April 19, 2012 by Chuck Bailey
This past Friday, the Earth Structure & Dynamics class assembled behind the Geology Department and then poured themselves into 3 vans and headed west to the Appalachian Mountains. Counting supernumeraries we totaled 36 people, which qualifies as a mobile mob.
On this trip, students practice and hone their skills doing geology in the field and become familiar with the tectonic history of the Appalachian Mountains. Most years the trip runs in early April – a beautiful time of year in the mountains, but a time of year in which the weather can range from outright frozen to quite delightful. This year’s trip reveled in warm and dry weather. We camped in a valley cradled by Cambrian quartz sandstones – it was glorious.
Just how does this operation go down? Typically, we pull up to an outcrop or an overlook and the students debouch from the vehicles. I hand out sheets of paper with an array of tantalizing (or perhaps not so tantalizing) questions. Students work in teams of two and answer the questions; after the work is complete we discuss our answers and try to place the outcrop into the regional tectonic framework. We spent most of Saturday afternoon hiking and doing geology along the western slope of the Blue Ridge Mountains and from those data constructed a cross section of the geologic structure beneath us.
Back in the Paleozoic, when I was a student, we’d arrive at an outcrop and the professor would 1) prod us with questions until somebody shouted out the correct answer or 2) give us a mini- (or not so mini) lecture while we all stood around, passive and bored. My little worksheets are intended to get everybody thinking about geology. Students typically adopt a team name for the weekend, here is a sampling from this year’s trip: Gneiss-Gneiss Baby, team Diamonds in the Rough, team Paxtram, team Cococrisp, and who could forget team Big Boudins.
The days are running out on the spring semester and students are mighty busy, but this weekend field trip, far from the madding crowd, is an important piece in the education of W&M geologists as no amount of classroom or lab work can replace the hands-on experience of doing geology in the field.
March 6, 2012 by Chuck Bailey
My first post as a W&M blogger came after our Utah field season during the summer of 2008. Indeed, we lived the high life that July, conducting geologic research on the Fish Lake Plateau, a broad and broken highland situated nearly 2 miles above sea level. My undergraduate research students: Trevor Buckley, JoBeth Carbaugh, and Graham Lederer have graduated and moved on to success in graduate school and careers beyond the academy.
One of the areas in which we worked was Mount Hilgard, an iconic mountain that tops out at just over 3,500 meters (11,500 feet). What makes Mount Hilgard iconic is its curious shape. From the north the peak rises to a craggy summit while from the east or west it is asymmetric with a gentle southern flank and a steep northern rampart.
Mt. Hilgard owes its shape to the underlying geology, the peak is capped by a 250 meter thick sequence of volcanic rock erupted from gassy volcanoes 24 million years ago. The volcanic rocks unconformably overlie an older sequence of thinly-bedded mudstone and limestone. During the last 5 million years these rocks were uplifted, faulted, and tilted such that they are inclined about 10˚ towards the south. Gravity, water, and ice conspired to erode the bedrock: the volcanic rock stands as a hulking edifice that, over time, sheds copious debris onto the weak foundation of sedimentary rock below.
On July 23rd, 2008 Graham Lederer and I set off on our last traverse for the summer and climbed Mt. Hilgard to collect samples. On our descent we transited a block field composed of rubble accumulated at the base of the cliffs above, hiking down a block field is precarious work. A small bedrock outcrop (C99) provided a respite from the tortured descent. Further downslope (C100) my field notes tersely read, “bloody block field with mosquitoes- no bedrock here”. Much to our chagrin, surficial deposits effectively masked the underlying bedrock.
Take a look at the latest imagery (from July, 2011) of Mt. Hilgard available on Google Earth. A big chunk of the mountain’s east side moved downslope in the spring of 2011. In common parlance it’s a landslide, to a geologist it’s a debris flow. In 2008 we’d crossed the same patch of ground that later became entrained in this flow. Geology is a science in which time and place are important. Our traverse was in the wrong place, but at the right time!
This mass movement came downslope with enough velocity to flow up and over a small hill and continue a bit further down the other side. The flow terminates in a series of lobes about 800 meters (1/2 mile) below its origin. To put the scale of this mass movement into context, I plotted the outline of the flow onto an image of William & Mary’s campus. In this scene, the flow originated between Sadler Center and the Integrated Science Center. It would have flowed east, taking with it most of Old Campus, including the Geology Department, and grinding to a halt just before reaching North Boundary Street. The Mt. Hilgard event likely moved half-a-million cubic meters of material—that is quite a pile. These mass movement events play a large role in shaping this alpine landscape.
This summer I’ll be back on the Fish Lake Plateau with William & Mary students and we’ll head for the eastern slope of Mount Hilgard. Here we’ll do geology—examining the newly exposed bedrock and quantifying the 2011 mass movement event. No doubt we’ll stand on the debris flow surface and wonder, that if the time and place had both been wrong, would we have been able to outrun this debris flow as it roiled down Hilgard’s menacing flank? What do you think?
