Download Divergent Plate Boundaries

Rocks, Sediment and Soils
Products of an Active Planet
Earth’s structure leads to
intense geologic activity
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Inner core: Solid iron
Outer core: Liquid iron,
convecting (magnetic field)
Mantle (Asthenosphere) :
Solid iron-magnesium
silicate, plastic, convecting
Crust (Lithosphere): Rigid, thin
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Crust:
Rigid,
Thin
Mantle:
Plastic,
Convecting
O, Si, Al, Fe, Ca, Na, K, Mg…
47%, 28, 8,
5,
4,
3,
3, 2
TDE pg.8-11
The Major Lithospheric Plates
And directions of movement
(TDE pg.5)
Geologic Phenomena at Plate
Boundaries
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Divergent Plate Boundaries: where plates
move away from each other new oceanic crust
is generated from melting mantle material that
cools and forms Basaltic Rock (a.k.a. Mafic rock)
 Iron-rich
 Silicon-poor
Lithosphere
 Dense
Lithosphere
Simplified
 Young
Block
Asthenosphere
TDE pg.15-20, 32-36
Diagram
Geologic Phenomena at
Divergent Plate Boundaries
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Divergent Plate Boundary
New Oceanic Crust
Forming at Mid-Ocean Ridge
Shallow
Earthquakes
Fissure
Eruptions
Crust
Lithosphere
Asthenosphere
Magma
Generation
Locations of Divergent Plate Boundaries
Mid-Ocean Ridges
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East Pacific Rise
Mid Atlantic Ridge
Mid Indian Ridge
Mid Arctic Ridge
(Mid-Arctic Ridge)
Divergent Plate Boundaries and
Hotspots Beneath Continents
Flood Basalts
on Uplift
 Shallow
earthquake activity
 Volcanic activity
Fissures eruptions
 Lava floods, and volcanoes
 Hot, non-explosive
 Basaltic rocks formed (i.e.
iron-rich/silica poor)
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 The
Rocks (igneous)
Basalt bedrock is usually formed in the shallow crust by
recent (<~200mya) volcanic activity at divergent plate
boundaries or hotspots (>1,000oC)
 E.g., volcanoes, feeder dikes, volcanic stocks, basalt floods
Hawaii, Ship Rock NM, Devils Tower AZ, Yellowstone WY
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E.g., Red Sea and
East African Rift Valleys
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Thinning crust, flood
basalts, long lakes
Rift
Valley
Shallow
Earthquakes
Linear sea, uplifted
and faulted margins
Oceanic Crust
Rift
Valley
Passive continental
shelf and rise
Fig. 19.21
Fig. 19.22
Formation of Mafic
Igneous Rocks
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Mafic Magmas
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Hot (>1000oC)
Non-Viscous (runny, flows easily)
“Dry” (no H2O or C02)
Mafic Rocks
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Usually Extrusive, Fine-grained,
Mafic (Basalt) rock forms oceanic
crust, Shield Volcanoes and Flood
Basalts
If Intrusive, course-grained mafic
rocks are formed Gabbro.
If intrusive, Dikes and Sills more
common. (Plutons don’t form)
Sill
Dike
Dike
Dike
Geologic Phenomena at
Plate Boundaries
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Convergent Plate Boundaries: where plates
move toward each other, oceanic crust and the
underlying lithosphere is subducted beneath
the other plate (with either oceanic crust or
continental crust)
Oceanic Trench
Lithosphere
Lithosphere
Subducted
Plate
Asthenosphere
Simplified
Block
Diagram
TDE 37-43
Formation of Magma
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How are rocks melted?
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1. Heating ■ 2. Depressurization
3. Increase water content
4. Increased silica content
Where do rocks melt?
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Subduction zones (Silicic
and Intermediate)
Mantle Plumes (“Hot Spots”)
not only at
Divergent
Boundaries
Hot and
Low Pressure
Mafic
Hot and
High Pressure
Formation of Silicic
Igneous Rocks
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Silicic (a.k.a, felsic) Magmas
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Cool (<700oC)
Viscous (sticky, doesn’t flow easily)
Gaseous (steam of H2O and C02)
Composite Volcano
Silicic Rocks
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Usually intrusive, course-grained,
Silicic (Granite) to Intermediate
(Diorite) rock forms plutons
If extrusive, fine-grained rocks
formed by explosive volcanoes
Rhyolite or Andesite Volcanoes
Also injects surrounding rocks
with silica laden steam
Batholith of Plutons
Dikes: Intruded near a pluton
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Silica rich fluids are
injected into cracks
in all directions
Discordant: cutting
across layers
Ores, rare elements
and minerals, gems
Geologic Phenomena at
Convergent Plate Boundaries
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Volcanic Activity
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Explosive, Composite
Volcanoes
Granitic rocks formed
(iron-poor/silica-rich)
Shallow earthquakes
near trench
Shallow and Deep
Earthquakes over
subduction zone
Physiographic Features at
Convergent Plate Boundaries
Island Arcs
(chains of volcanic Islands),
Chains of Volcanoes
Oceanic Trenches
The “Ring of Fire”
A ring of convergent plate boundaries on the Pacific Rim
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New Zealand
Tonga/Samoa
Philippines
Japanese Isls.
Aleutian Island
arc and Trench
Cascade Range
Sierra Madre
Andes Mtns.
Fujiyama
Pinatubo
Composite Volcanic Arcs (Granitic, Explosive)
Basaltic Volcanism (Non-Explosive)
Depth of Earthquakes
at convergent plate boundaries
Seismicity of the Pacific Rim 1975-1995
0
33
70
150
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Shallow quakes at
the oceanic trench
(<33km)
Deep quakes over
the subduction zone
(>70 km)
300
500
800
Depth
(km)
The 3 rock types form at
convergent plate boundaries
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Sedimentary Rock:
Sediments (e.g., ocean
sand, silt, and clay) are
compressed cemented
(lithified)
Metamorphic Rocks:
rocks are compressed,
heated and change
minerals but do not melt
Igneous Rocks: When
rocks melt, Magma is
formed, rises, cools and
crystallizes. Below surface Intrusive. LavaExtrusive
Transform Plate Boundaries
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Where plates slide parallel to each other:
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Transform faults are created
Mid-ocean ridges are offset
If occurring beneath a continent the
continent is sheared and faulted
Transform Plate Boundaries
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Examples
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Every mid-ocean
ridge is offset along
by transform faults
San Andreas Fault
Shallow
earthquakes
are generated
Volcanic activity is
rare
See Figure 19.27
http://pubs.usgs.gov/publications
/text/San_Andreas.html