from PlanetEarth Website
The whole ocean floor is made up of volcanic oceanic crust - isn't it?
Well, probably not. Roger Searle
The Earth's central core is surrounded
by a mantle made of
peridotite rock, with a thin
surface crust. The 6km thick oceanic crust covers 60 per cent of the
Earth's solid surface.
As the plates separate they are stretched and faulted, producing the remaining 10-20 per cent of spreading.
But towards the end of the twentieth century, more and more places were discovered where it seemed the 'crust' was missing and the mantle itself was exposed on the sea floor. Many of these places contain structures called 'Oceanic Core Complexes' (OCCs).
Intensive worldwide study has shown that
OCCs consist of broad domes, 10-20km across and 1-2km high. Cores
and dredges show they mostly contain peridotite (mantle) or
gabbro (crust), but the actual distribution of these rock
types was unclear.
This means OCCs are effectively giant, long-lived faults (called 'detachments').
We assembled an international team of geologists and geophysicists, and were privileged to take the first scientific cruise of NERC's new research ship RRS James Cook.
We sailed from Tenerife in March 2007,
four years after first forming our plans.
More and more places were discovered
where it seemed the 'crust' was missing
and the mantle itself was exposed on the sea floor.
During the seven days it took us to steam 3400 km towards the Mid-Atlantic Ridge we reviewed previous work.
We refined the focus of our study to a region about 1800km east of Barbados. Here, Debbie Smith of Woods Hole Oceanographic Institution had recently mapped many OCCs.
This region offered the promise of extensive peridotite with a chance to investigate, for the first time, active core complexes.
A multibeam echo-sounder uses a fan of over 100 narrow beams to map a swathe of depths several kilometers wide.
Data from many tracks are combined in a grid to make a digital terrain model.
This can be displayed in a variety of
ways, including oblique 3-D views with shading to suggest
illumination. The shapes revealed are powerful indicators of
geological structures like volcanoes and fault scarps.
A powerful way of viewing these data is to 'drape' the sidescan image over a 3-D view of the topography, and use special software to 'fly' around the resulting image.
above the axis of the
The sidescan image shown above is viewed as though from several kilometers above the axis of the Mid-Atlantic Ridge, looking away from the plate boundary.
It shows an actively forming OCC, with a
smooth dome whose striated surface is emerging from older seafloor
at its foot. We interpret the smooth dome as the surface of an
actively slipping detachment fault. The fine, bright, curved line at
the foot of the dome is thus the current tectonic plate boundary.
This nicely confirms the numerical models that predict a reduction in melt supply at OCCs.
Peridotite reacts easily with water to
produce the extremely weak minerals talc and serpentine, which
lubricate and weaken the fault and allow it to keep slipping for
millions of years, becoming a 'detachment'.
The detachment fault starts to move back
towards its original position on the mid-ocean ridge. Eventually,
magma starts to be injected into the fault again, re-establishing
vigorous volcanism at the ridge axis and stopping the fault from
slipping any further.