February 22, 2012 by Chuck Bailey
We are deep into the spring semester and my teaching/administrative duties are gobbling up most of my weekdays and nights. There is hardly a moment for research during the week, so research gets done on the weekends. I spent this past Saturday in the field searching for Hylas, the Hylas Fault Zone that is, not Hylas of Greek mythology. As the story goes, the mythological Hylas journeyed with the Argonauts, but while on an errand to fetch water he was abducted by nymphs. His mates searched in vain and Hylas was left to his fate. In 1976, Andy Bobyarchick described a zone of both ductile and brittle fault rocks near the small crossroads of Hylas, Virginia (~25 km northwest of Richmond) and the Hylas Fault Zone entered the geological literature.
Jump forward three and a half decades and John Hollis, a W&M geology major, has commenced his thesis research on understanding the brittle deformation history of the Hylas Fault Zone. During the past six months, John sought out exposures of bedrock in stream valleys and quarries to understand the structural geology of these rocks. On Saturday we put a canoe into the North Anna River: a floating traverse across the eastern Piedmont in search of the Hylas Fault Zone.
For February, the weather could not have been better (although a bit on the chilly side for nymphs). The North Anna was similarly cooperative, flowing at a comfortable rate that easily floated the boat, but left most of the bedrock outcrops above the waterline. We glided by a veritable menagerie of metamorphic rocks in the enigmatic Goochland Terrane, examining thinly layered gneisses, coarsely banded gneisses, pegmatitic gneisses, biotite-garnet schists, and amphibolites. We measured the orientation of ductile and brittle structures in these impressive channel and riverside cliff outcrops.
Halfway into the trip we lunched at a large low outcrop of biotite-garnet gneiss cut by two generations of brittle fractures. Immediately northeast of the river stood the stone foundations and derelict chimney of Jericho Mills. In May 1864, Jericho Mills formed a major crossing point for the Union Army as it pressed south against the Confederate Army. A photo dated May 24th 1864 by Timothy O’Sullivan shows a pontoon bridge spanning the North Anna just upstream from the mill and the same outcrop upon which we lunched, measured structures, and collected a sample 148 years later. For me, this nexus of human history, geologic history, and now my personal experience defies easy description but is somwehere between surreal and cool. O’Sullivan’s photos illustrate that the terrain bordering the North Anna was mostly open country during the Civil War: today that same terrain is cloaked in forest and the vestiges of past human activities are greatly muted. On this tranquil float, it was difficult to conceive that we transited a battleground on which more than 5,000 Americans were either killed or wounded during those four days of fighting long ago.
The North Anna flows with a subdued gradient across the Piedmont, but in the last few kilometers before arriving at the Coastal Plain the river drops through a sequence of rapids that define the Fall Zone. These rapids got our attention as we coursed quickly past the bedrock in whitewater. After negotiating the big Fall Zone rapid we caught our breath on the broken rocks on the east side of the Hylas Fault Zone. These rocks have enjoyed more deformation than most: when these rocks were hot, but not molten, they were sheared and squeezed. Later these rocks were fractured, faulted, and mineralized. A suite of narrow jagged veins angle through the rock, these veins look suspiciously like pseudotachylite. Pseudo what? Pseudotachylite is a dark glassy fault rock formed by frictional heating, melting, and subsequent quenching. These veins may record paleoearthquakes in the Hylas Fault Zone.
Just below the rapids, we quietly glided over the Fork Church Fault, the eastern boundary to the Hylas Fault Zone. Although we never saw the Fork Church Fault, it is a whopper of a normal fault with thousands of meters of slip and it forms the boundary fault to the Taylorsville Mesozoic basin. We were delighted by the mossy outcrop of boulder conglomerate that signaled our arrival in the Triassic rift basin. The character of the North Anna changed dramatically as well: the Fall Zone rapids were behind us, bedrock outcrops became sparse, and the river birches formed a nearly unbroken canopy above the river. Another research adventure complete, we pulled the canoe off the water in growing twilight.
We learned much from our float trip. John has a heap of new data to chew on and incorporate into his thesis. Some of our observations don’t jive with previous studies and we’ve got a host of new questions about the regional geology. Geologists are fortunate in that the field is commonly where we go to collect our primary data. Although time consuming, fieldwork is often sublime and beautiful.
February 3, 2012 by Chuck Bailey
William & Mary is celebrating its 319th birthday this weekend. For an institution of higher learning in the western hemisphere, 319 years certainly qualifies as venerable. Although what qualifies as old depends on your perspective; geologists typically take a long view on time. It’s easy to do when you consider that the Earth’s history stretches out to well over four billion years.
Consider the photograph below, a little snippet of bedrock cropping out high above Harris Cove in the Blue Ridge Mountains of western Virginia. The rock is a gneiss (pronounced- nice!), a curiously colorful granitic gneiss. This is an ancient rock, it crystallized and cooled into a granite, far below the Earth’s surface, some 1.15 billion years ago. It was later metamorphosed and transformed to a coarsely-foliated gneiss. By the late Neoproterozoic (~550 million years ago) the rock was exposed at the surface and then buried beneath a sequence of lava flows and sedimentary deposits. At some point the rock underwent another transformation and the iron-bearing minerals were altered into a new mineral, epidote—a distinctive greenish silicate, while the feldspar turned pink. Indeed, it’s an old rock with quite a history.
