An Enigma in Mongolia

Near the geographic center of Asia, north of China and Tibet and south of Russia, the Hangay and trans-Hovsgol mountain ranges cover about 425,000 square kilometers (164,000 square miles) of Mongolia and look out over an arid and mostly treeless steppe.

Occupying half of this region and embedded in its center, the Hangay Dome rises to heights of 4,000 meters (13,000 feet)—about 1,500 meters higher than the surrounding topography.

High topography in continental interiors is unusual, says Anne Meltzer, and this makes the Hangay a unique opportunity for scientists who study the processes that shape the earth’s surface.

Meltzer, professor of earth and environmental sciences, leads an international team of two dozen researchers who have spent six years studying the Hangay region with a National Science Foundation grant. The team has expertise in geomorphology, geochronology, thermochronology, paleoaltimetry, biogeography, petrology and geochemistry, and includes scientists from the Mongolian Academy of Sciences.

In addition to exploring the earth’s geologic history, says Meltzer, the team hopes to shed light on the connections among continental deformation, the development of topography and global climate.

“A high topography influences the circulation of the atmosphere,” she says. “The region of Mongolia influences the movement of air masses and storms on a global scale with far-reaching impacts on weather and climate. Also, in the early 1900s, there were four magnitude-8 earthquakes on faults that pass on either side of the Hangay. Large magnitude earthquakes are rare within continents. Starting in 2005, there were swarms of smaller earthquakes around Ulaanbaatar, the capital of Mongolia, where most of the country’s population, government, and business is located.

“Our work sheds light on the processes and structures that generate large earthquakes. The Mongolians take a keen interest in this.”

Mongolian topography

In her work, Meltzer buries seismometers at strategic sites in the Hangay region. The instruments can detect earthquakes equal to or larger than magnitude 5 anywhere in the world. The instruments recorded the devastating earthquake in Nepal in April 2015, as well as the recent nuclear test in North Korea in 2016. The instruments are sensitive enough to also pick up the footsteps of a passing goat. The elastic waves generated by earthquakes propagate through the earth, says Meltzer, and by imaging them it is possible to open a window into the subsurface of the earth, from near the surface to deep into the interior. One puzzle that intrigues Meltzer and her colleagues is the dramatic difference between the height of the Hangay Dome and the elevation of the surrounding landscape. The relatively high topography of the Dome, she says, is a feature more commonly found near the edges of continents, where the thick, rigid tectonic plates making up the earth’s outer layer collide.

“We know,” says Meltzer, “the plates are in motion at the earth’s surface, colliding to form mountains and subducting [sliding beneath each other] to form volcanic regions. Generally the interior regions of continents do not deform and have elevations closer to sea level. But in the Hangay we see high topography, and the question is why.”

The researchers have examined several factors that might answer that question.

“The continental crust of the Hangay Dome looks to be thicker than the average continental crust,” says Meltzer. “This helps support the topography. If you look deeper inside, the lithospheric plate [the crust and rigid part of the upper mantle] appears anomalously thin, and the deeper, warmer part of the mantle is closer to the surface. This could account for the relative height of the Hangay Dome.”

The team has concluded that the high topography is a function of several processes. Early plate collisions, says Meltzer, thickened the crust, producing a state of gravitational equilibrium between the earth’s crust and mantle that allows the crust to “float” at a high elevation.

“It’s like an iceberg,” says Meltzer. “We see the tip of the iceberg but not the larger mass below the water. With the Hangay, we see a high mountain, but underneath is a root of low-density material propping it up. Its relative thickness makes the crust more buoyant and therefore capable of supporting a high topography like that of the Hangay in the interior of a continent.”

A second process, says Meltzer, could be a dynamic flow of material inside the mantle that generates high topography.

Meltzer is looking to start a new research project in Indonesia. “The relative motion of the Indian, Australian, and Eurasian plates provides an opportunity to study the transition from plate subduction to continental collision.”

The country is at considerable risk from seismic hazards. “We hope to get undergraduates involved from Lehigh and Indonesia. One outcome from this research would be a better understanding of the seismic hazards and how to mitigate the risk of these hazards.” 

Anne Meltzer is the first Francis J. Trembley Chair in Earth and Environmental Sciences. She studies earthquakes and the structure of the earth through naturally and artifically generated seismic waves. Meltzer has conducted research in Pakistan, Tibet, Mongolia, North and South America, and the Caribbean. She is a former chair of the board of directors of the Independent Research Institutions for Seismology (IRIS) and received her Ph.D. from Rice University.

Story by Kurt Pfitzer

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