Lecture 3

Emergence of ocean basins

we used to be one big continent = continetal dift first established in 1858, antonio snider-pellegrini, noted similiar foosils also 1831-1914 edward suess seen fern fossils present acorss all southern continents

supercontinent - gondwana and tethys ocean

land briges between continents.

edward - tries to explain oceans via cooling and sinking of materials with diff densities

Alfred wgeener 1880-1930

matches continents at 200m isobath - proposes continental drift

theory largely reject - lack mechanism

evidence - plant fossils

-animal fossils

-geological formations such as coal bed

- Paleoclimatological evidence from glacial deposits - s gondwana

emergence of plate tectonics - continental drift evidence

-formation of mountain range via compression & folding

-Collision between plate throws up mountian ranges

- Rift valleys and transform fualts consistent with tension from sea floor spreading

crustal motion

Four key observations

Geographical location of earth quakes - epicentres show plate boundaries

Geographical variability of crustal heat flow - decreases away from spreading centre

Geographical variability of thickness of deep-sea sediment - thickness increases away from spreading centre

Magnetic anomalies- orientation reverses - shows regular patterns away from spreading centre

Seismic Earthquakes are

concentrated

•Oceanic ridges

•Transform faults

•Trenches

•Island Arcs

Earthquake

magnitude

•Highest

at trenches- compression

•Lowest

at ridges- spreading

Geomagnetism

the earths magnetic field reverses approx. every 0.1 to 1 Ma

this is seen in basaltic rocks - solidified magma from volcanic eruptions - they are ricj in iron = ferromagnetic material - retains orientation od magnetic field upon cooliing - terrestrial rcords also document the reverse

Vine-matthews-morley Hypothesis

Independently developed in the 60- hypothesis

-new ocean crust formed at ocean ridgea

-magnetic orientation preserved

- magnetometer records should reveal magnetic anomalies - magnetic orientation should show striped pattern parallel to ridges

volcanic chains

oceanic crust moves over hot spot = hawian chain - youngest is the most active - oldest is subside. - subsudence and erosion eventually turn islands into guyots - emperor seamount chain

pacific ring of fire

Earths most active volcanoes driven by friction associated with convergence/subduction

these are common ocean-ocean convergence