Fault Behaviour

Unusual Fault Behavior

Some geologic faults that appear strong and stable, slip and slide like weak faults. Now an international team of researchers has laboratory evidence showing why some faults that 'should not' slip are weaker than previously thought. Their findings are detailed in the December 17, 2009 edition of the journal Nature.


The Case of Low-Angle Normal Faults

"Low-angle normal faults--faults that dip less than 45 degrees--are a problem," said Chris Marone, professor of geosciences, Penn State. "Standard analysis shows that these faults should not slip because it is easier to form a new fault than to slip on this orientation."

However, field evidence shows that low-angle normal faults do slip. One explanation is that they act more like weak faults, slipping and sliding. Previous laboratory experiments indicated that the fabric of the rocks in the fault had too much friction for the faults to slide easily. The researchers wanted to test the material in the fault in a form closer to what occurs naturally.


Fault Friction - Laboratory Testing

"The standard way to test the friction of the rocks in a fault is to take some of the rock and grind it up into a powder," said Marone. "The powder is then tested in an apparatus that applies shear forces to the materials measuring the amount of force it would take to move sides of the fault."

These conventional measurements indicated that low-angle normal faults have too much friction to move, but the reality is that they do move.


The Zuccale Fault

Cristiano Collettini, researcher at Geologia Strutturale e Geofisica, Universita degli Studi di Perugia, Italy, got his way because of an unusual low-angle normal fault on the Isle of Elba. The Zuccale fault sits exposed on the beach so it is easy to gather large amounts of rock.

"Cristiano was insistent on checking the shear forces under conditions as close to the natural situation as possible," said Marone. "I thought what he wanted to do was impossible because the rocks cannot easily be cut into a shape that we can work with. We need a prismatic wafer of the material."

The researchers first ground the material and tested the mechanical properties of the powder in the conventional way. The ground powder did produce sufficient frictional forces to prevent slipping. This is what they expected.

"Normally the rock we use from fault zones comes from below the surface and we only get small amounts to work with," said Marone. "With the samples from Elba we could use a had rotary cutter and carve a wafer from the rock with the same orientation that would slip in the ground."

The wafers were about two by two inches square and an eighth of an inch thick. The researchers found that material prepared in this way was very weak when sheared in one direction moving almost like a deck of cards pushed in opposing directions.


The Role of Talc and Montmorillonite Clays

The reason for the low friction is small patches of talc and clays like montmorillonite that allow the material to slide.

The researchers, who also include Andre Niemeijer, former Penn State postdoctoral fellow now at Instituto Nazionale Di Geofisica Vulcanologia, Rome, and Cecilia Viti, Universita degli Studi di Siena, Italy, note, in the Dec. 17 issue of Nature that "fault weakness can occur in cases where weak mineral phases constitute only a small percentage of the total fault rock and that low friction results from slip on a network of weak phyllosilicate-rich surfaces that define the rock fabric."

Talc and many chemical varieties of clay minerals these clays are often found in fault materials. In areas where connected layers of weak minerals occur, geologists have concluded that the connections create weak fabric that enables easy slippage. But when clays or talc create intermittent flake-like surface coatings, they provide far more slip than when they are simply powdered. The discontinuous flakes and coatings that the researchers found were previously considered insufficiently complete to weaken the fault.

"These low-angle normal faults do not look like they will do anything but creep along, but they could have earthquakes," said Marone. "There are places in central Italy, for example, where faults like this have had small earthquakes."


Application to Other Faults

"These findings may be widely applicable to other fault settings, depending on the local stresses creating the faults," said James Dunlap, program manager in NSF's Geosciences Directorate. "In this way, this work is exciting and timely for those in the earthsciences community, as it may well prove advantageous to society, especially for those communities, on or near fault lines, most susceptible to earthquakes."

Carbonate Lava

Carbon Dioxide Becomes Solid at Surface of Oldoinyo Lengai

East African Rift Volcanoes

Science has unearthed the secret to what might have been alchemy at Oldoinyo Lengai volcano in Tanzania.

There, in the ancient East African Rift at a place known to local Maasai people as the Mountain of God, Oldoinyo Lengai spews forth carbon dioxide-laden lavas called carbonatites. The carbonatites line the volcano's flanks like snowballs.


The World's Only Carbonatite Eruptions

Oldoinyo Lengai is the only place on Earth where carbonatites currently erupt--and where carbon dioxide from a volcano doesn't vanish into thin air as a gas.

In a paper published this week in the journal Nature, scientists report the results of a study of Oldoinyo Lengai's volcanic gas emissions, sampled by the team during a carbonatite lava eruption.


How Carbonatite Magma is Produced

"We now know the origin of one of the most peculiar magmas on Earth," said William Leeman, program director in the National Science Foundation (NSF)'s Division of Earth Sciences, which funded the research. "These scientists have found that, based on new studies of the chemistry of gas emissions at Oldoinyo Lengai, a very small amount of melting of Earth's mantle, akin to that beneath mid-ocean ridges, can produce carbonatites."

The carbonatites consist of high amounts of carbon dioxide, some 30 percent. Unlike most lavas that are liquid at temperatures above 900 degrees Celsius (1,652 F), carbonatites are much cooler and erupt at only 540 degrees Celsius (1,004 F). However, they're extremely fluid, with a viscosity like that of motor oil.


Sampling Volcanic Gases

"We were able to collect pristine samples of the volcanic gases because Oldoinyo Lengai was erupting and under tremendous magma pressure at the time," said Tobias Fischer, a volcanologist at the University of New Mexico and lead author of the paper. "There was minimal air contamination."

"The gases reveal that the carbon dioxide comes directly from the upper mantle, just below the East African Rift," said David Hilton, a geochemist at the Scripps Institution of Oceanography.

"These samples of mantle gases allow us to infer the carbon content of the upper mantle where the carbonatites are produced."

It's about 300 parts per million, a concentration virtually identical to that measured below mid-ocean ridges.


Nephelinite Magmas

The finding is significant, said geochemist Bernard Marty of the CNRS-CRPG (Centre National de la Recherche Scientifique-Centre de Recherches Pe'trographiques et Ge'ochimiques in France), "because it shows that these extremely bizarre lavas and their parent magmas, called nephelinites, were produced by melting of typical upper mantle minerals--which don't have a high carbon dioxide content."

Previous research, mainly based on laboratory experiments, suggested that a higher carbon dioxide content is needed to produce nephelinites and carbonatites.


High Sodium Magmas

"Oldoinyo Lengai magmas also contain an unusually high amount of sodium, up to about 35 percent," said Pete Burnard, a geochemist at CNRS-CRPG.

"It's this sodium content that makes the Lengai carbonatites solid rather than gas at the surface. At all other volcanoes on Earth, carbon dioxide 'degasses' into the atmosphere without forming the sodium-rich carbonatite magmas of Oldoinyo Lengai."

Not all Oldoinyo Lengai's carbon dioxide becomes carbonatite, however. Like other volcanoes, Oldoinyo Lengai does emit carbon dioxide into the atmosphere as a gas.

The scientists conclude that the upper mantle below the continents and the oceans is a uniform and well-mixed reservoir in which the compositions and abundances of carbon dioxide and other gases like nitrogen, argon and helium are essentially the same.

Hilton, Marty and Burnard are co-authors of the Nature paper.

The University of New Mexico Research Allocations Committee (RAC), Institut National des Sciences de l'Univers, Centre National de la Recherche Scientifque (INSU-CNRS), and Programme Intérieur de la Terre also funded the research.