You are here
Is Erosion Helping the Himalayas Grow?
Is Erosion Helping the Himalayas Grow?
A Literature Review of River Incision and Uplift in the Himalayas
“The Roof of the World,” the Himalayan Mountains, is home to some of the highest peaks in the world. The Himalayan Mountains formed from the subduction of continental plates that continues today to increase mountain elevations. High-altitude climates have an increasing effect on erosional denudation, transforming landscapes and altering geomorphic processes that govern orogeny. Unprecedented rapid uplift rates in the Himalayan Mountains are perplexing scientists. It is hypothesized that erosion may actually be responsible for growth of Himalayan Mountains. River valleys continually gouged out of the mountainous terrain by fluvial erosion create the potential for rising mountain peaks.
Molnar and England (1990) began by looking at the driving forces of mountain uplift and the influence of climate change on mountain geomorphologic processes. The highly concentrated weight of mountain ranges causes isostatic depression of the crust and mantle. As a result, continental crust thickens under mountain ranges. However, Molnar and England noted that the mountain ranges bordering the Tibetan Plateau increased in elevation at a rate so fast as to be unaccountable solely by crustal thickening. The evidence Molnar and England found for the accelerated uplift did not necessarily reflect a general surface uplift from crustal thickening, but rather it was the result of isostatic compensation of rapid erosion and bedrock unroofing. Erosion by both rivers and glaciers lowered mean elevations and thus, underlying crust thinned. Isostatic compensation then raised the remaining peaks on the sides of the incised valleys to greater heights than before (Fig. 1). While some scientists believe tectonic uplift and chemical weathering from uplift and erosion triggered the Quaternary ice ages (Cornwell et al. 2003), Molnar and England concluded that it was the Cenozoic climate change that increased valley incisions and induced mountain peak uplift in mountain ranges all over the world. Molnar and England’s findings spurred a deeper exploration into further explaining why and how mountain uplift is related to isostatic response to valley incision.
Figure 1. Illustration of isostatic uplift of mountain peaks in response to isostatic compensation. (A) Erosion of deep valleys in an initially level plateau leads to (B) a landscape with lower mean elevation with mountain peaks rising about the elevation of the original plateau. Source: Montgomery, D. (1994). Valley incision and the uplift of mountain peaks. Journal of Geophysical Research, 99, 13,914.
Nanga Parbat, the ninth highest mountain on Earth, resides at the western end of the Himalayas in Pakistan. It lies just south of the Indus River, which is responsible for mass fluvial erosion and valley incision that forms some of the steepest relief on the planet (Cornwell et al. 2003). As the slopes of Nanga Parbat steepen, they become less stable and eventually fail under gravitational forces. The denuding of the Nanga Parbat massif occurs through a process coined “unroofing.” Schroeder and Bishop (2000) attribute the unroofing of the massif to geomorphologic erosional processes having occurred over millions of years. The gravitational collapse of Nanga Parbat’s oversteepened slopes exhibited outward as mass movement. Much of the debris from rockslides and landslides fell into the river, creating a partial barrier that narrowed and obstructed the river’s natural flow. As a result, the river gradient and velocity increased and effectively removed the debris, equating to high rates of denudation. Landslides, such as the Liachar-Indus landslides of 1840-1841, may dam a river for several months, resulting in catastrophic floods that carry large, eroded sediment loads downstream. Torrential rains from orographic precipitation, rapid snow melt, avalanches, and glacial erosion all contribute to erosion of mountain slopes. Much of the Nanga Parbat landscape’s major alteration is attributed to major glaciations, mass movement, and running water from glacial meltwater streams during the late Quaternary. The erosion-initiated uplift of Nanga Parbat’s peak into high, permafrost zones where colder temperatures increased rock strength through joint freezing and produced cold-ice glaciers, thus resulted in less mass-movement based denudation. The denudation that continues to occur is responsible for unroofing the Nanga Parbat Himalaya, while at the same time produces steep relief and deep valley gorges.
As important as erosive processes such as landslides, rockslides, and glacial scour, may be to uplift by removing weight from the underlying crust, downstream surface uplift from fluvial incision is an additional influential factor. Montgomery (1994) concurs with the assessment that climate change from the late Cenozoic accelerated valley erosion, which may be responsible for up to twenty to thirty percent, 1500 meters, of the present elevation of the Himalayan peaks. But of the processes contributing to valley incision, Montgomery states that transport laws for erosion by fluvial processes have the strongest basis. Erosion for fluvial incision is proportional to stream power, which is proportional to water discharge and valley slope. Montgomery modeled the downstream pattern of river incision in the Himalaya based on a plateau edge, mimicking the Tibetan Plateau that borders the Himalayan Mountains. Montgomery found that calculated profiles showed significant uplift of mountain peaks at the edge of the plateau, where the Karnali River is incised up to 4500 meters, and a decreasing relief downstream. Montgomery’s findings supported the hypothesis that the great height of the Himalayas is due to river incision.
River incision, such as the Indus Gorge that runs through the Nanga Parbat massif, has a profound effect on mass wasting and erosion, but also effects the structuring of the crust and lithosphere. Zeitler et al. (2001) produced a study examining the advective heat flow in crust and its ensuing role in mountain uplift. Crust will weaken and thin as the upper crust is eroded away or incised by rivers. As valley incision weakens the crust, advective heat flow from deep inside the earth is attracted to the weakening area as the potential energy attempts to reestablish equilibrium between the highlands and the lowlands. The increasing topographic gap between the valley gorge and mountain peak causes the geothermal gradient to increase. In response to the differential geothermal gradient, crustal flow is diverted from the isothermally compressed weak crust to the mountain massif to prevent strain from further weakening the already-compromised crust. The diversion of the advective heat flow in the crust results in mountain uplift as well as a production of weaker crust under the massif. The downcutting river and high topographic relief provide the rapid erosion required to keep the system in place. This thermal-mechanical-erosional process, termed a “tectonic aneurysm” (Fig. 2), is one of the driving mechanisms of growing elevation from river incision and erosion of the Nanga Parbat and many other Himalayan mountain massifs.
