Peter Molnar on Why the Tibetan Plateau is So High
Peter Molnar is a Professor of Geological Sciences at the University of Colorado at Boulder. He has worked on a great many subjects, but especially on how mountain ranges are built and how climate is affected by topography and crustal movements over geological time. His research on mountain ranges has focused on the high terrain in Asia - the Himalaya, Tibet, and the Tien Shan, all of which he has studied extensively in the field. He explains why he thinks that bits of hot mantle are dripping off the bottom of the Tibetan lithosphere and how this can account for the 4,500-meter height of the Tibetan plateau.
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Podcast Illustrations
All images courtesy of Peter Molnar unless otherwise indicated
The elevation map shows the enormous extent of the high Tibetan plateau.
Generic Mapping Tools, GLOBE & ETOPO1
Present-day Tibetan Plateau
Geological field trip campsite in Tibet, with part of Bukadaban (6,860 m) in the Kunlun mountains in northern Tibet. This series of three images were all captured north of the Jangtang mountains in the Hoh Xil basin (see topographic profile below).
Bukadaban, looking northeast across LiXie Wudan lake in northern Tibet.
Driving along the shore of Yinma lake.
To the west of Lhasa, these folded sedimentary rocks overlain by volcanic rocks from the late Cretaceous provide evidence for the formation of the Transhimalaya mountain range well before India collided with Tibet around 50 million years ago.
Courtesy of Mike Searle
Paleoaltimetry of Tibet
Hypothesized topographic profiles across Tibet based on a range of paleoaltimetry indicators.
Fossil Indicators of Elevation History
Palm fossil found in the Lhunpola basin between the Transhimalaya and the Jangtang mountain ranges. It was recovered in a 25 million-year-old ancient lake sediment at a present-day elevation of 4,655 meters. Since such tropical plants cannot withstand the cold climate at high elevations, the fossil indicates the basin was no more than about 2,000 meters in elevation at the time.
Courtesy of T. Su
Fossil plants from the Middle Eocene (about 47 million years ago) in central Tibet at a present-day elevation of 4,850 meters. They represent a humid subtropical ecosystem that would have flourished at a land surface height of up to 2,400 meters with a monsoon season and a mean temperature of about 19 °C. The scale bars are 10mm (A,C,D,H,N,O,R,S), 5mm (B,G,I,J,P,Q), and 2mm (E, F, K, L, M).
Su et al., PNAS (2020), 117, 32989
Normal Faulting
Pervasive north-south normal (extensional) faults indicate that the Tibetan plateau is extending in an east-west direction and subsiding, perhaps by as much as about 1,000 meters over the past 15 million years. This suggests that the plateau may have been as high as about 5,500 meters in the mid-Miocene. The image shows recent normal faulting in the Ama Drime massif in southern Tibet.
Jessup et al., (2008) Geology, 36, 587-590
Isostasy and the Foundering of Hot Lithospheric Mantle
The crust is less dense than the mantle. The lithosphere consists of crust and lithospheric mantle. Below the lithosphere is the asthenosphere, which is mantle that is hot enough to convect. The crust and the mantle are distinguished by their composition, i.e., they have different mineral assemblages. The crust contains silica-rich minerals, such as feldspar and quartz, and the mantle contains minerals rich in iron and magnesium, such as olivine and pyroxene.
Vertically exaggerated cross‐section through the upper mantle, showing crust and mantle. The asthenosphere is a weak layer that underlies the stronger mantle lithosphere. The lithosphere includes the coldest uppermost part of the mantle and the crust on top. The thicknesses of both crust and mantle lithospheres vary from place to place but, in general, where the crust is thick, the surface stands high to form a mountain belt or high plateau. The gradation in tone through the lithosphere indicates a downward decrease in strength to the asthenosphere.
Sequence of cartoons showing the growth and collapse of a mountain range built by horizontal compression and thickening of the lithosphere (b). The lithospheric root, composed of thick, cold, dense mantle lithosphere, breaks off and sinks. It is replaced by the ambient hotter mantle of the asthenosphere, indicated by the light shading in (c) where thickened mantle is shown in (b). The remaining lithosphere then rises, with the surface rising as indicated by the dark shading at the top of the thick crust.
The cartoon shows how horizontal compression induces thickening of the crust and mantle lithosphere. Since the mantle lithosphere is denser than the underlying asthenosphere, it is gravitationally unstable, and blobs of mantle lithosphere detach and sink.
Simulation of the collision of India with Asia. The crust is shown as tan, and the mantle is shown in red, with the lithospheric mantle in dark red and the weaker, underlying asthenosphere shown in light red. In the first part of the movie, the Indian plate moves north toward southern Eurasia and its mantle lithosphere subducts beneath southern Eurasia. Slices of the crust of India become detached and stacked atop one another to form the Himalaya. After the initial collision, India continues to penetrate into Eurasia causing Eurasian lithosphere to thicken. Thickened crust builds the Tibetan Plateau, and the gravitationally unstable, thickened lithospheric mantle forms blobs that drop off into the asthenosphere. The remaining Tibetan lithosphere above the thinned lithosphere rises a bit higher. Peter Molnar thinks this combination of crustal thickening and removal of mantle lithosphere explains the rising of the Tibetan Plateau over the past 25-15 million years.
Courtesy of Tanya Atwater
Editor’s Note: Peter Molnar passed away in June 2022.