Prof. J. Renton
Outline lecture notes** (JA)
- Recap on important words/terms
- Earth has inner iron core, peridotite mantle, brittle, 40mi thick outer crust
- Lighter things always float on denser ones
- Basaltic oceanic crust, granitic continental crust
- Plastic asthenosphere 100-150mi down can flow, above it is lithosphere (crust + upper mantle)
- Convection cells in asthenosphere have alternating rising/falling streams, creating tension/compression in crust
- Tensional forces break brittle lithosphere into plates. Under continental crust they create cracks up to surface -> rift zone
- Rising asthenosphere liquifies to low-viscosity granitic magma, rises up thru cracks, meets groundwater -> hot springs, fumaroles. Eventually magma surfaces -> small lava flows, fire fountains. Descending lava cools -> cinders, which collect -> cinder cone.
- Rift zones in Albuquerque (aborted) and East African Rift (central part collapsed -> rift valley)
- When one end of rift reaches ocean, seawater spills in (Red Sea) -> linear ocean
- When other end breaks, forms an island and starts new ocean (with oceanic ridge)
- Pangea broke up this way. Alfred Wegener matched up rock types, fossils, glacial deposits/striations between S America and Africa.
- If oceans growing, crust must be consumed. At deep sea trenches, compressional forces force denser oceanic lithosphere down under lighter continental lithosphere -> zones of subduction.
- Oceanic lithosphere melts -> viscous granitic and runny andesetic lavas.
- Andesitic lavas rise to surface -> arc volcanoes, continental arc if trench near coast, island arc if further offshore
- Granitic magma rarely reaches surface, big blobs attach to edge of continent, adding granitic rocks to continent.
- Always (denser) oceanic lithosphere subducts below (lighter) continental, never other way round.
- Opening oceans create new ocean bottom, so oldest oceanic crust ~200m years old and nearest to continents.
- Pacific Ocean closing on 3 sides, eaten up faster than new ocean floor created.
- Crust formed 3.5-4bn years ago, some continental rocks dated to 4.1bn years old. They don't subduct so are never recycled, plus extra added by rising granitic magma.
- J. Tuzo Wilson proposed 500m year cycle of supercontinent formation and breakup. Continents drift apart for 250m years and then come together to form new, different supercontinent.
- Assuming plate tectonics started 3bn years ago, now in 6th cycle. Atlantic Ocean still opening, Indian as big as it will get, Arctic "resting", Pacific closing.
- Americas will only travel another 8-900mi West, not even as far as Hawaiian Islands
- Called the Wilson Cycle
|Plate Tectonics Info||description [Wikipedia]|
|Plate Tectonics Revision notes||first 3 pages [BBC BiteSize]|
|GeolSoc Plate Tectonics||description [British Geological Society]|
|Wilson Cycle||description [Wikipedia]|
|Geology Page Wilson Cycle||description [Geology Page]|
** more detailed lecture notes available to group members on request.