Summary Divergent Boundaries – Mid- Ocean Ridges (MOR) Separates 2 tectonic plates Tectonic plates move apart – extensional forces New oceanic crust generated - ~ 5 cm/yr Earthquakes – shallow, low energy

MOR - Ophiolite Complex

Pillow Basalts in the Ocean

Pillow Basalts on the Continents

Sheeted Dikes

Gabbro

Convergent Boundaries Plates come together Ocean-Ocean Convergence Ocean-Continent Convergence Continent-Continent Convergence

Subduction Zones Ocean-ocean collision or ocean-continent collision Density differences in crust forces one plate to descend under the other plate More dense plate descends Descends at an angle between 15 o to 75 o with an average of  45 o Subduction of plate creates a deep-ocean trench

Deep Ocean Trenches Range in depth between 8 – 12 km 100’s km wide and 1000’s km long

Additional Terminology Accretionary Wedge – sediment that has accumulated as sediment has been “scraped” of off the subducting plate Forearc – region lying between the trench and the magmatic arc Backarc – region lying behind the magmatic arc

Ocean-Continent Collision Ocean crust is more dense (3.0 g/cm3) than continental crust (2.8 g/cm3) Ocean Crust subducts Within the mantle, plate begins to melt Melt has basaltic to intermediate composition Hot melt (magma) is less dense (more buoyant) than surrounding material and rises Ascending magma incorporates continental crust – composition becomes intermediate to felsic Creates linear zone of igneous activity (volcanoes or intrusions-plutons) called a Continental (Magmatic) Arc

Ocean-Continent Example – Cascade Subduction Zone

Ocean-Continent Example – Peru-Chile Subduction Zone

Ocean-Ocean Convergent Zone

Ocean-Ocean Collision Similar to Ocean-Continent Convergent Zone except no influence of Continental Crust Magmas are basaltic to intermediate in composition Ascending magma creates linear Volcanic Island Arc

Continent-Continent Convergent Zone (Collision) Is preceded by Ocean-Continent Convergence Density between plates is similar – neither will subduct Crust buckles, fractures, shortens, thickens due to compressional stress Creates large mountains of folded and faulted rocks.

Migration of India & Collision Into Eurasia

Formation of the Himalayan Mts.

Mt. Everest

Ocean Evolution

Earthquakes & Convergent Boundaries Earthquake depth ranges from shallow ( 300 km) High magnitude (high energy) – 95% of all earthquake energy is released from convergent zone earthquakes Wide zone, but with a pattern

Transform Boundary

Transform Faults Plates slide past one another Crust is neither produced nor destroyed Facilitate plate movement Connect segments of MOR Transition zone between MOR and Subduction zone Transition/Connection between two Subduction zones

Transform Faults to Facilitate MOR Movement

Transform Faults Found in both ocean crust and continental crust Can create compressional structures (folds) and extensional structures (basins)

San Andreas Fault

Earthquakes are shallow, less than 10 km in depth Small magnitude (low energy) Common Narrow pattern along MOR Earthquakes and Transform Boundaries

Boundary Locations

Similarities-Differences of Boundaries Transform DivergentConvergent 1. Shallow Earthquakes 2. Intermediate Earthquakes 3. Deep Earthquakes 5. Crust generated 6. Crust destroyed 7. Compression Forces 8. Extension Forces 9. Volcanoes 10. Mountains

Hot Spots

Produced by rising plume of mantle material (mantle plume) They are stationary – plates move over the hotspots

Basalts produced by hot spots are chemically different from basalts produced at MOR

Hot Spot Example – Island Chains

Island Chain Formation

Hot Spot Under Hawaii Notice linear layout of islands Could determine plate movement rate for data Island continually being formed

Hot Spot Example – Continental Yellowstone

Continental Hot Spot Mechanics

Volcanism and Continental Hot Spots

Plate Tectonics – Driving Mechanisms Driven by the release of internal energy in the form of heat Proposed mechanisms –Convection cells –Differences in density and forces generated by plates

Forces Active on Plates Slab-pull – sinking of dense ocean crust pulls the plate Ridge-push – gravity pulls plate of of the MOR Basal drag – resistance of plate from underlying asthenosphere Mantel resistance – subducting plate is resisted in the asthenosphere and mesosphere Friction – resistance along transform faults and between converging plates