Field geologists take oodles of rock pictures and invariably we include some marker to provide ‘scale’ for the picture. I’ve used rock hammers, lens caps, pocketknives, and even doughnuts to provide scale. Coins are a time-honored standard, in part because the local currency imparts a certain authenticity to your whereabouts (e.g. loonies in Canada, kangaroo dollars in Australia, and Eva Perón pesos in Argentina). Take another look at the coin I used for scale. Just whose faces are those emblazoned on the not-quite-round coin? Why, it’s our very own King William (GVLIELMVS) & Queen Mary (MARIA) on a weathered threepence from the days of the Glorious Revolution in the late 17th century. The coin is 18 mm in diameter (roughly the size of a dime) and a gift from my coin-collecting father.
It’s nice (or is that gneiss?) to be old and good to see William & Mary out and about! Happy Charter Day.
January 23, 2012 by Chuck Bailey
One of the courses I am teaching this term is Earth Structure & Dynamics (GEOL 323), a second-level geology course, and a required class for all Geology majors. This course combines structural geology, tectonics, geophysics, and a pinch of historical geology. Thirty-four students enrolled in this year’s class, that’s plenty, and free seats are hard to come by in the classroom.
As you might guess from the course title, I want students to be well versed at recognizing and understanding how earth structures form. What is an earth structure? Anticlines, reverse faults, half-grabens, stretching lineations, and boudinage (to name a few). This requires three-dimensional thinking, and 3D thinking requires much practice. On Friday, I projected an image of a craggy mountain side in the Needles Mountains of southwest Colorado and asked the class to 1) sketch the geology, 2) identify the salient geologic structures, and 3) work out the temporal history of the geologic events. I annotated the image such that different rocks and surface landforms are illustrated. Talus is rocky debris that has accumulated at the base of the cliffs: I want the students to “see through” the talus to visualize the underlying bedrock structures.
The limestone crops out above both the gneiss and quartzite, and it is the only one of the bunch that’s not been metamorphosed, therefore it is the youngest rock in the scene. But which of the metamorphic rocks is the oldest? The class struggled with this one. From the image alone the age relations are not discernible, but notice the quartzite is weakly metamorphosed (in contrast to the gneiss, a well-metamorphosed rock) and the original layering (bedding) is evident. Based on this information the quartzite is younger than the gneiss.
The boundaries between the different rock types are structures (referred to as geologic contacts). The limestone overlies the older metamorphic rocks along an unconformity, whereas the gneiss and the quartzite are juxtaposed across a fault (but just what type of fault?). Many students overlooked the significance of these geologic structures, but with practice, these abominations will stop!
The photo is two dimensional, but how best to help bring out the third dimension? Google Earth to the rescue—we zoomed from space into the Needle Mountains to about the same vantage point as the field photo was taken. With the tilt, pan, and zoom function it was easy to see the steep slopes of talus and that the limestone forms an erosional cap above the older metamorphic rocks. We will use overlays and tours in Google Earth to hone our visualization skills in the coming weeks. I want the Earth Structures students to translate from 2D to 3D without a hitch, by the end of the semester they’ll be dreaming in 3D!
So what is the temporal history of the geologic events? Here is my interpretation from oldest to youngest:
- formation of the protolith for the gneiss
- burial and metamorphism to form the gneiss
- uplift and erosion
- deposition of sand that forms the protolith to the quartzite
- burial and metamorphism affects the gneiss and creates the quartzite
- faulting juxtaposes the gneiss and quartzite
- deposition of the limestone
- minor tilting
- erosion to form the modern topography
- I take the picture
What the Geology 323 students worked out was the relative timing of geologic events, a fundamental skill for earth scientists. The next logical step would be to determine when, in absolute terms, these events occurred—that comes later in the semester when we delve into geochronology.
January 9, 2012 by Chuck Bailey
In my last post I discussed the Virginia earthquake that shook eastern North America on August 23rd, 2011. Here is the second part of that story: unfortunately the answer to the question I pose in the title is not particularly satisfactory. We cannot answer the question about whose fault is it (or more precisely which fault was it?) because the quake occurred along a buried and previously unrecognized fault in the Virginia Piedmont.
What caused the Quake?
Simply put, rocks in central Virginia are under stress, and on August 23rd, 2011 the stress level exceeded the local rock strength and a previously unknown fault ruptured. The maximum compressive stress that produced the earthquake was likely oriented west-northwest/east-southeast. What is the cause of that stress? Virginia lies astride eastern North America’s passive margin and is thousands of kilometers from active tectonic boundaries. The vast majority of the world’s earthquakes occur at or very near tectonic plate boundaries. The causal mechanism for the Virginia quake is not clear. As Seth Stein at Northwestern University puts it, parts of eastern North America may be better described as a “passive-aggressive” margin. We all know that passive-aggressive types are difficult to deal with and sometimes hard to understand—the Virginia Piedmont is similar. There is much left to learn about these intraplate earthquakes.
