Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Ancient glaciers still affect the shape of North America, say scientists

15.12.2005


Long after the disappearance of the glaciers that once covered much of North America, the land they rested upon is still recovering from their weight – and the slow movement of this recovery includes horizontal motion never seen before, say Purdue scientists.

The research team, led by Eric A. Calais, has found that a large swath of territory in the Northeast is slowly moving southward in relation to the rest of the continent. This region, once covered with massive ice sheets heavy enough to deform the very stone they rested upon, has long been known to be rising slowly in response to the glaciers’ retreat, but this response has now been observed to have a horizontal component as well. The southward movement does not appear to be dramatic – in most places it is on the order of a millimeter per year or less – but it calls into question a view that earth scientists have held about crustal plates for three decades, namely that these large, rocky chunks of the planet’s surface are rigid objects.

"Our findings do not disprove this view entirely, but they encourage us to see that deviations to the rule can occur," said Calais, an associate professor of geophysics in Purdue’s College of Science. "We are now in a position to measure just how rigid these plates are, and in some areas, such as those still rebounding from the glaciers, we are finding movement we had never seen. This movement contains critical clues regarding the viscosity of the earth’s interior, which is impossible to measure directly. The movement may even influence seismic activity in places such as Quebec, which is from plate boundaries but has nonetheless suffered from strong earthquakes."



The findings appear in the current issue of the scientific journal, Nature. Members of the team hail from seven different institutions, including the University of Arkansas’ G. Mattioli, the University of Wisconsin’s C. DeMets, CNRS France’s J.-M. Nocquet, Northwestern University’s S. Stein, the Georgia Institute of Technology’s A. Newman and the University of Memphis’ P. Rydelek.

Earthquakes often strike areas near plate boundaries, where large chunks of the earth’s outer shell grind against one another and create powerful waves that shatter the nearby surface. The resulting cracks in the land, such as California’s San Andreas fault, are well-known earthquake source spots that result from the North American plate’s contact with the nearby Pacific plate. But less understood are the powerful, but thankfully rare, earthquakes that strike the interiors of continents. For example, the New Madrid Seismic Zone, which stretches from southeastern Missouri to Memphis, Tenn., lies thousands of miles from plate boundaries but has experienced several large quakes in 1811-12, raising local earthquake insurance premiums in the region to levels comparable to those in San Francisco.

"We were looking for evidence that such areas are still deforming today in hope to better define their seismic potential," Calais said. "Past attempts to settle the argument have turned up conflicting data, but recent improvements in Global Positioning System technology have allowed us to measure the continent’s rigidity with greater accuracy. We found that such local deformations are too slow to be picked up by our instruments but serendipitously discovered how the continent deforms at a larger scale."

The Global Positioning System, or GPS, uses 24 satellites to make precision measurements of locations on the Earth’s surface. In addition to the familiar application of helping travelers navigate in unfamiliar territory, GPS can measure physical alterations to the territory itself, with an accuracy of about a millimeter per year. Earth scientists have placed hundreds of GPS stations around the continent to assist with studies of how the figure of the planet changes over time. Some of those changes, Calais said, are already well understood – or so the researchers thought.

"We already knew that parts of North America are slowly rising due to an effect we call post-glacial rebound," he said. "The weight of the ice sheets that once covered the region was large enough to deflect the surface beneath it, lowering its elevation. Now that the ice sheets are gone, the land is recovering slowly – so slowly that it is still rising today, 10,000 years later."

What the team’s study showed, however, was that the region is not merely rising in elevation, but also expanding outward at a slow but noticeable rate.

"You might imagine trying to flatten out a plastic bowl by pushing it up from the bottom," Calais explained. "As you push up in the middle, it rises, but it also forces the edges outward. That’s very likely the sort of effect we’re observing because other explanations do not fit the data very well."

Calais cautions, however, that the observed movement is quite small compared to other typical earthquake-generating tectonic activity; land along the San Andreas fault, for example, moves 30 times faster per year or more than the newly measured movement. The scientific significance of the study, he said, lies primarily in the discovery of a new behavior in the Earth’s crust.

