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Rifting to Ocean Basin Evolution – Key Points

Embryonic Rift Basins

  • Continental extension starts when mantle plumes heat and dome the continental crust.

  • Doming splits into linear arms ("triple junction").

  • Normal faulting drops crustal blocks ⇒ rift valleys; basaltic (lava) eruptions common.

  • Key modern examples are the East African Rift, Midland Valley (Scotland), and Basin & Range (USA).

East African Rift

  • NE–SW fault‐bounded valleys with volcanic centre & uplifted flanks.

  • Ol Doinyo Lengai: only active carbonate‐lava volcano on Earth; sits on the flank of a larger basaltic edifice.

  • The region close to Olduvai Gorge (the earliest hominid fossils).

Afar Triangle & Shield Volcanoes

  • Triple-junction between the African, Somali, and Arabian plates.

  • Red Sea & Gulf of Aden mid-ocean ridges push the Arabian Plate north.

  • Crust extremely thin ⇒ voluminous basaltic outpourings build broad shield volcanoes (e.g., Erte Ale).

  • Erte Ale hosts rare persistent lava lakes (active since 1906).

Juvenile Ocean Basins

  • Continued rifting generates oceanic basalt in the basin centre → continents drift apart.

  • Basalt (denser) sits lower; initial rift-flank highs cool & subside.

  • Periodic marine incursions create narrow seaways (e.g., the Red Sea, the Gulf of Aden, and the Gulf of California).

Red Sea Highlights

  • Elongate, semi-isolated basin; cliffs shed sediment that forms coastal shelves.

  • Restricted circulation ⇒ intense evaporation ⇒ thick salt layers.

  • Gravity data reveal dense basement topography beneath the younger sedimentary drape.

Mature Ocean Basins & Sea-Floor Spreading

  • Ongoing spreading at mid-ocean ridges adds new oceanic crust, widening the basin (e.g., the Atlantic).

  • Basalt erupts along linear fissures; magnetite grains lock ambient magnetic polarity.

  • Alternating normal/reversed polarities create symmetrical magnetic "stripes" about the ridge axis.

Atlantic Ocean & Iceland

  • North–South Mid-Atlantic Ridge mirrors continental margins; transform faults accommodate offsets.

  • Iceland = exposed ridge segment above mantle plume; faults visible at Thingvellir allow walking inside the ridge.

Hydrothermal Circulation

  • Seawater percolates through ridge faults, heats, and vents back, forming nutrient-rich black smokers.

  • Supports extremophile ecosystems; linear ridge topography imaged (e.g., Explorer Ridge, BC).

Age of the Ocean Floor & Plate Dynamics

  • New crust forms at ridges, moves away, cools, densifies, and is eventually subducted.

  • Fast vs slow-spreading ridges control crustal age gradients.

  • Oceanic crust ≤ 210 Ma; continental interiors > 3 Ga, younger toward margins via accretion.