In the days after the August quake some postulated that fracking operations may have been responsible for the Virginia earthquake. Hydraulic fracturing, or fracking, is a process by which fluids are pumped into a well and pressurized until the surrounding bedrock, at a specified depth, is fractured. By increasing the fluid pressure, the effective stress on the subsurface rocks is reduced (in essence the pressurized fluid “pushes back” on the natural stresses generated from the overlying and surrounding rocks). When the effective stress exceeds the tensile strength of the rock, fracturing occurs. Hydraulic fracturing is being used to greatly increase the permeability of subsurface formations in the Appalachian basin to aid natural gas extraction. Many thousands of gas wells have been drilled in Pennsylvania, West Virginia, and Ohio over the past five years and the region has experienced nothing less than an energy boom.
Pumping fluids into the subsurface has triggered earthquakes (e.g. the Rocky Mountain Arsenal in the 1960s, the Rangely, Colorado oilfield between 1969-1973; the Basel, Switzerland geothermal field in 2006, to name a few), but these earthquakes are proximal to the injection sites (<10 km). Many localities where fracking and/or fluid injection into wells is occurring are rightly concerned about the prospect of seismic activity.
This is not the case in the central Virginia Piedmont as the nearest hydraulically fractured gas wells are nearly 200 kilometers away in West Virginia. Most hydraulic fracturing in the Appalachian basin occurs at depths of 1,000 to 2,000 meters (~4,000 to 8,000 feet); the hypocenter for the Virginia quake was considerably deeper, between 4 and 5 km depth (2.5 to 3 miles). The subsurface geology between the Appalachian basin (layered sedimentary formations) and the Virginia Piedmont (complexly deformed metamorphic rocks) is strikingly dissimilar as well. Let’s be clear, hydraulic fracturing was not responsible for the 2011 Virginia earthquake.
While the August temblor was larger than any previously recorded in the region, this quake was nothing unusual as a number of Virginia faults have slipped in the recent geologic past (remember, however, that recent for a geologist can be a million years or so). In my early years as a W&M faculty member, I worked on a project with my colleague Rick Berquist (Virginia Division of Geology and Mineral Resources) in which we examined reverse faults in the eastern Piedmont that juxtapose old igneous rocks over Neogene marine sediments. The earthquakes that caused these faults to slip occurred in the last few million years and the rupture occurred pretty much at the Earth’s surface. Although they seem novel to us, earthquakes are nothing new in Virginia.
Finding Fault at the North Anna Power Station
The shaking from the 2011 Virginia earthquake was vigorous and the nuclear reactors at the North Anna Power Station automatically shut down. Strong motion accelerometers at the North Anna site recorded peak accelerations of 2.6 m/sec2. The Earth’s gravity accelerates objects at about 9.8 m/sec2 (32 ft/sec2), so jolts from the Virginia quake accelerated the ground at just over 25% of Earth’s gravity (g). These values need to be put into perspective; most people think only of acceleration in terms of how fast an automobile ramps up its speed when the accelerator pedal is crushed. Consider a muscle car that rushes from 0 to 60 mph in 5 seconds- that’s an acceleration of 18 ft/sec2 (just 2/3s of a g) but more than sufficient to push you back in your seat. The ground acceleration coupled with the high frequency of the shaking from the Virginia quake was significant and conspired to do plenty of damage in Louisa County.
Part of the concern is that the shaking at the North Anna Power Station exceeded its original design value. I went back and examined the reports prepared by engineering firms for Vepco (the forerunner of Dominion, the power company that runs the North Anna facility) back in 1969 and 1971. In the section on Operating Basis Earthquake they write “On the basis of the seismic history of the area, it does not appear likely that the site would be subjected to ground motions above Intensity V during the life of the proposed facility.” As I noted in the previous post, the shaking experienced in central Virginia from the 2011 quake reached Intensity VII to VIII. Oops!
The report also states, “that the maximum horizontal ground acceleration… at the site due to such a shock would be no more than 12% of gravity”. The 2011 event exceeded that by a factor of two, but by all accounts the facility endured the shaking with relatively modest damage and the safety systems and procedures worked as designed. Although there were problems with the first seismic evaluation, we should be mindful that the report was written prior to the modern seismological network that has characterized the Central Virginia seismic zone over the past three decades.
In 1970, William & Mary Geology professors Steve Clement and Bruce Goodwin along with John Tyler Community College professor John Funkhouser visited the site and recognized a fault cutting through the bedrock in the excavations. At the North Anna site four approximately circular holes were excavated for the reactor foundations (although only two reactors were constructed), each hole was about 40 m (125 ft) in diameter and the total distance between the excavations from unit 1 to unit 4 was ~300 m (1000 ft). The fault, a east-northeast striking structure dipping 40˚ to 50˚ to the north, cut through all four excavations. Go figure.
The geological consultants hired to evaluate the site either never recognized the fault or chose not to disclose its presence. The original site assessment glossed over salient geological structures. Vepco’s Final Safety Analysis Report (from March 1973) stated “Faulting at the site is neither known nor is it suspected: all available information tends to confirm the continuity of the strata”.
