Carl E. Renshaw - Dartmouth Earth Sciences

Experimental Geomechanics

Why are there deep earthquakes?  How are the polar ice sheets breaking apart?  How did the lineaments we see on other planets form?

Theses and other similar questions motivate our work in experimental geomechanics.  Simply put, we seek to understand how rocks breaks.

Fractures are the most ubiquitous structural feature in rock, occurring in all rock types and settings.  They strongly control the Earth's hydrologic systems by controlling the shapes of coastlines, drainage systems, lakes, and subsurface flow pathways.  They are also responsible for many of our most spectacular scenery, including Yosemite, Zion, Arches National Parks, among others.

Geologists have argued over the origins of rock fractures for more than a century.  In their review on "Progress in Understanding Jointing Over the Past Century, Pollard and Aydin note that as early at 1882 Grove Karl Gilbert, participant in the Powell Surveys , charter member of the U.S. Geological Survey, former President of the Geological Society of America, 

"stirred a controversy by asserting that no 'satisfactory explanation has ever been given on the origin of the jointed structure in rocks' and he had none to propose."

Despite this a long period of study and the obvious critical importance of understanding rock fracture to such essential processes as earthquakes, natural resource recovery, and climate change, there remains much we still do not understand about rock failure.  The link below explores just one of the many examples that demonstrate our limited understanding of rock fracture.

Deep Earthquakes

Why do we think we can make progress in advancing our understanding of rock fracture where others have not?  The answer is simple.  We have one critical advantage -- in our work we can use ice as a model material for rock.


The advantage of using ice is that its transparent.  This means we can directly observe the failure process.  And in this case, seeing is not just about believing, but also about understanding.

Below is an example of our observations of fracture growth in ice (Iliescu and Schulson, Acta Materialia, 52:5723, 2004).


Our experiments are done in Dartmouth's one-of-a-kind ice research laboratory.

Does what we observe in ice apply to rock?  While there are important differences between ice, both are crystalline solids and with respect to brittle failure we not been able to detect any significant differences between the failure process in the two materials.  Ice is about an order of magnitude weaker than rock, but when we account for these differences in material properties, the graph below shows that the variation in failure strength, for example, is indistinguishable from rock (Renshaw and Schulson, Nature, 412:897, 2001)


You can learn more about our work in the references below.  More are on the way.

Schulson, E.M., Fortt, A.L., Iliescu, D., Renshaw, C.E., On the role of frictional sliding in the compressive fracture of ice and granite:  Terminal vs post-terminal failure, Acta Materialia, in press..

Schulson, E.M., Fortt, A.L., Iliescu, D., Renshaw, C.E., Brittle failure envelope of first-year Arctic Sea Ice, Journal of Geophysical Research – Oceans, in press.

Renshaw, C.E., Schulson, E.M., Plastic faulting:  Brittle-like failure under high confinement, Journal of Geophysical  Research, 109(B9):B09207, 2004.

Renshaw, C.E., Schulson, E.M., Universal behaviour in compressive failure of brittle materials, Nature, 412:  897-900, 2001.