Circulation in the Solid Earth& Plate Tectonics

Geos 110 Lectures: Earth System ScienceChapter 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)