Commenting on their 1970 visit (which occurred 3 years prior to the Final Safety Analysis Report) Steve Clement noted that “the fault was a classic example that I’d expect most undergraduate students to recognize” and Bruce Goodwin said of a Vepco representative “he’d have to have been asleep not to know there was a fault”.
A supplemental report was issued in August 1973 that focused on the geology of the now disclosed fault. The report opens with what I’d describe as a politic statement “During excavation for Units 3 and 4 at the North Anna Power Station, a minor shear zone of a type commonly found in the Piedmont was exposed. While technically a fault, since relative motion has occurred along this zone, it is not considered in any way to represent a fundamental geologic structure of the region.”
They really did not want to call it a fault. But a fault is what it is. Nobody wants a fault under a nuclear power plant.
The discussion now focused on whether the fault was a seismically capable structure. The Nuclear Regulatory Commission defines a capable fault as one that has experienced movement at or near the ground surface at least once within the past 35,000 years or movement of a recurring nature within the past 500,000 years or as a fault with instrumentally determined seismicity associated with it. Determining the capability of a fault is not necessarily easy. The supplemental Dames & Moore report is extensive and they employed an array of techniques to determine the age of faulting. Their conclusion, which was supported by a number of geologists with no vested interest in the power station, was that the fault was a minor normal fault that had reactivated an older reverse fault and that the last motion on the structure occurred ~200 million years ago. These findings were in lock-step with the 1970s-era understanding of eastern North America’s geologic history. The power station (Units 1 and 2) was built and Vepco did pay a modest fine for making false statements.
Nearly forty years on and one moderate earthquake later what can be concluded of the fault under the North Anna Power Station?
- This east-northeast striking fault is a minor structure in the architecture of the Piedmont
- The fault was not favorably oriented to have been reactivated by the stresses that produced the 2011 quake
- The gouge within the fault developed in the brittle upper crust and in the presence of abundant fluids
- The kinematics of youngest structures in the fault zone are consistent with reverse slip (not normal faulting)
- Faulting is younger than previously supposed
My conclusions are based on evaluating the existing reports, maps, and photos of the site. It would be glorious to study the exposures present in the early 1970s when the site was excavated and exposed. Today, there are more and better techniques for evaluating slip on faults as well as determining when those faults last slipped. Perhaps, the jolt from 2011 Virginia earthquake was sufficient for geologists to abandon some long-held assumptions about the passive character of eastern North America and reexamine the region anew.
December 22, 2011 by Chuck Bailey
As the year comes to a close it is a fine time to reflect on the 2011 Virginia earthquake. It’s been four months since the Virginia earthquake jolted eastern North America, and we now know more about what happened. This moderate-size (Mw=5.8) quake–felt by millions of people from Alabama to Quebec–caused significant damage in Louisa County, cracked both buildings and nerves in Washington D.C., and served notice that there is still some kick left in these ancient rocks.
What Happened on August 23rd?
At 1:51:04 p.m. (EDT) a fault ruptured at a point some 4 to 5 km (2.5 to 3 miles) below the Earth’s surface in Louisa County, Virginia (~60 km northwest of Richmond). As one side of the fault slid past the other, seismic waves radiated outward from the source area. The primary waves (P-waves) raced away at nearly 6 km/second: sweeping through Richmond 11 seconds after the quake, passing through Williamsburg in 20 seconds, and arriving at the West Coast in about 5 minutes. The primary waves were followed by shear waves and salvos of surface waves, these were the jolts that people felt. On the William & Mary campus; shaking perceptible to humans lasted about 20 seconds. At the North Anna Nuclear Power Station, 21 km from the epicenter, peak ground accelerations reached ~250 cm/sec2, more than sufficient to damage unreinforced masonry structures in the epicentral region.
The Virginia temblor was a moderate earthquake. Worldwide there have been 344 earthquakes of magnitude 5.8 or greater this year, which averages out to about one quake of this size (or larger) per day somewhere in the world. What makes this quake special is that it was the largest quake to rock the eastern United States in over a century and was felt by more people than any other quake in U.S. history. At the recent American Geophysical Union meeting, Shao and others report a seismic moment of 5.75 x 1017 Newton meters for the quake, which translates into ~35 terajoules of energy released (for comparison, World War II-era atomic bombs packed an energy punch of 50 to 90 terajoules).
The nature of seismic wave first arrivals at seismic stations helped define both the geometry and type of fault that slipped. The diagram to the right is a first motion diagram, in essence a stereographic projection that forms a visual representation of the fault style and defines two possible fault orientations for the Virginia earthquake. For the uninitiated these diagrams are confusing, geologists commonly refer to these diagrams as beachball diagrams.
Based on the pattern of first arrivals, the fault that slipped was a reverse fault striking to the north or northeast and dipping moderately either to the west or southeast. With these data alone the fault cannot be uniquely determined- it could be either plane. The P- and T- axes represent the axes of maximum contraction and extension respectively; in essence the Earth’s crust in central Virginia was shortened in an approximately east/west direction from the quake movement.
Within a day or so after the earthquake, seismologists from Virginia Tech and the U.S. Geological Survey had an array of portable seismometers installed in central Virginia. This equipment recorded hundreds of aftershocks. Most of these aftershocks were small (M= 1-3), but Louisa County residents certainly felt them.
