Learning Plate Tectonic Geography Learning Plate Tectonic Geography

Brushing up on basic geography will help you learn Plate TectonicsOnce you know your basic geography (continents and major mountain ranges) and ocean basin features (Mid Ocean Ridges, Oceanic Trenches) you can

-Learn the 7 major plates-Learn the types of plate boundaries-Learn why those features are where they are

II. Introduction . Introduction (cont.)(cont.)

• E. Forces shaping the Earth at the surface and from within– 1. Surficial Processes

Solar energy and gravity shaping the landscape

– 2. Internal Processes Internal energy and forces that buckle and break Earth’s crust

If External Processes Only?If External Processes Only?

• Question: – If 550 million tons of rock

are broken down and transported to the sea from the United States each year,

– Why has our continent not been worn flat after the billions of years of its existence and

– Why haven’t the oceans been filled in?

Mississippi River Delta

Mississippi River Drainage Basin

Erosion, transport, deposition

Mississippi River Delta

1. Surficial Processes1. Surficial ProcessesResults of the “Results of the “External Heat EngineExternal Heat Engine””

• Weathering– Chemical and

Mechanical Breakdown of solid rock into sediments

• Erosion– Removal of rock and

sediment from source by Gravity, wind, water, ice

Surficial Processes Surficial Processes (cont.)(cont.)Results of the “Results of the “External Heat EngineExternal Heat Engine””

• Transport over large distances by water, wind and ice.

Surficial Processes Surficial Processes (cont.)(cont.)Results of the “Results of the “External Heat EngineExternal Heat Engine””

• Deposition of large amounts of sediment in seas and oceans

2. Internal Earth Processes Results of the “Internal Heat Engine”

• Evidence of internal energy and forces working on our earth.– Intricate landscapes– Volcanoes– Earthquakes– Geothermal

Gradients (deeper is hotter)

Another Question: What is the source of all this energy?

3. Formation of Earth

http://www.psi.edu/projects/planets/planets.html

Birth of the Solar System

Nebular Theory – nebula compresses– Flattening of spinning nebula

and collapse into center to form sun

– Condensation to form planets, planetesimal, moons and asteroids during planetary accretion around 4½ billion years ago

– (Meteorites are iron-rich and rocky fragments left over from planetary accretion)

www.psi.edu/projects/planets/planets.html

www.geol.umd.edu/~kaufman/ppt/chapter4/sld002.htm

Orion Nebulawww.hubblesite.org

Formation of the Planets• The mass of the center

of the solar system began nuclear fusion to ignite the sun

• The inner planets were hotter and gas was driven away leaving the terrestrial planets

• The outer planets were cooler and more massive so they collected and retained the gasses hence the “Gas Giants”

Gas Giants

Terrestrial Planets

www.amnh.org/rose/backgrounds.html

Differentiation of the Planets• The relatively uniform iron-

rich proto planets began to separate into zones of different composition: 4.6mya

• Heat from impacts, pressure and radioactive elements cause iron (and other heavier elements) to melt and sink to the center of the terrestrial planets

The zones of the earth’s interior

Further Differentiation of Earth

• Lighter elements such as Oxygen, Silicon, and Aluminum rose to form a crust

• The crust, which was originally thin and heavy (iron rich silicate) Like today’s Oceanic crust,

• Further differentiated to form continental crust which was thicker, iron poor and lighter Figure 1.7, the zones of the earth’s interior

Composition of Earth and Crust 

Element(Atomic #)

 Chemical Symbol

 % of Earth

% of Crust(by Weight)

Change in Crust Due to Differentiation

Oxygen (8) O 30 46.6 Increase

Silicon (14) Si 15 27.7 Increase

Aluminum (13) Al <1 8.1 Increase

Iron (26) Fe 35 5.0 Decrease

Calcium (20) Ca <1 3.6 Increase

Sodium (11) Na <1 2.8 Increase

Potassium (19) K <1 2.6 Increase

Magnesium (12) Mg 10 2.1 Decrease

All Others   ~8 1.5  

Crust and MantleLithosphere and Asthenosphere

• The uppermost mantle and crust are rigid solid rock (Lithosphere)

• The rest of the mantle is soft but solid (Asthenosphere)

• The Continental Crust “floats” on the uppermost mantle

• The denser, thinner Oceanic Crust comprises the ocean basins Figure 1.7, Detail of crust and Mantle

• The Lithosphere is broken into “plates” (7 major, 6 or 7 minor, many tiny)

• Plates that “ride around” on the flowing Asthenosphere

• Carrying the continents and causing continental drift

Litho-spheric Plates

Plates Shown by Physiography

Types of Plate Boundaries

• -Convergent -Divergent -Transform

Lithospheric Plates and Boundary types

Three Types of Plate

Boundaries

• Divergent |

• Convergent |

• Transform

e.g., Pacific NW

• Where plates move away from each other the iron-rich, silica-poor mantle partially melts and

Divergent Plate Boundaries

Asthenosphere

Lithosphere LithosphereSimplified Block Diagram

• Extrudes on to the ocean floor or continental crust

• Cool and solidify to form Basalt: Iron-Rich, Silica-Poor, Dense Dark,

Fine-grained, Igneous Rock

Characteristics of Divergent Plate Boundaries

Oceanic Crust

Magma Generation

• Divergent Plate Boundary– Stress: Tensional extensional strain – Volcanism: non-explosive, fissure eruptions,

basalt floods– Earthquakes: Shallow, weak– Rocks: Basalt– Features: Ridge, rift, fissures

