Rocks, Sediment and SoilsProducts of an Active PlanetEarth’s 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…

Crust: Rigid, Thin

Mantle: Plastic, Convecting

47%, 28, 8, 5, 4, 3, 3, 2 TDE pg.8-11

The Major Lithospheric Plates And directions of movement (TDE pg.5)

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

Geologic Phenomena at Plate Boundaries

Asthenosphere

Lithosphere LithosphereSimplified Block Diagram

New Oceanic CrustForming at Mid-Ocean Ridge

Oceanic Crust

Lithospheric Plate MovementMagma

Generation

Geologic Phenomena at Divergent Plate Boundaries

Divergent Plate Boundary

Lithosphere

Asthenosphere Welling up of hot mantle rock (solid but soft)

Fissure Eruptions

Shallow Earthquakes

Locations of Divergent Plate BoundariesMid-Ocean Ridges East Pacific Rise Mid Atlantic Ridge Mid Indian Ridge Mid Arctic Ridge

(Mid-Arctic Ridge)

Eas

t Pac

ific

Ris

e

Mid

-Atla

ntic

Rid

ge

Indian

Ridge

Mid-

Divergent Plate Boundaries and Hotspots Beneath Continents

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

Shallow earthquake activityVolcanic activity

Fissures eruptions Lava floods, and volcanoes Hot, non-explosive Basaltic rocks formed (i.e.

iron-rich/silica poor)

Flood Basalts on Uplift

Fig. 19.21 Fig. 19.22

Rift Valley

Passive continental shelf and rise

Rift Valley

E.g., Red Sea and East African Rift Valleys

Thinning crust, flood basalts, long lakes

ShallowEarthquakes

Linear sea, uplifted and faulted margins

Oceanic Crust

Mafic Magmas Hot (>1000oC) 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 don’t form)

Formation of Mafic Igneous Rocks

DikeDike

Dike

Sill

Sill Sill

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)

LithosphereLithosphere Simplified Block Diagram TDE 37-43Asthenosphere

Subducted Plate

Oceanic Trench

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

MaficHot andHigh Pressure

Hot andLow Pressure

Formation of Silicic Igneous Rocks

Batholith of Plutons

Composite Volcano

Silicic (a.k.a, felsic) Magmas Cool (<700oC) Viscous (sticky, doesn’t 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 volcanoesRhyolite or Andesite Volcanoes

Also injects surrounding rocks with silica laden steam

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

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

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

Physiographic Features at Convergent Plate Boundaries

Composite Volcanic Arcs (Granitic, Explosive)Basaltic Volcanism (Non-Explosive)

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.

Indonesia

Fujiyama

Eas

t Pac

ific

Rise

Pinatubo

Ande

s M

ount

ains

Casc

ade

Rang

e

Aleutian

Island Arc

Siarra Madre

Jap

anes

e Is

ls.

New Zeala

nd

Phill

ipin

es.

Depth of Earthquakes at convergent plate boundaries

Seismicity of the Pacific Rim 1975-1995 03370

300

150

500

800

Shallow quakes at the oceanic trench (<33km)

Deep quakes over the subduction zone (>70 km)

Depth(km)

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. Below surface Intrusive. LavaExtrusive

MetamorphicRocks

Sedi

men

tary

Roc

ks

Magma

IgneousRocks

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

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