The aftershock pattern clearly reveals the fault geometry: the earthquake occurred on a northeast—striking fault that dips about 50 to 55˚ to the southeast. Click on the link below to watch a short animation. The aftershocks are blue spheres, notice how they mostly line up neatly along a plane—that is the fault that slipped. The big red sphere (the August 23rd quake) plots off the plane, that quake was located by a regional network of seismometers and is not as accurately located as the aftershocks pinpointed by the locally deployed array of seismometers.
During the quake the southeastern side of the fault (hanging wall) was shoved upward with a maximum displacement of about 1 meter. The total rupture length along the fault was likely 5 to 10 kilometers. There was no rupture at the surface because displacement across the fault did not propagate all the way to the Earth’s surface. The 2011 earthquake occurred along a blind, and previously unrecognized, reverse fault in the Virginia Piedmont.
Geology of the Piedmont
The earthquake occurred in the Piedmont, a region of complex geology that is the metamorphic core of the Appalachian Mountain system. Some of these rocks originated far from North America and were later crushed against the continental margin during tectonic collision and faulted to their current location. In the past twenty years geologists have distinguished many different terranes in the Piedmont: terranes are blocks of crust with distinct geologic histories and are bound by major faults or tectonic sutures. The difference between terranes is well illustrated on the aeromagnetic map displayed in the animated map sequence below.
The 2011 Virginia earthquake occurred in the Chopawamsic terrane. Rocks in this terrane formed as volcanic and plutonic rocks in a continental arc during the Ordovician Period (~470 to 450 million years ago). This arc was likely outboard of ancient North America and was later accreted to the continent. In the late Paleozoic (300 to 280 million years ago), during the massive tectonic collision that created Pangaea, these rocks were squeezed and baked (deformed and metamorphosed) into gneisses and schists. The Chopawamsic terrane is bound on the northwest by the Brookneal/Shores fault zone and on the southeast by the Spotsylvania fault zone. Our kinematic studies of these fault zones indicate that they experienced simultaneous right-lateral wrenching and shortening when they were active in the Paleozoic. In essence, the Spotsylvania fault zone moved the Goochland terrane to the southwest and the Brookneal/Shores fault zone moved the Chopawamsic terrane to the southwest relative to the western Piedmont.
In the Triassic Period (220 to 195 million years ago) Piedmont terranes were fractured and broken during rifting which created sedimentary basins, such as the Culpeper and Richmond basins. This rifting ultimately opened the Atlantic Ocean. Traditionally, geologists have viewed the Piedmont as a relatively static region whose tectonic heyday was long past. Today, it is a gently rolling landscape mantled by thick soils, the product of slow erosion for millions of years and a seeming dearth of tectonic activity.
But as my colleague David Spears at the Virginia Division of Geology and Mineral Resources has pointed out, there are subtle clues in the rock structure of the central Piedmont that suggest recent tectonic activity. The 2011 quake was a not so subtle reminder that David is correct and we need to get our boots on the ground and eyes on the outcrop to study the region in more detail.
The Central Virginia Seismic Zone
The Central Virginia seismic zone is a region of moderate but persistent seismic activity. The first recorded quake occurred in 1774 near Petersburg and was felt throughout Virginia and North Carolina. The largest historical quake (prior to the 2011 temblor) in the central Virginia region took place in 1875 and is estimated to have been a magnitude 5.0. Estimating both the size and exact location of historic earthquakes is difficult. Geologists use the Modified Mercalli Intensity Scale to estimate the size of historical earthquakes based on eyewitness accounts and damage reports. This is a 12-point scale that employs roman numerals, with a II being a quake so small that only few people felt it, a IV being felt by many people indoors, a VI being felt by all with some damage to plaster and masonry, a VIII causes considerable damage to structures, a X destroys most structures and the ground is thoroughly cracked, and a XII equals total damage. The intensity of damage decreases away from the epicenter. The 1875 quake reached an intensity of VI to VII in central Virginia; the 2011 quake had a maximum intensity of VIII in Louisa County whereas in Williamsburg the quake’s intensity was a IV.
By the late 1970s a regional array of permanent seismic monitoring stations helped better locate and measure earthquakes in the southeastern United States. Over the past three decades there have been 47 quakes with a M≥2 in central Virginia (22 of those are aftershocks from the 2011 quake). These quakes are widely distributed and rarely correlate to mapped faults (see the animated map above). The focal mechanisms are consistent with slip on reverse faults at depths between 4 to 10 kilometers. At these depths, the rock is warm (70˚ to 200˚ C or 160˚ to 400˚ F), but solid and behaves in a brittle fashion when placed under stress.
There is more to tell, but my research students counseled me to curb my enthusiasm, as blog posts should not be too long. In the next post I’ll discuss the possible causes of the 2011 earthquake and tell the lurid history of finding fault at the North Anna Nuclear Power station.
