Iceland Bathymetry

I've always been interested in bathymetry - well, at least since needing to produce meaningful "location map" inserts for well logs at work. I've used things ranging from a geology server on Cornell.edu 's system to (since I left work) a tool called GeoMappApp (GeoMapApp (www.geomapapp.org) / CC BY).

This morning someone posted on Twitter a partial picture of Heezen & Tharpe's 1968 seabed topography map (to be more precise, Heinrich Berann's 1977 painting of Heezen & Tharpe's 1968 cross-ocean profiles), which prompted a question by another user about the "ring around Iceland". Which is why I'm interrogating the GIS.

Firstly, we need a bit motre detail on the bathymetry. Note that we're over 50 years later than the data Heezen and THarpe were working with, so we've a lot more data than they had. But it has to be said, Iceland has always valued - and therefore mapped - it's nearshore fishing grounds. So their data in the mid-1960s was probably pretty good for the first 300-odd km.

Bathymetry map of North Atlantic, centred on Iceland. White line is the associated N-S depth profile.

The white line shows the position of this profile. Note that sealevel is 3/4 up the vertical axis. Also note the vertical exaggeration is 125× ; distances are south from the origin, near Scoresby Sund on the East coast of Greenland.

You can see a definite "surface" around Iceland close to shore - between km marks 600 and 700 - which is very typical for around islands. (Damn, I forgot to put a scale on the bathymetry plot.) It's where wave action breaks up rock and moves boulders around until the sediment "falls off the edge" into the abyssal deeps. (This also appears on the next profile, alonf the Scottish margin, which is mainly composed of multi-billion-year old high-grade metamorphic rocks - much more consolidated than fresh lavas which erupted into seawater.)

From 700 to 1200 km from the start point, you can see that the seabed level (above some particular level in the mantle) declines failry steadily. Not uniformly, it looks more like what the mathemeticians call an "asymptotic" curve - always approaching a certain lline, but never quite getting there. If you, dear reader, want to play with the numbers yourself, I have the data file along this track line under the name "IS-profile-NS.txt". Some header lines :

GMRT Grid Version 4.11
Longitude Latitude Distance (km) Elevation (m)
-20.88 69.50 0.0 -378.21484
-20.88 69.48 2.9305084 -374.4984
-20.88 69.45 5.864622 -379.54816

(The original file is tab-separated, not space-separated, but that shouldn't cause any significant problems.

The general opinion of this profile is that the predictable decrease in the seabed level is the result of the deper parts of the crust slowly cooling as they move away from the heat inputs at the mid-ocean ridge. That's very compliant with the rest of physics - density changes with temperature, Archimedes and his post-bath streak through the streets of Syracuse, all that jazz.

But I have to admit that for the specific case of Iceland, I had to select my profile fairly carefully, because to the NW and SE are complicating structures - the Fareoes (DK : "Far Islands") ridge and the Iceland-Greenland ridge. These are the surface traces of the movement of the comtinental shelves and newly-created oceanic crust over the margins of the Icelandic hotspot, which has resulted in the accumulation of considerable thicknesses of lavas on the surface (I've drilled oil wells on the UK side of this region ; basalt and eroded lava beds and intrusions are common for the top several kilometres.) and sub-surface heating from some flow away from the hotspot. The textbook image of a hotspot is that they're circularly symmetrical, but as seismic data improves it is becoming increasingly clear that they're not simple, or circularly symmetrical. Or even, verticallt straight. By coincidence I was reading a paper on the topic just a few days ago ("Imaging deep-mantle plumbing beneath La Réunion and Comores hot spots: Vertical plume conduits and horizontal ponding zones", Dongmo Wamba et al., Sci. Adv. 9, eade3723 (2023) 25 January 2023) which gives a much more geological "feeling", complex structure. That this SW Indian Ocean example shows plumbing structures in the mantle of several thousand km size N-S, E-W and Up-Down feels - to me - more realistic than simple cartoons of circular structures. In the volume of the Earth, nothing is the "spherical cow in a vacuum" that physicists (stereotypically) start with ; everything has a history, which affects it's present and future. Complications of the "Icelandic hotspot" stretch at least as far as the island of Lundy in the Bristol Channel, the line of "Tertiary Volcanic Districts" (as the regional geological memoirs are titled) from Ulster to Arran to Mull to Skye to the Forties oilfield (intimately associated with the whole Central North Sea oil province), a slew of seamounts and dead volcanoes between Orkeny and the Faroes, the Faroes themselves, and I literlally do not know what is further up the Norwegian Atlantic coast. Really quite comparable with that paper's Comoros-Mayotte-Reunion-Marion-Crozet-Kerguelen sub-crustal plumbing.

