Index of ".grd" files
---------------------

anom_124s_tot_set1_cut2_v1.grd:

	The is file contains gravity anomalies derived from Seasat, Geosat,
	ERS-1 and TOPEX/POSEIDON altimeter data, and shipborne gravity data.
	A technique called least-squares collocation has been employed to
	construct such a combined gravity field. The data clearly show the
	different spreading morphologies of the Reykjanes Ridge, from the
	axial high near to Iceland and its associated hotspot to the median
	valley south of 60 degree north that is more typical of slow spreading
	ridge. Also observed are ridges that lie sub-parallel to the axial
	high and taper southwards towards the axis. These ridges are seen to
	cross the magnetic isochrons.

flex_20_2828.grd:	Flexure of plate under Canary Islands.
rup_5x5_topo_cut.grd:	Bathymetry in similar region

mfhaw.25e01f.grd: 	Flexure of plate under Hawaii
thaw_tl5.grd:		Bathymetry over same region

grav.grd:		Gravity anomaly ofver Weddell sea, Antarctica.

agpba_30x30.grd:	Bouger gravity map of Africa.

The following three grd files are all gravity maps of different parts
of the world's mid-Ocean ridge system south of 30S, made from the
recently declassified GEOSAT GM dataset. GEOSAT's altimeter made
measurements of the height of the sea surface (geoid) at extremely
fine track spacing (~3-4km) for 18 months between 1985 and 1986; the
gravity variations have been computed using a FFT method, and the
results are comparable in accuracy to the best ship gravity data. The
global coverage of the satellite means that *any* part of the ocean
floor may be investigated in this way, and so comparisons can be made
between different ridges at different spreading rates to further our
understanding of Mid Ocean Ridge dynamics.

cunha.g.g7x45.grd:	Mid-Atlantic Ridge 32 degrees South
	The location of the spreading axis together with the close spacing of
	Fracture Zones is clearly shown; and the orthogonal geometry of the
	typical ridge-transform-ridge spreading system is obvious. The gravity
	signal differs from the topography particularly in how it reveals the
	flexural response of the lithosphere to applied loads, both positive
	(rift mountains) and negative (the deep trenches along Fracture
	Zones).

sp.g.g7.grd:		 Southeast Indian Ridge 78 East, 
				Amsterdam and St.Paul Islands 

	Here the location of the axis is much less clear and is the subject of
	debate.  The Southeast Indian Ridge passes the two volcanic islands of
	Amstaerdam and St.Paul, and the region is a localised hotspot. There
	is a clear trace of a fracure zone running from 78E 37S to the corner,
	and trending along the spreading direction. However the trace becomes
	much more diffuse and not so strictly aligned to the spreading
	direction just to the west of the islands. There is also distinct
	asymmetry to the trench-scarp system that extends continuously from
	top right to bottom left, an example of flexure, the characteristic
	wavelength of which can be used to determine the effective Elastic
	Thickness of the Lithosphere. Gravity rather than topography must be
	used for this.


bv.8586g.35x.grd:	Bouvet Island and Fracture Zone, 
				Southwest Indian Ridge 2E 

	This is a smaller (test) region, showing principally the Bouvet
	Fracture Zone and the nearby Bouvet Island. The highs that flank the
	deep trench of the Fractue Zone are a flexural response, as is are the
	lows that surround the Island. Note also the contrast in the signal
	between the actively slipping part of the Transform Fault and the
	stable region away from the axis (ie north of -53.5 in this view).
	The location of the axis at the north end of the Fracture Zone can be
	seen exiting in a southeasterly direction.

geoid_230270-3010.grd:	Geoid at the East Pacific Rise
	Short wavelength (<200 km) geoid undulations reflect sea floor
	topography whilst longer wavelength features reveal deeper
	lithospheric processes within the earth. The marine equipotential
	surface, called the geoid, corresponds to mean sea level and this map
	shows the south Pacific geoid derived from combined Seasat, Geosat and
	ERS-1 satellite altimetry data.  Whilst the East Pacific Rise
	spreading centre (between 250-255) is associated with a geoid high,
	this map shows an almost circular series of highs and lows with
	wavelength 700 km. Curiously this is the thickness of the earth's
	upper mantle - are these anomalies the surface expression of upper
	mantle convection ? Faint geoid lineations trending N80W are thought
	to represent small-scale convective instabilities beneath the
	lithosphere.

geoid_200240-3010.grd:
	Four major fracture zones are visible trending N70E. Oceanic islands
	and seamounts show up as short wavelength positive (red) features.
	Faint geoid lineations trending N80W are thought to represent
	small-scale convective instabilities beneath the lithosphere.