"We still do not know how this behavior is connected to earthquakes," Calais said. "It is a relatively slow motion, and it’s too soon to say whether it affects fault lines far away in Missouri or anywhere else for that matter. But, more interestingly, this behavior is telling us something about the strength of the interior of our planet, a fundamental property we have no way of measuring directly. The stronger it is, the longer it takes for post-glacial effects to dissipate.

"We are starting to decipher a subtle message left at the earth’s surface by processes happening thousands of kilometers under our feet. The planet still has lessons to teach us."

This research was funded by the U.S. Geological Survey.

Writer: Chad Boutin, (765) 494-2081, cboutin@purdue.edu

Source: Eric Calais, (765) 496-2915, ecalais@purdue.edu

Purdue News Service: (765) 494-2096; purduenews@purdue.edu

Chad Boutin | EurekAlert!
Further information:
http://www.purdue.edu

More articles from Earth Sciences:

nachricht Novel method for investigating pore geometry in rocks
18.06.2018 | Kyushu University, I2CNER

nachricht Decades of satellite monitoring reveal Antarctic ice loss
14.06.2018 | University of Maryland

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: AchemAsia 2019 will take place in Shanghai

Moving into its fourth decade, AchemAsia is setting out for new horizons: The International Expo and Innovation Forum for Sustainable Chemical Production will take place from 21-23 May 2019 in Shanghai, China. With an updated event profile, the eleventh edition focusses on topics that are especially relevant for the Chinese process industry, putting a strong emphasis on sustainability and innovation.

Founded in 1989 as a spin-off of ACHEMA to cater to the needs of China’s then developing industry, AchemAsia has since grown into a platform where the latest...

Im Focus: First real-time test of Li-Fi utilization for the industrial Internet of Things

The BMBF-funded OWICELLS project was successfully completed with a final presentation at the BMW plant in Munich. The presentation demonstrated a Li-Fi communication with a mobile robot, while the robot carried out usual production processes (welding, moving and testing parts) in a 5x5m² production cell. The robust, optical wireless transmission is based on spatial diversity; in other words, data is sent and received simultaneously by several LEDs and several photodiodes. The system can transmit data at more than 100 Mbit/s and five milliseconds latency.

Modern production technologies in the automobile industry must become more flexible in order to fulfil individual customer requirements.

Im Focus: Sharp images with flexible fibers

An international team of scientists has discovered a new way to transfer image information through multimodal fibers with almost no distortion - even if the fiber is bent. The results of the study, to which scientist from the Leibniz-Institute of Photonic Technology Jena (Leibniz IPHT) contributed, were published on 6thJune in the highly-cited journal Physical Review Letters.

Endoscopes allow doctors to see into a patient’s body like through a keyhole. Typically, the images are transmitted via a bundle of several hundreds of optical...

Im Focus: Photoexcited graphene puzzle solved

A boost for graphene-based light detectors

Light detection and control lies at the heart of many modern device applications, such as smartphone cameras. Using graphene as a light-sensitive material for...

Im Focus: Water is not the same as water

Water molecules exist in two different forms with almost identical physical properties. For the first time, researchers have succeeded in separating the two forms to show that they can exhibit different chemical reactivities. These results were reported by researchers from the University of Basel and their colleagues in Hamburg in the scientific journal Nature Communications.

From a chemical perspective, water is a molecule in which a single oxygen atom is linked to two hydrogen atoms. It is less well known that water exists in two...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Munich conference on asteroid detection, tracking and defense

13.06.2018 | Event News

2nd International Baltic Earth Conference in Denmark: “The Baltic Sea region in Transition”

08.06.2018 | Event News

ISEKI_Food 2018: Conference with Holistic View of Food Production

05.06.2018 | Event News

 
Latest News

Novel method for investigating pore geometry in rocks

18.06.2018 | Earth Sciences

Diamond watch components

18.06.2018 | Process Engineering

New type of photosynthesis discovered

18.06.2018 | Life Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>