December 14, 2011 by Chuck Bailey
The Alberene Dream Team spent the summer of 2011 in the field working to understand the geology of the eastern Blue Ridge. As summer turned into the fall semester, the team compiled their data and started to analyze the buckets of rocks we’d collected during the field campaign. In the Geology Department we cut rocks into small chips that were then glued to a glass slide, sawed once again, and finally ground down until the thickness of the rock is ~30 micrometers. For obvious reasons these microscope slides are called thin sections. Light typically passes through the thin sliver of rock and under the microscope the mineralogy and texture of the sample are revealed in a beautiful and colorful fashion. What follows is a report from each member of the Dream Team on the microscopic character of some eastern Blue Ridge rocks and what can be gleaned from thin section observations.
Alex Johnson- Yes, it is very Gneiss, just don’t take it for Granite
It was a strange twist of fate that I, the youngest member of the dream team, chose a thin section of the oldest rock unit in the study region—the Blue Ridge basement complex. The rock originally crystallized as a granite some one billion years ago. Large feldspar phenocrysts bear witness to the rock’s igneous heritage. But, a billion years is a long time and geologic hijinks happens—this rock is best described as a biotite-bearing granitoid gneiss. Biotite, muscovite, epidote, and sphene form a posse of metamorphic minerals that overprint the original texture. Quartz, an original igneous mineral, has recrystallized into strain-free aggregates with tidy and straight grain boundaries. Alas, the poor feldspars. To call them disturbed would be too kind, destroyed too cruel, but somewhere in between it lies. The feldspars have been saussuritized, hydrothermally altered to a fine-grained mixture of epidote, sericite, and zoisite. That’s what a billion years and a couple of orogenic events gets you.
Andrea Jensen- Soaking up the soapstone
Ultramafic rocks are characterized by very low silica content and are dominated by dark colored minerals with plentiful iron and magnesium. They are igneous rocks associated with magmas derived from the mantle deep within the Earth. The Alberene quadrangle is bursting with ultramafic rocks that were originally pyroxenites dominated by the mineral pyroxene with lesser amounts of olivine and plagioclase feldspar. These pyroxenites have since undergone low grade metamorphism and been transformed into carbonate-bearing soapstones. The soapstone in the Alberene area is so extensive that it has long been quarried—native peoples used this soft stone for heat resistant bowls and pipes; modern uses include chemically resistant counter tops, architectural flagstone, and as a medium for sculpture.
Controversy surrounds the origin of the ultramafic rocks in the eastern Blue Ridge. Some workers suggest these are differentiated mantle-derived magmas that intruded thinned continental crust during rifting, whereas other geologists posit that these rocks are altered bits of the mantle thrust into place during tectonic collision. Regardless of how they formed, these rocks have since enjoyed metamorphism. Soft minerals such as chlorite, talc and carbonate replaced original minerals.
Molly Hahn- A dark and dirty rock
The graphitic schist illustrated below forms layers within metasedimentary rocks of the Lynchburg Group. In hand sample it is a fine-grained black rock with well-developed foliation and rusty patches. In thin section, the majority of the rock is dark and opaque with bands of gray, elongated quartz grains and reddish brown biotite grains. The banding and elongation of minerals is called foliation—the result of mineral recrystallization and realignment during deformation. The dark materials are a mixture of graphite, a soft fine-grained platy mineral composed entirely of carbon, and iron oxide minerals. Just what is the source of carbon in this rock? One possible source is organic matter, perhaps the remains of microscopic organisms, that was deposited as seafloor mud in a dank anoxic environment. The Lynchburg Group is a thick sequence of mostly marine rocks deposited during the Neoproterozoic (~700 million years ago) long before complex shelled organisms had evolved. As the rock was buried and later metamorphosed the volatile elements in the organic matter were expelled and the carbon crystallized into graphite. The fine-grained iron oxide minerals likely came from oxidation of pyrite (iron-sulfide) during metamorphism and/or later weathering near the Earth’s surface.
Kevin Quinlan- Mistaken identity
The Scottsville Basin, located a few kilometers south of the Alberene quadrangle, is a body of sedimentary rocks that formed during the breakup of Pangaea some 200 million years ago. My research focuses on describing how the rocks in the basin have deformed over time. This sample came from a low outcrop in the James River and contains abundant quartz and plagioclase feldspar. These mineral grains are rounded, evidence that they were deposited as detrital sediments. Individual grains are held together by a fine-grained matrix, giving the rock a clastic texture. This texture is a good indication that the sample began as a sedimentary rock, probably sandstone. Notice, however, that the matrix between grains is dominated by brightly colored, elongate crystals. This is the mineral muscovite, formed when clay minerals in mud are heated to at least 350 °C at pressures that occur 10 to 20 kilometers below the Earth’s surface. Under these conditions the rock experiences metamorphism. Hence, the rock is best described as metamorphosed sandstone. Metamorphism in the Piedmont happened during the late Paleozoic (~300 million years ago). The Scottsville Basin formed after Paleozoic metamorphism. However, existing geologic maps place this meta-sandstone within the basin. Because we know that this rock is too old to be part of the basin, it’s reasonable to assume that the boundary of the basin has been improperly drawn on old maps. This finding will help me in my research goal of more accurately delineating the basin’s borders.
