How many years can a mountain exist, before it is washed to the sea?
Are you familiar with these lyrics? Despite exceeding many human lives our curiosity and intelligence drive us to answer this question. If you have already started thinking how can a mountain exist before it is washed to the sea, you obviously half way to becoming a geomorphologist. Let’s start with the lyric, which points out the basic principal of geomorphology perfectly. Three important variables in geomorphology are ‘uplift’ ,’ climate’ and ‘time’, it means that the variation of tectonic uplift and climate caused the diversity of the landscape. The initial control on mountain growth is ‘tectonic’, while the processes of destroying a mountain are related to climate. And who can help the mountain find the way to sea? Yes, the river! The river system is always changing. River reorganization, caused by the change of tectonic and climate condition, would effect not only species connectivity but also human settlements, and when the fluvial system fails to remove mass from uplifting region, the internally drained basin will store gravitational potential energy in the crust, this process may influence the growth of orogens in turn. With the basic theory in your mind now let’s talk about something which have already happened, which is happening, and which is going to happen in huge river system around the world.
Huge rivers usually originate from the mountain region, if we use spatial variations as alternative of the temporal variation, when orogens develop from initial stages of inversion to full accretion, ( e.g. from the Atlas Mountains to the Eastern Cordillera of Colombia, then to the Pyrenees), the river pattern would change from structure-controlled longitudinal pattern (which means rivers flow parallel with the structure of the mountain) to slope-controlled transverse river pattern (which means rivers are vertical with the mountain range). Geologic evidences such as irregular network shape, wind gap, sediment provenance, are always used in river evolution reconstruction. Correspondingly there are also some ways to predict how river system will change, you can easily do that, just follow! Imagine that there are two river basins sharing a common divide in front of you, if the erosion rate of one river basin irrs much higher than the other, the river basin which erodes faster will get bigger and the other one will become smaller via divide migration; if one of the river is too aggressive, it’s possible for one river to capture other’s upstream area. There are two indexes proposed to indicate this process, chi and mean local relief, both can be extracted from the digital elevation model (DEM). Each divide is supposed to migrate from high erosion rate to low erosion, from lower chi to higher chi, and from the higher mean local relief to lower mean local relief.
Now here is a task for you, 14 river basins in the Northern Qilian Mountain (NE Tibetan Plateau) are selected, then the chi and mean local relief across each divide were extracted, and the direction of divide migration indicated by both indexes were compared with the erosion rate, the results are shown in the Fig.1, do you have any idea about how does the river pattern look like? Actually this task is for me so I’ll give the solution. The divides migration pattern shows that, the transverse-dominated basins in the mountain front seem to expand at the expense of longitudinal-dominated basins. Why does this happen? The growth of the Northern Qilian Mountain will result in a progressive increase of the regional slope and the rainfall gradients, thus the difference of erosion rate will appear, then pushing the river adjusting itself to get used to the new tectonic and climatic condition.
Fig.1. Map of χ (a) and mean local relief(b) for 14 basins in the Northeastern Qilian Mountain. Heavy white lines in (a) and heavy red lines in (b) show drainage outlines. The relationships between χ anomaly and cross-divide mean local relief with the erosion rate was plotted in (c) and (d) respectively. The divide which the migration direction could be predicted correctly by χ anomaly and mean local relief is red in (c) and (d), otherwise is black.(Pan et al.,2018, Geomorphology, under review)
By far we already know at least one thing would happen after we all die——river reorganization! But, why should I care about that? I mean I am not the fish living the river basin, no matter how river reorganizes itself, my house would never get bigger. Most of us will not live more than 100 years, should we care about the millions years changes of the earth? There are many answers for us to stare into infinity and geomorphology is my way. Learning geomorphology really helps me figure out how should I live my life as a moment of forever, just like one of my favorite poem “The Garden” by Jacques Prevert, “One morning / In winter’s light / In Montsouris Park / In Paris / On earth / This earth / That is a star”. After many generations of outstanding works in geomorphology, we still have many unknowns. So let’s keep exploring, but don’t forget, this earth is just a star of the universe.
[Homepage image: Qilian mountains by Stefan Wagener]