Taking a profile perpendicularly across the Ridge direction, from the mountain rim of Greenland, through Iceland, over the Faroes to the Shetland Islands, shows a more complicated set of elements :

Within Iceland, dips to almost sealevel speak to marine and ice erosion to that (approximate) base level (probably with some filling by sediments, also accumulating to approximately sealevel), then continuing to the East (increasing distance from the start point), between about 700km and 1300km the seabed sinks along a similar shaped profile to the N-S profile, though at a devreased rate. Then is a interval going above sealevel - the Faroes - which appear to have either a lot of build-up, or some thermal support from below. Then there is a trough (not a subduction trench, but probably fault-controlled) which reaches down to approximately meet the previous "thermal sinking" curve. Then there is the Scottish continental slope, and the wave-cut surrounds of the continentals shelf around a relatively small island group. (Text file of plot data is IS-profile-NWSE.txt)

This final set of images is across the Reykjanes ridge, a few hundred km SW of Iceland, and somewhat away from the complications of the hotspot.

Here the decrease in seabed level away from the ridge is much more clearly symmetrical about the ridge axis. The axis is still a thousand metre tall range above the "abyssal plain" - compare it with the Greenland mountains above sealevel.

The data file for this plot is "/home/aidank/winxferdir/Portable/geomapapp/GMA outputs/IS-ReykjanesRidge-Profile-NWSE.txt"

I've uploaded all the images and data files to a folder on "Box", but I'm not sure how that is visible from the outside world. https://app.box.com/s/skg097kuinerqy52b89xvuain2d7qweb Suck it and see!

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There's an annoyance - the Wikipedia page for Marie Tharpe cites her as having "discovered the Mid-Atlantic Ridge", while the page for the Mid-Atlantic Ridge correctly cites the discovery to the Challenger oceanographic survey voyage in 1872 - some decades before Tharpe, and probably her parents, were born. I'm fed up with correcting Wiki and being reverted, so do what you want with the correction. I'm not disputing Tharpe's contribution, but the first awareness of a tall mid-Atlantic underwater topography feature came from laying trans-Atlantic telegraph cables in the 1860s, and has been incredibly well reported. Amongst people who know what physics is. Well, ::SHRUG:: if someone more dedicated to Wikipedia wants to fix it, feel free.

Sector collapse

When we were doing the Vulcanology trip to Tenerife, a couple of the stops were to examine the faults bounding the Guimar (SE coast) and Santa Cruz (N coast) collapses. Always worth thinking about, even without the fears stirred up by that Portsmouth (?) hazard research centre.
Well, Prof Ceiling Cat Emeritus has been posting about his current jaunt around Hawai'i, and one look at the geography of Oahu made me think "sector collapse" again.

Oh dear, that's not good looking. That looks like lumps of islands 10km by 20km which have broken off and slid over 50km down the seabed slope.
What does the profile look like? (following the white line in the bathymetry plot)

10km NE-SW by 20km NW-SE by 1.5km thick. That's a big chunk of rock. The tsunami that hit the Pacific coasts (and particularly the British Columbia to Washington section) would have been ... unhealthy to see. I wonder what the date was.

For comparison, here's the most recent slump from the North side of Tenerife.

(250m bathymetry contours, bolded at 1000m intervals, for all images) The characteristic "lumps on the sea floor" of a slump can be seen. In the profile you can estimate the thickness of the largest lump, though this slump seems to have fragmented more than the Hawai'i example above.

 To a first approximation, say 5km by 3km by 0.25km. More detailed mapping with sonar shows that a lot of the seabed has rough areas which are interpreted as earlier generations of slump. Upwards of 20 slumps have been identified around the Canaries archipelago. 
Probably the most recent slump around Tenerife (unless it has "gone" while I'm typing) is from the side of the island facing Gran Canaria.

 The profile shows how steeply the islands drop off away from the volcanic centres.

Note the level of the inter-island gap - 2.5km below sea level - compared to the abyssal plain to the North at over 3.5km below sea level. There is around a kilometre of fill in this gap which hasn't accumulated to the North.

 Since the turn of the millennium there has been considerable speculation about the possibility of major landslides from the flanks of volcanic islands in general, and the Canary archipelago in particular. While concern about the particular claims concerning a West-flank collapse of La Palma have somewhat abated, there are certainly major landslip features around ocean islands. The recent (22 December 2018) flank collapse of Anak Krakatoa in the Indonesian archipelago killed over 400 people, making the point that these things do indeed happen.