Researchers from the Geography, Prehistory and Archaeology department and the Geology department of the EHU, and the Geology department of the USAL have quantified, for the first time, the difference in the relief of the two slopes of the mountain range that extends from Castro Valnera (on the border between Burgos and Cantabria) to the Pyrenees. Since the formation of the range, there has been differential erosion of the slopes, which has resulted in the crust of the Atlantic slope uplifting more than that of the Mediterranean slope.

Similar in a way to boats that float higher in the water when weight is unloaded, the level of the Earth’s crust rises as it loses the weight of the surface rocks. The collision of tectonic plates results in the formation of mountain ranges, but their relief continues to rise afterwards. In fact, the tectonic plates float over the Earth’s mantle, on the asthenosphere (a layer that extends from the base of the crust at 100 km to a depth of 660 kilometres) and, according to Iván Martín Martín, researcher in the Geography, Prehistory and Archaeology department of the University of the Basque Country-EHU, “when the load on the tectonic plates lessens, the relief gradually rises, in a similar way to boats, to maintain a certain stability.” This is known as isostatic adjustment, and it is particularly pronounced in areas where glaciers are disappearing, for example.

Understanding isostatic adjustment is fundamental for understanding the changes in sea level and the evolution of the Earth’s landscape. The Basque–Cantabrian Range separates the two watersheds in the north of the Iberian peninsula; that is, one flows into the Cantabrian Sea and the other into the Mediterranean Sea. Therefore, this mountain range forms a dividing line between two distinct slopes: the Atlantic and Mediterranean. The aim of the EHU’s research group is to explain the contrast between both slopes.

Iván Martín has estimated that from the time the Basque–Cantabrian Range was formed, up to the present day, 2,019 km³ of rock has been eroded on the Atlantic slope and 1,408 km³ on the Mediterranean slope. According to the EHU researcher, “differential erosion has taken place; the Atlantic slope has suffered more erosion, which has caused a greater isostatic uplift. The relief of the Atlantic slope has risen by approximately 255 m, while the relief of the Mediterranean slope has risen by 167 m, representing a difference of 88 m between the two slopes.” He also adds that “although the erosion is considerable, the erosion discharge will continue in the future; that is, more surface rocks will erode and, as a result, the relief will continue to rise.”

The rivers of 35 basins on both slopes were analysed (13,190 km²) using both conventional and modern methods. “To date, globally, not many studies have been carried out 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 the rivers, and there are hardly any studies on fluvial erosion”, he commented.

A significant evolutionary contrast in a very small area

An example of this is the cliffs of Jaizkibel, the Punta Galea cape and the Nervión waterfall: “In these cases, the uplifting of the relief is highly evident”, he says. “There is a marked contrast in the evolution of the topography in a very small area, at a distance of around 40 or 50 kms”, enthuses the EHU researcher.

During the research, the discharge of the slopes was calculated for the first time, as well as its consequences. Martín pointed out that, thanks to this research “we now understand how the rivers on both sides were formed; how the relief has changed; why there is a difference between the two slopes; why the rivers on the Atlantic slope run crosswise and those on the Mediterranean slope are longitudinal; why there are plains at certain altitudes though not at lower altitudes, etc.”

The Basque–Cantabrian Range, consisting of folded marine sediments, was formed as a result of the Iberia-Eurasia collision. “Rivers are as sensitive as the veins in our body. They can easily fall out of balance and they retain the imprint of everything that happens to them”, he says. It is known that the river network of the Atlantic slope is more developed than that of the Mediterranean slope: the rivers are shorter, have a greater slope, and it rains more in this area. Furthermore, “it is important to understand the sea level changes throughout the Earth’s history”, explains the researcher. “Ultimately, when the sea level falls, rivers can be thrown out of balance, the slopes become more pronounced, and erosion increases. As the Mediterranean Sea is further away, there is less erosion on its slope. However, around 6 million years ago the Mediterranean Sea dried up (a phenomenon known as the Messinian salinity crisis), the sea level fell by more than a kilometre and there was significant erosion of the rivers on the Mediterranean slope.”

The researcher also focused on the tilt of the uplift: “It is true that there has been an uplift, but it has not been entirely vertical, as it is tilted. In general, everything tilts westwards. However, in the central area (which includes Bilbao and Vitoria-Gasteiz, where the Zadorra and the Nervión rivers are), the tilt is quite mild. It has been observed that the isostatic uplift is greater in small basins. In fact, it is highly unlikely that a large river will experience changes, but it is more likely in small rivers. It is interesting to observe how the smaller rivers tilt towards the opposite side.”

Additional information

This study forms part of the doctoral thesis of Iván Martín Martín. The thesis directors are Antonio Tarriño and Fernando Sarrionandia of the Mapping and Geographic Information Systems Service (SGIker) of the EHU, and Pablo G. Silva, of the University of Salamanca. Iván Martín studies the geomorphological evolution of the Zadorra river and has analysed the entire territory and observed how the relief has changed in the area as a whole.