Locations of Divergent Plate Boundaries

Mid-Ocean Ridges

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

Fig. 1.10

(Mid-Arctic Ridge)

Eas

t P

acifi

c R

ise

Mid

-Atla

ntic

Rid

ge

Indian

Ridge

Mid-

030

70

150

300

500

Divergent Plate BoundariesRifting and generation of shallow earthquakes (<33km)

Depth(km)

03370

300

150

500

800

• 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, basalt floods, long lakes

ShallowEarthquakes

Linear sea, uplifted and faulted margins

Oceanic Crust

Fig. 2-15Pg. 40

Fig. 2-16Pg. 41

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)

• Wet crust is partially melted to form silicic (Silica-rich, iron-poor, i.e., granitic) magma– Stress: Compression– Earthquakes– Volcanism– Rocks– Features

Lithosphere

Simplified Block DiagramAsthenosphere

Subducted Plate

Oceanic Trench

Plate Movement

Magma Generation

Volcanic Arc

Shallow and Deep Earthquakes

Lithosphere

Fig. 2-17Pg. 42

Convergent Plate Boundary e.g., Pacific Northwest

• Volcanic Activity– Explosive, Composite

Volcanoes (e.g., Mt. St. Helens)

– Arc-shaped mountain ranges

• Strong Earthquakes– Shallow near trench– Shallow and Deep over

subduction zone• Rocks Formed

– Granite (or Silicic) • Iron-poor, Silica-rich • Less dense, light colored

– Usually intrusive: Cooled slowly, deep down, to form large crystals and course grained rock

Fig. 2-18Pg. 42

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

The “Ring of Fire” (e.g., current volcanic activity)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.

• Also: Himalayans to the Alps

Indonesia

Fujiyama

Eas

t P

acif

ic R

ise

Pinatubo

An

des

Mo

un

tain

s

Cas

cade

Ran

ge

Aleutian

Island Arc

Siarra Madre

Jap

anes

e Is

ls.

New Z

ealand

Ph

illip

ines

.

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)

• Each major plate caries a continent except the Pacific Plate.• Each ocean has a mid-ocean ridge including the Arctic Ocean.

– Divergent bounds beneath E. Africa, gulf of California• The Pacific Ocean is surrounded by convergent boundaries.

– Also Himalayans to the Apls

Major Plates and Boundaries

Iceland

Kilimanjaro

RedS

ea

Gulf of

Aden

Etna

Visuvius

Eas

t A

fric

an

Rif

t

Mid

-Atl

anti

c R

idg

e

Mid

-Ind

ian R

idg

e

Divergent Plate BoundariesRifting and Formation of new Basiltic Oceanic Crust

Oceanic Crust*– Thin (<10 km)

– Young (<200my)

– Iron Rich (>5%) / Silica Poor (~50%)

– Dense (~ 3 g/cm3)

– Low lying (5-11 km deep)

– Formed at Divergent Plate Boundaries

Composite Volcanic Arcs (explosive)

Basaltic Volcanism (non-explosive)

*Make a “Comparison Table” on a separate page

Convergent Plate BoundariesFormation of Granitic Continental Crust

Continental Crust– Thick (10-50 km)

– Old (>200 m.y. and up to 3.5 b.y.)

– Iron Poor (<1%) / Silica Rich (>70%)

– Less Dense (~ 2.5 g/cm3)

– High Rising (mostly above see level)– Formed at Convergent Plate

Boundaries

Oceanic Crust– Thin (<10 km)

– Young (<200 my)

– Iron Rich (~5%) /

Silica Poor (~50%)

– Dense (s.g. ~3 x H2O)

– Low lying (5-11 km deep)

– Formed at Divergent Plate Boundaries

Isostatic Adjustment

• Why do we see, at the earths surface,

– Intrusive igneous rocks and– Metamorphic rocks – Formed many km deep?

• Thick, light continental crust buoys up even while it erodes

• Eventually, deep rocks are exposed at the earth’s surface

• Minerals not in equilibrium weathered (transformed) to clay

• Sediments are formed

Transform Plate Boundaries• Offset Mid-

ocean ridges

• May cut continents – e.g. San

Andreas Fault

Fig. 2-21Pg. 44

The Hydrologic Cycle Works with

Plate-Tectonics to

• Shape the land– Weathering

clay, silt, sand…

– Erosion– Transport– Sedimentation

• Geologic Materials– Sediments– Sedimentary

Rocks

The 3 rock types form at convergent plate boundaries

• Igneous Rocks: When rocks melt, Magma is formed, rises, cools and crystallizes.

• Sedimentary Rocks: All rocks weather and erode to form sediments (e.g., gravel, sand, silt, and clay). When these sediments accumulate they are compressed and cemented (lithified)

• Metamorphic Rocks: When rocks are compressed and heated but not melted their minerals re-equilibrate (metamorphose) to minerals stable at higher temperatures and pressures

MetamorphicRocks

Sedi

men

tary

Roc

ks

Magma

IgneousRocks

The RockCycle

Igneous and Sedimentary Rocks at Divergent Boundaries and

Passive Margins• Igneous Rocks (basalt)

are formed at divergent plate boundaries and Mantle Hot Spots. New basaltic, oceanic crust is generated at divergent plate boundaries.

• Sedimentary Rocks are formed along active and passive continental margins from sediments shed from continents

• Sedimentary Rocks are formed on continents where a basin forms and sediments accumulate to great thicknesses. E.g., adjacent to mountain ranges and within rift valleys.

See Kehew, Figure 2.30