Although it may seem hard to believe, parts of the Earth’s crust have been leaking into a lower mantle, a phenomenon that has caused deformations in different parts of the Earth’s surface.
Recently, a group of Earth scientists from the University of Toronto (U of T) discovered that this phenomenon known as lithospheric trickle has been occurring for millions of years under the Andes mountain range.
As explained in the research, lithospheric drip occurs when parts of the lowest layer of Earth’s outer envelope thicken and begin to leak into the lower mantle when it warms to a certain temperature.
As the fragments sink into the lower mantle, a basin first forms at the surface that then rises as the lower weight breaks off and sinks deeper into the mantle. This results in an upward rolling of the land mass over hundreds of kilometres.
The process
This process gives rise to significant surface deformations such as basins, crustal folds, and irregular uplifts.
Although the process is a relatively new concept in the decades-old field of plate tectonics, several examples of lithospheric drift have been identified around the world.
Scientists at the U of T have confirmed that several regions of the central Andes in South America formed in the same way. And they’ve done it using materials available at any hardware and art supply store.
“We have confirmed that a deformation on the surface of an area of the Andes mountain range has a large part of the lithosphere below underpinned,” says Julia Andersen, a PhD candidate in the U of T Department of Earth Sciences. and lead author of a study.
Geoscientists have used the sedimentary rock record to track changes in the surface elevation of the Central Andes since the Miocene epoch, approximately 18 million years ago.
Seismic imaging provides a remote image of the Earth’s interior much like an ultrasound scan of the human body, illuminating new insight into the trickle structures of the lithosphere.
Following the study, the researchers believe that the dripping of the lithosphere is most likely the cause of the observed deformations in the Andes.
“The discoveries show that the lithosphere may be more volatile or fluid than we thought.”
— Russell Pysklywec, Professor of Earth Sciences at U of T, study co-author and Andersen’s PhD supervisor
Julia Andersen explains:
Some of the implications of this are that even without Plate Tectonics (which Earth has), the Earth’s surface could continue to warp and shift due to processes occurring in the lithosphere and mantle.
-This also has implications for other planets in our solar system that do not have plate tectonics, such as Venus.
We see features on the surface of Venus called the Corona, as well as other small-scale mountainous regions (much smaller than the Andes), and these could have formed due to a process such as lithospheric dripping.
julia andersen
PhD candidate in the U of T Department of Earth Sciences and lead author of a study
-What led you to study lithospheric dripping?
In geophysical studies and data (particularly seismic tomography data) there are cases where part of the mantle lithosphere has been removed (or has disappeared) and the mantle has risen to replace it. We study lithospheric drip because it is a mechanism that can explain how the lithosphere has been removed from the mantle.
-What is lithospheric dripping and how does this process occur on Earth?
In most cases, the lithosphere “floats” on top of the mantle because it has a lower density (think of oil floating on water). However, high pressures caused by tectonic events (such as mountain building) can create cases where the lithosphere becomes denser than the mantle. This causes it to sink (or drip) into the mantle like cold syrup. If thick syrup is poured into water, it will drip to the bottom of the glass, the same physics is going on here.
-How have the Andes been specifically affected by this phenomenon?
In the interior (plateau region behind the mountains) of the Andes, there are several sedimentary basins. We believe that several of these basins are formed by drips. A mountain building event creates a lot of pressure, creating many cases where the lithosphere can be more dense than the mantle, so it leaks. When it drips, the bark is washed down, creating a depression. These depressions are filled with sediments (soil, etc.) that eventually harden into sedimentary rocks. While it is pulling down it is also pulling in so the crust above the trickle thickens making little mountains (much smaller than the Andes).
-How does the model you have created work and what conclusions have you obtained after putting it into operation?
In the model we’ve created, we simply use what we know about physics and fluid dynamics to make it work. We know that if we put a dense fluid on top of a less dense one, it will sink (or leak) and gravity contributes to this in the model. We also know that the Earth’s upper crust doesn’t flow, it behaves more like glass than syrup, so we use sand to mimic that behavior. What we discovered from these experiments is that when the lithosphere leaks, it deforms the surface above the leak in a similar pattern each time. This is the same way the Arizaro Basin in Argentina has been deformed for millions of years. This led us to believe that the Arizaro Basin, and others, have been created by lithospheric trickling. We also verified in the experiments that sometimes a lithospheric drip does not deform the surface. This tells us that sometimes the lithosphere could be leaking below the earth’s crust, but there are no signs on the surface that this is happening.
