Rocks, Sediment and Soils Products of an Active Planet

1
Rocks, Sediment and SoilsProducts of an Active
Planet
Crust Rigid, Thin

• Earths structure leads to intense geologic
  activity
• Inner core Solid iron
• Outer core Liquid iron, convecting (magnetic
  field)
• Mantle (Asthenosphere) Solid iron-magnesium
  silicate, plastic, convecting
• Crust (Lithosphere) Rigid, thin
• O, Si, Al, Fe, Ca, Na, K, Mg

Mantle Plastic, Convecting
47, 28, 8, 5, 4, 3, 3, 2
TDE pg.8-11
2
The Major Lithospheric Plates And directions of
movement (TDE pg.5)
3
Geologic Phenomena at Plate Boundaries

• 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
• Dense
• Young
• TDE pg.15-20, 32-36

Lithosphere
Lithosphere
Simplified Block Diagram
Asthenosphere
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Geologic Phenomena at Divergent Plate Boundaries

• Divergent Plate Boundary
• Lithosphere
• Asthenosphere
  

New Oceanic Crust Forming at Mid-Ocean Ridge
Fissure Eruptions
Shallow Earthquakes
Oceanic
Crust
Magma Generation
Lithospheric Plate Movement
Welling up of hot mantle rock (solid but soft)
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Locations of Divergent Plate BoundariesMid-Ocean
Ridges
(Mid-Arctic Ridge)

• East Pacific Rise
• Mid Atlantic Ridge
• Mid Indian Ridge
• Mid Arctic Ridge

Mid-
Mid-Atlantic Ridge
East Pacific Rise
Indian
Ridge
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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)

• The Rocks (igneous)
• Basalt bedrock is usually formed in the shallow
  crust by recent (lt200mya) volcanic activity at
  divergent plate boundaries or hotspots (gt1,000oC)
• 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

• Fig. 19.21
• Fig. 19.22

Thinning crust, flood basalts, long
lakes Shallow Earthquakes Linear sea, uplifted
and faulted margins
Rift Valley
Rift Valley
Oceanic Crust
Passive continental shelf and rise
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Formation of Mafic Igneous Rocks

• Mafic Magmas
• Hot (gt1000oC)
• Non-Viscous (runny, flows easily)
• Dry (no H2O or C02)
• Mafic Rocks
• 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 dont form)

Sill
Sill
Sill
Dike
Dike
Dike
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Geologic Phenomena at Plate Boundaries

• 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
Simplified Block Diagram TDE
37-43
Lithosphere
Lithosphere
Subducted Plate
Asthenosphere
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Formation of Magma

• How are rocks melted?
• 1. Heating 2. Depressurization
• 3. Increase water content
• 4. Increased silica content
• Where do rocks melt?
• Subduction zones (Silicic
• and Intermediate)
• Mantle Plumes (Hot Spots)
• not only at
• Divergent
• Boundaries
• Mafic

Hot and Low Pressure
Hot and High Pressure
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Formation of Silicic Igneous Rocks

• Silicic (a.k.a, felsic) Magmas
• Cool (lt700oC)
• Viscous (sticky, doesnt flow easily)
• Gaseous (steam of H2O and C02)
• Silicic Rocks
• 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

Composite Volcano
Batholith of Plutons
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Dikes Intruded near a pluton

• Silica rich fluids are injected into cracks in
  all directions
• Discordant cutting across layers
• Ores, rare elements and minerals, gems

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Geologic Phenomena at Convergent Plate Boundaries

• Volcanic Activity
• Explosive, Composite Volcanoes
• Granitic rocks formed (iron-poor/silica-rich)
• Shallow earthquakes near trench
• Shallow and Deep Earthquakes over subduction zone

14
Physiographic Features at Convergent Plate
Boundaries

• Island Arcs
  Chains of Volcanoes
• (chains of volcanic Islands), Oceanic
  Trenches

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The Ring of Fire A ring of convergent plate
boundaries on the Pacific Rim

• New Zealand
• Tonga/Samoa
• Philippines
• Japanese Isls.
• Aleutian Island arc and Trench
• Cascade Range
• Sierra Madre
• Andes Mtns.

Composite Volcanic Arcs (Granitic,
Explosive) Basaltic Volcanism (Non-Explosive)
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Depth of Earthquakes at convergent plate
boundaries
Seismicity of the Pacific Rim 1975-1995
0 33 70

• Shallow quakes at the oceanic trench (lt33km)
• Deep quakes over the subduction zone (gt70 km)

150
300
500
800
Depth (km)
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The 3 rock types form at convergent plate
boundaries

• 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.

Sedimentary Rocks
Metamorphic Rocks
Igneous Rocks
Magma

• Below surface? Intrusive. Lava?Extrusive

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Transform Plate Boundaries

• Where plates slide parallel to each other
• Transform faults are created
• Mid-ocean ridges are offset
• If occurring beneath a continent the continent is
  sheared and faulted

19
Transform Plate Boundaries

• Examples
• 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