Learning Plate Tectonic Geography Brushing up on basic geography will help you learn Plate Tectonics Once 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

I. Introduction (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? 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 Processes Results of the “External 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 (cont.) Results of the “External Heat Engine” Transport over large distances by water, wind and ice.

Surficial Processes (cont.) Results of the “External 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 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)

planets/planets.html ppt/chapter4/sld002.htm Orion Nebula

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

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 Increase  Silicon (14)Si Increase  Aluminum (13)Al<18.1 Increase  Iron (26)Fe355.0 Decrease  Calcium (20)Ca<13.6 Increase  Sodium (11)Na<12.8 Increase  Potassium (19)K<12.6 Increase  Magnesium (12)Mg102.1 Decrease  All Others ~81.5

Crust and Mantle Lithosphere 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 Simplified 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 (Mid-Arctic Ridge) East Pacific Rise Mid-Atlantic Ridge Indian Ridge Mid-

Divergent Plate Boundaries Rifting and generation of shallow earthquakes (<33km) Depth (km)

Fig Fig Rift Valley Passive continental shelf and rise Rift Valley E.g., Red Sea and East African Rift Valleys Thinning crust, basalt floods, long lakes Shallow Earthquakes Linear sea, uplifted and faulted margins Oceanic Crust Fig Pg. 40 Fig Pg. 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 Diagram Asthenosphere Subducted Plate Oceanic Trench Plate Movement Magma Generation Volcanic Arc Shallow and Deep Earthquakes Lithosphere Fig Pg. 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 Pg. 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 East Pacific Rise Pinatubo Andes Mountains Cascade Range Aleutian Island Arc Siarra Madre Japanese Isls. New Zealand Phillipines.

Depth of Earthquakes at convergent plate boundaries Seismicity of the Pacific Rim 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 Red Sea Gulf of Aden Etna Visuvius East African Rift Mid-Atlantic Ridge Mid-Indian Ridge Divergent Plate Boundaries Rifting and Formation of new Basiltic Oceanic Crust Oceanic Crust* –Thin (<10 km) –Young (<200my) –Iron Rich (>5%) / Silica Poor (~50%) –Dense (~ 3 g/cm 3 ) –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 Boundaries Formation 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/cm 3 ) –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 H 2 O) –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 Pg. 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 Metamorphic Rocks Sedimentary Rocks Magma Igneous Rocks

The Rock Cycle

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