Hi I'm Peter Clift, professor of geosciences at Louisiana State University.
In another video, I talked about how the Asian monsoon forms
and how it affects the climate of southern Asia.
In this video, I'm going to tell you more about how the Asian monsoon has affected the geology of Asia
I'll tell you what we know about the modern Asian monsoon started
and how this is related to the rise of the Himalayas and the Tibetan plateau,
as well as how the monsoon mountain interactions is linked to erosion in the region.
The modern Asian monsoon began sometime in the Cenozoic era,
the last 66 million years of Earth history.
Determining when in the Cenozoic the Asian monsoon began and then intensified
requires examining the geological record more closely.
The monsoon greatly affected the climate of Asia and this climate history is preserved in Cenozoic sediments.
These sediments are exposed onshore and also found in the nearby Indian Ocean and South China Sea
where they have been sampled through scientific drilling.
For many years researchers interpreted the Cenozoic sediment record
to indicate that the monsoon started around 8 million years ago.
This conclusion was based on microfossils in Arabian Sea sediments
linked to oceanic upwelling and phytoplankton blooms there, caused by the monsoon.
More recently, evidence has come to light of an earlier changing climate that may
have been caused by an intensifying monsoon.
This involves a switch from arid to humid conditions
in southwest China and Myanmar that began around 36 million years ago.
In a nutshell, we're still trying to figure out when and how
the modern Asian monsoon was established.
One thing we do know is that the Asian monsoon is
controlled by the rise of land in southern Asia.
Especially the world's highest mountains and plateaus:
the Himalayas and the Tibetan plateau.
Geoscientists are still collecting and interpreting evidence about how
and when this uplift happened.
Uplift began with the collision of India and Asia starting about 50 million years ago
and continues today.
So if we understand more about how and when the Himalayas and the Tibetan plateau rose,
we'd know more about how that affected the Asian monsoon.
Here's an interesting story about how something that happened deep in the earth between 30 and
25 million years ago affected the monsoon.
The northern movement of India to collide with Asia was caused by a subduction zone
along the southern margin of Asia.
This subduction zone dipped north beneath what is now the Himalayas and Tibet.
Shortly after the collision began, the subducting Indian plate broke
and the deeper part of the sinking plate no longer pulled the upper part of the plate down.
The loss of pull from the dense slab allowed the remaining Indian plate to rebound
and the Himalayas to quickly rise by as much as a kilometer higher.
This rise strengthened the south Asian monsoon.
As a result, an important feedback occurred.
Increased rainfall from the stronger monsoon
led to increased erosion of the mountains,
and so to more sediment being carried by
the Indus, Ganges, and Brahmaputra rivers into the deep Indian ocean.
Strong uplift and intense erosion allow deep crust to rise to the surface and be exposed as
the metamorphic rocks of the high Himalayas today.
It is the intense summer monsoon rains along the Himalayan mountain front
which distinguishes it from the dry Tibetan plateau to the north.
I hope you have enjoyed learning more about the Asian monsoon,
with a dramatic history recorded in the sedimentary rocks of Asia and its surrounding seas.