Figure 2. Cartoon illustrating dynamics of a tectonic aneurysm. Source: Zeitler, P., Meltzer, A., Koons, P., Craw, D., Hallet, B., Chamberlain, C., Kidd, W., Park, S., Seeber, L., Bishop, M., & Shroder, J. (2001). Erosion, Himalayan Geodynamics, and the Geomorphology of Metamorphism. GSA Today, 7.
In addition to the rate of river incision, climate is one of the most significant factors in determining the amount of erosion. The annual, summer monsoon season occurs when air masses, tracking over the Bay of Bengal, pick up moisture, causing tropical depressions, and, unable to pass over the Himalayas, drop tremendous amounts of precipitation over the Himalayas. Hodges (2006) proposed that monsoon rainfall influences how energy transfer takes place within the depths of the Himalayan-Tibetan system. Much like the “tectonic aneurysm” theory, where advective heat flows towards the weakest crust, the lower crustal flow of Tibet flows southward in the direction of least resistance towards the Himalayan front range, which has experienced extreme surface erosion due to high monsoon rainfall. The flow of crust towards the front range results in uplift, which in turn results in more erosion, thus creating a positive feedback system. Hodges discovered that rapid uplift is correlated with high topographic relief and steep river gradients, producing a large change in elevation over a short horizontal distance. High rates of erosion are responsible for the steepening slopes of Himalayan massifs; current erosion in the Himalayan range is largely driven by monsoons. To establish that channel extrusion has been responsible for the unusually rapid uplift of the Himalayan Mountains, upwards of a few millimeters per year, Hodges measured uplift and erosion rates based on cosmogenic dating. To support rapid uplift, Hodges found that the annual rate of uplift, mean hillslope angle, and relative stream channel steepness all increased over millennia timescales (Fig. 3). Hodges also found that annual monsoon precipitation increased over the zone of unusually rapid uplift, further supporting his hypothesis that there exists a significant relationship between local climate and deformation. Furthermore, the correspondence of the region of high monsoon precipitation to the zone of channel extrusion is consistent with the hypothesis that channel extrusion is caused by range-front erosion in response to monsoon. As the “tectonic aneurysm” theory postulates, the extrusion activated by rapid erosion results in the diversion of crustal flow to the front range, which in turn causes uplift of the mountains so that the peaks can more readily intercept the monsoon as it tracks northward. Glacial erosive processes primarily drove past erosion of the Himalayas, but much of today’s erosion is climate-induced and has a notably strong correlation to the erosion driving steepening channel extrusion and mountain uplift.
While inertia-driven tectonic activity is still present in the Himalayas, the resulting thickening crust is insufficient in explaining the unparalleled rapid rates of uplift. Scientists looked to erosive processes to explain the phenomenon. River incision is a boundary condition for hillslope erosion. As the incision steepens, the mountain slope will become less stable and will erode at a higher rate, primarily through mass movement. The incising river channel coupled with upland glacial erosion of mountain tops lowers the mean elevation and weakens the underlying crust, allowing advective heat flow from the crust to move upward, into the mountain massif, resulting in mountain uplift. The rate of fluvial erosion, correlated to the rate of uplift, is directly affected by stream power, which is proportional to water discharge and valley slope. Therefore, as the slope angle increases, so does the rate of fluvial erosion and vice versa. Many of the erosive processes in the Himalayas today are driven by climate and are adversely affected by climate change. The main mechanism causing the rapid uplift in the Himalayas is increasing river incision, thus resulting in some of the world’s highest peaks, steepest slopes, and deepest valley gorges in the world.
Figure 3. Relationship of annual rate of uplift, mean hillslope angle, relative stream channel steepness, and annual monsoon precipitation to uplift. Hodges, K. (2006, August). Climate and the Evolution of Mountains: New studies of the Himalaya and the Tibetan Plateau suggest a deep relation between climate and tectonics. Scientific American, 78.
Cornwell, K., Norsby, D., & Marston, R. (2003). Drainage, sediment transport, and denudation rates on the Nanga Parbat Himalaya, Pakistan. Geomorphology, 55, 25-43.
Hodges, K. (2006, August). Climate and the Evolution of Mountains: New studies of the Himalaya and the Tibetan Plateau suggest a deep relation between climate and tectonics. Scientific American, 72-79.
Molnar, P., & England, P. (1990). Late Cenozoic uplift of mountain ranges and global climate change: chicken or egg? Nature, 346, 29-34.
Montgomery, D. (1994). Valley incision and the uplift of mountain peaks. Journal of Geophysical Research, 99, 13,913-13,921.
Shroder, J., & Bishop, M. (2000). Unroofing of the Nanga Parbat Himalaya. The Geological Society of London, 163-179.
Zeitler, P., Meltzer, A., Koons, P., Craw, D., Hallet, B., Chamberlain, C., Kidd, W., Park, S., Seeber, L., Bishop, M., & Shroder, J. (2001). Erosion, Himalayan Geodynamics, and the Geomorphology of Metamorphism. GSA Today, 4-9.