buoyancy for old ocean crust at trenches
Continental Orogenic Belts & Rock Cycle
• Continents are made of
older fragments that
were once at plate
margins
• Cycles of accretion,
rifting
• Uplift, erosion,
sedimentation,
metamorphism, renewed
igneous activity
• Oldest rocks ~4Ga seds!
Rock Cycle
• Rocks and minerals & the lements which comprise
them are recycled
• Mainly at Convergent margins: buoyancy,
thickening, erosion etc.
Wilson Cycle (Supercontinent Formation)
•
•
•
•
•
Plates move towards Subduction Zones
Continental crust is too thick and buoyant to subduct
Continents re-collide and amalgamate every ~500Ma
Pangea broke up at 205 Ma forming Atlantic
Eventually the Pacific will dissappear (300-400 Ma)