Science1 publisher3 min readPublished
River erosion lifted the Basque-Cantabrian Range's Atlantic slope 88 metres more than its Mediterranean side
A team at the University of the Basque Country and the University of Salamanca measured how much rock 35 river basins have carried off the Basque-Cantabrian Range, then converted that lost weight into metres of rebound.
The Scientist · Science desk
What happened
- Since the Basque-Cantabrian Range formed, relief on its Atlantic slope has risen about 255 metres and relief on its Mediterranean slope about 167 metres, a difference of 88 metres between the two.
- The group analysed the rivers of 35 basins on both slopes, covering 13,190 square kilometres, using conventional and modern methods.
- Geographers and geologists at the University of the Basque Country, with geologists at the University of Salamanca, say this is the first quantification of the relief difference between the two slopes, from Castro Valnera to the Pyrenees.
- The range is built of folded marine sediments raised by the collision of Iberia and Eurasia, and it divides rivers draining to the Cantabrian Sea from those draining toward the Mediterranean.
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Why it matters
- capability A measured value for rebound driven by river erosion gives modellers a case to test outside glaciated terrain, which is where Martin says almost all the existing isostatic work has been done.
- constraint The figures are cumulative since the range formed and no formation date accompanies them, so nobody can convert them into millimetres per year or set them against present-day uplift measurements.
- precedent If unloading accounts for a topographic contrast this sharp over 40 to 50 km, lopsided relief at other drainage divides becomes a quantity to measure before it is put down to the collision that built the range.
The eroded volumes and the uplift figures scale differently. Ivan Martin Martin of the University of the Basque Country estimates that 2,019 cubic kilometres of rock has come off the Atlantic slope and 1,408 cubic kilometres off the Mediterranean slope [5][6]. That is about 43 percent more volume on the Atlantic side [20]. The uplift figures differ by about 53 percent [21]. Volume becomes metres of rebound only after it is divided by the area it came off, and the published summary gives one combined area for the study, not a split by slope [24].
Take the combined area anyway. The two volumes add to 3,427 cubic kilometres [22], and spread across 13,190 square kilometres that is a mean thickness of roughly 260 metres of rock removed [23]. The two slope uplifts are 255 metres and 167 metres [7]. Whether each slope's rebound matches its own thickness of missing rock is the check worth doing, and it needs the area of each slope [24].
The physics is standard. Plates float on the asthenosphere, which runs from the base of the crust at 100 km down to 660 km [11]. Martin said: "when the load on the tectonic plates lessens, the relief gradually rises, in a similar way to boats, to maintain a certain stability" [9]. The effect is most pronounced where glaciers are disappearing [10], and that is where the literature sits. Martin said: "To date, not many studies have been conducted globally on this topic. They have mainly been carried out in glacial areas. However, almost all the isostatic adjustment that occurred in these areas was caused by the erosion of rivers, and there are hardly any studies on fluvial erosion" [12].
The work does not divide the range's total relief between mountain-building and rebound. Relief keeps rising after a collision has built a range [14], and the 255 and 167 metres are attributed to unloading [7]. No companion figure for the tectonic share appears in the summary, and no date is given for when the range formed, so the totals cannot be turned into a rate [24].
Where the contrast shows up in the field is the cliffs of Jaizkibel, the Punta Galea cape and the Nervion waterfall [16]. Martin said there is "a marked contrast in the evolution of the topography in a very small area, over a distance of around 40 to 50 km" [16]. He also credits the accounting with explaining why the rivers on the Atlantic slope run crosswise while those on the Mediterranean slope are longitudinal [18], a claim the summary states without supporting figures [24]. The study appears in Geomorphology [4].
On what comes next for the slopes, Martin said: "Although erosion is considerable, it will continue in the future; that is, more surface rocks will erode and, as a result, the relief will continue to rise" [17].
What to watch
- The Geomorphology paper's per-slope basin areas and error bars, which would let the volume-to-uplift conversion be checked independently.
- A date for the range's formation; without one the 255 m stays a cumulative total and never becomes a rate.
- Whether the same unloading accounting reproduces relief asymmetry at other non-glaciated drainage divides.