Untitled Flashcards Set

Layered basalts record magnetic changes over time Inclination and declination indicate change in position relative to magnetic pole.Palaeomagnetism measured in samples from other continents gave different pole positions for each continent = different polar wander paths.Multiple magnetic poles was not an option, therefore the continents must move relative to the magnetic poles and to each other • Continental drift was a reality (mid-1950’s) • The acceptance of continental drift focused research on the search for a viable mechanism • The answer came from detailed geophysical and geological studies of the ocean floor.Sea Floor Spreading • A theory based on studies of the topography and magnetism of the ocean floor • Ocean-floor exploration started during WW-II techniques and instruments used for research into marine geology during 1950s and 1960s • Data from marine geophysical (sonar) surveys in the North Atlantic used to construct x-sections of the ocean floor.Work by Tharp and Heezen identified a ridge 2-3 km high) running N-S along the centre of the North Atlantic Basin – the North Atlantic Ridge • Marie Tharp noticed that the axis of the ridge was characterised by a steep sided gap or “central rift” identical to those on x-sections of continental rift valleys; was the ocean floor splitting?Heezen then suggested they plot the position of all earthquakes recorded from the Atlantic basin • The earthquakes plotted along the line of the central rift and extended north and south away from their area of study – ridge not local • Next step was to plot earthquakes from all ocean basins this generated a feature continuous with the North Atlantic Ridge that was present in all ocean basins– Mid Ocean Ridge System.The Mid Ocean Ridge System was defined by shallow focus earthquakes; medium and deep focus earthquakes were associated with the Pacific Ocean Basin – define the ocean trenches.The ocean basins are geologically very active ❑All ocean basins have a “Mid-Ocean Ridge” characterised by thin crust, high heat flow, slow seismic velocities: hot, low density material below ❑Isolated volcanic islands like Iceland are located on the ocean ridge system ❑Margins of ocean basins are characterised by normal geophysical signatures indicating cold, rigid crust ❑The Pacific Ocean basin has deep ocean trenches and volcanic island arcs (e.g. Philippines, Japan) ❑The deep ocean trenches are surface expression of an inclined zone of shallow, intermediate and deep earthquakes ❑The foci of these earthquake foci extend down to 720km and define the Wadati-Benioff ZoneSea Floor Spreading Hypothesis • Proposed in 1906 by Prof Harry Hess (Princeton) to explain the link between the mid-ocean ridges and the deep ocean trenches • Mid-ocean ridges were underlain by hot low density rock (magma); Ocean trenches were characterised by cold dense rock (solid) associated with an inclined zone that extended to depths of 720km into the mantle • Hess argued these features complemented each other and interpreted the MORs as the sites where mafic magma erupted from the upper mantle and solidified to form new ocean crust