Chuck Bailey- The Monticello mystery
One of our research targets is an intriguing rock reported to crop out in the vicinity of Thomas Jefferson’s Monticello. A number of older geologic studies describe an alaskite, a felsic igneous rock composed primarily of K-feldspar (K for the element potassium) and quartz, intruding the greenstones of the Catoctin Formation. Alaskite, as a rock name, is anachronistic and modern petrologists refer to these rocks as alkali-granites. We located a string of linear outcrops of a fine-grained feldspar rich rock to the south of Monticello. The microscopic view reveals the rock is loaded with K-feldspar with lesser amounts of quartz, opaque minerals, and plagioclase. But it is the texture of the rock that tells the important story—this rock has a clastic texture, composed of rounded to angular fragments of sand grains in contact with one another. Clastic textures are diagnostic of sedimentary rocks. The Monticello ‘alaskite’ is a sedimentary rock, not an igneous rock as previously reported, that forms a layer within the sequence of Catoctin Formation lava flows. The source of this clast-supported arkosic arenite, the proper name for this rock, is a mystery as the K-feldspar could not be derived from the mafic Catoctin Formation. The older granitic basement complex, exposed to the west of the Catoctin Formation, does contain some K-feldspar, but what processes would concentrate the K-feldspar to such a degree? That’s the nature of research—as one question gets answered, others arise.
November 18, 2011 by Chuck Bailey
Fall has reached its full crescendo in Williamsburg; leaves are a riot of orange, scarlet and russet, the temperature has dropped, and frost has been sighted on more than a few pumpkins. The National Weather Service has officially declared the 2011 growing season over. The growing season ends when the first freeze occurs (i.e. the air temperature drops to or below 0˚ C).
The Weather, Climate, and Change class has taken a close look at the first freeze dates for Williamsburg. The histogram below is a time series, with 111 years of records from 1900 to 2011, that illustrates the first freeze date in Williamsburg for a given year. The earliest date for a frost was October 2nd (which occurred in both 1946 and 1947) and the latest frost occurred on December 1st (in 2009), the mean date for the first frost is just about November 1st. Are any trends evident from the time series?
Over the last 111 years Williamsburg’s first freeze occurred with equal frequency in October and November, but in the recent past the first freeze almost always occurs in November. Only once in the past 14 years did the first frost occur earlier than November 1st. What, if anything, might this mean?
Growing season, the time between the last frost in the spring and the first frost in the fall, is an important metric of a locale’s climate. In Williamsburg the mean growing season is an ample 203 days, but has varied between 156 days (in 1946) and 263 days (in 2010). The plot below is the revered (or is that reviled?) scatter plot; each orange diamond illustrates the growing season for a particular year. Do any patterns or trends jump from the graph?
The variation from year to year is significant and makes discerning any temporal patterns difficult. When in doubt it’s always fun to fit a line to a data set. With a few clicks of the mouse I ran a linear regression and produced a best-fit line for the data. The best-fit line has a gentle positive slope (0.146), indicating that the growing season in Williamsburg has increased over time. According to the regression, the growing season is increasing by ~1.5 days per decade (just over two weeks in a century). But the correlation coefficient (R2) for the best-fit line is quite low (0.06), an expected result given the wide variation in growing season length from one year to the next. Many people may understandably be skeptical of this trend.
Another way to consider these data is to take a five-year running average (incorporating the 2 previous years and 2 following years as well). This produces a smoothing effect that reduces the overall variation in growing season length. My eye is drawn to a dip in growing season in the mid-1940s and the perceptible lengthening of the growing season starting in the 1990s.
For many people the decade-scale is a handy interval in which to categorize time (e.g. the Roaring 20s, the Swinging 60s, etc.). Consider Williamsburg’s growing season averaged by decade. The 1940s take the prize for shortest mean growing season (185 days) while the 2000s come in with the longest (225 days). The thin black lines and bars (whiskers) extending above and below individual data points are a standard deviation: the longer the whisker the greater the variation.
Here’s the question I put to the students: is there a significant difference between decadal growing seasons? The standard deviations between the 1940s and 2000s don’t overlap; we can reasonably conclude that the growing season is significantly different between these decades. There is much overlap between the standard deviations in the other decades. Can we make sense of these data? Are the trends real or significant? My Weather, Climate, and Change students debated all of this.
It was a contentious debate because time series data can be complex and teasing out patterns is not trivial. But consider this: since the start of the 21st century the shortest growing season in Williamsburg has been 207 days, during the 30 year interval between 1960 and 1989 only six years (20%) had growing seasons as long as 207 days. Five of the Top Ten longest growing seasons occurred in the last decade. Since the 1980s, Williamsburg’s growing season is about 3 weeks longer than it was in the 1980s, the last frost in spring generally comes earlier and the first frost in fall arrives later. We could have an early frost next year and a short growing season, but the trend towards longer growing seasons is clear.
What the class hasn’t debated yet is just what could be causing these longer growing seasons. Is it global warming, an urban heat island effect, or maybe a decadal-scale climate oscillation? But let’s not get ahead of ourselves! Our discussion on the driving processes comes next…