Week 5 - Convergent Plates

North Face of Mount Everest from the Tibet Autonomous Region of China. Courtesy of ©Pichugin Dmitry/Shutterstock.com

My travels this week take me to the Himalayan mountain range and Tibetan plateau. They formed when the Eurasian Plate and Indian Plate collided around 50 million years ago. The collision continues today. When these two gigantic masses of continental lithosphere met head-on, neither were able to sink, because of their buoyancy. Solid rock crumpled and faulted at these boundaries. Slivers of rock were shoved on top of each other, which formed the mountain range (Convergent Plate Boundaries, 2017). This is referred to as a converging plate boundary because when two similar density plates collide, they are forced up causing fold mountains, instead of subducting. The collision still active today is causing the Himalayas to grow and India is still drifting.

Continental vs. Continental Plate Convergence, courtesy of USGS.

India was a large island 225 million years ago, separated from Asia by the Tethys Sea, and located off the Australian coast. India began drifting northward when Pangaea separated approximately 200 million years ago. Scientists have reconstructed the northern journey of India, by studying history of the mass and the Tethys (Convergent Plate Boundaries, 2017).

Artist's conception of the 6,000-km-plus northward journey of the "India" landmass (Indian Plate) before its collision with Asia (Eurasian Plate), courtesy of USGS.

  

The rise of the Himalayas has been rapid, peaks like Mount Everest have ascended to more than nine kilometers. The two land masses are still impinging and moving towards one another. The Himalayas are ascending more than one centimeter annually. It is believed that the Eurasian Plate is stretching rather than shoving upward, because of gravity (The Himalayas: Two continents collide, 2015).

Sunset view of towering, snow-capped Mt. Everest, from the village of Lobuche (Solu-khumbu), Nepal. Photograph by Gimmy Park Li, courtesy of USGS

 

Scientists have found pink sandstone layers that contain magnetic mineral grains or magnetite, that recorded the pattern of the earth’s magnetic field changing polarities back and forth. Plant and animal fossils have also been discovered in these sandstones, indicating that the Tethys Sea flooded the region sporadically. Compared to Tibet’s arid climate today, this is evidence of the significant changes that have taken place due to plate movement. India is still drifting north and compressing into the Asian continent with immense pressure. One primary consequence of this is Earth’s crust has massive pressure building up, causing earthquakes along several faults, scarring the landscape along the way (The Himalayas: Two continents collide, 2015).

The Himalayan mountain range spans through the borders of Pakistan, India, Nepal, Bhutan, and China. The crust beneath is still in the compression process. The Himalayas are an active fold mountain, the most common type in the world, they are created when two or more tectonic plates collide, causing rock and debris to warp and fold into rocky outcrops, hills, and mountains, through a process called orogeny. It takes millions of years for an orogeny event to create a fold mountain. These are often related to the continental crust, they develop at convergent plate boundaries, and are also referred to as continental collision or compression zones. This is where the tectonic plates collide. A set of stresses focused on one point in rock is known as compression. These zones are where tectonic plate activity drives crustal compression at the edge of the formation, therefore, most fold mountains are located on the edge or prior edge of continental plate boundaries.  Continental crust rocks on the edge are often less stable and weaker than those found on the continental interior, making them more inclined to warping and folding. Anticlines and synclines are up-and-down folds as a product of compression, they are the most common. “An anticline has a ∩-shape, with the oldest rocks in the center of the fold. A syncline is a U-shape, with the youngest rocks in the center of the fold” (Evers, 2015).

A reverse fault refers to dip-slip faults where the hanging block moves up relative to the footwall block. A type of reverse fault where the dip is less than 5 degrees is called a thrust fault. Horizontal shortening of the crust is caused by compressional stresses are what create both reverse and thrust faults, which are most common along convergent plate boundaries. Colliding plates with compressional forces create folds and thrust faults that thicken and shorten the crust to produce mountainous topography such as the Himalayan range (Lutgens, 2016, p. 303).

Folded layers of Himalayan rock in a mountainside, beside the Leh-Manali Highway, near Gushal in Himachal Pradesh, India, courtesy of https://www.flickr.com/photos/jace/3803843672

References

        Convergent Plate Boundaries. (2017). United States Geological Survey. Retrieved from https://geomaps.wr.usgs.gov/parks/pltec/converge.html

        Evers, J. (2015). Fold mountains. National Geographic Society. Retrieved from https://www.nationalgeographic.org/encyclopedia/fold-mountain/

The  Himalayas: Two continents collide (2015). United States Geological Survey. Retrieved from https://pubs.usgs.gov/gip/dynamic/himalaya.html

    

        Lutgens, F. K, Tarbuck, E. J., & Tasa, D. (2016). Essentials of geology. (13th ed.). Upper Saddle River, NJ: Prentice Hall (Pearson). ISBN-10: 0134446623; ISBN-13: 9780134446622

Refe