Mantle Plumes
Are hotspots really hot?

Seismic detection of these relatively narrow features is difficult, and observations are often subject to a variety of interpretations.  I have been involved with two types of projects focusing on the structure of mantle plumes, or "hotspots." ScS reverberations can be used to study the whole mantle structure and provide good control on mantle discontinuity depth and whole-mantle travel time. I pair this approach with receiver function analysis at the same location, to gain better insight into structural variability beneath ocean islands. My recent work on mantle plumes has involved studies beneath Hawaii and Samoa.

I also participated in an interdisciplinary, multi-institution project resulting from the 2006 CIDER workshop in Santa Barbara, CA.  We analyzed mantle potential temperature and seismic transition zone thickness jointly at a variety of "hotspots" and ridges to examine differences in the mantle beneath these regions.  In short, we observed an association of thin transition zones with higher potential temperatures beneath hotspots. We concluded that these regions are therefore associated with an increased temperature through transition zone depths that likely extends into the lower mantle as well.

Geoscience Education
MAESTRO: Math and Earth Science Teachers Resource Organization

The Mathematics And Earth Science Teachers' Resource Organization (MAESTRO) is a partnership project between James Madison University and Harrisonburg City and Page County Public Schools in Virginia. MAESTRO focuses on improving public Earth System Science literacy, through a regional collaboration of in-service and preservice teachers with STEM faculty, as well as the strategic goals of developing the future geoscience workforce and supporting regional networks that strengthen geoscience education. The partnership is combining mathematics and Earth science instruction in middle and high schools by developing an integrated mathematics and Earth systems science approach to instruction in Grades 6 and 9, where Earth science concepts are typically taught, along with pre-algebra and algebra. This curricular integration is intended to enhance the mathematical skills and confidence of students through concrete, Earth systems-based examples, while increasing the relevance and rigor of Earth science instruction via quantification and mathematical modeling of Earth system phenomena.

The research component of MAESTRO is focused on the impact of local field-based case studies on student performance in, and perception of, mathematics and Earth science, as well as the impact of Lesson Study on the teachers' ability and willingness to integrate STEM concepts in the classroom. It is anticipated that the proposed integration across grade bands will first strengthen students' interests in mathematics and science (a problem in middle school) and subsequently reinforce the relevance of other sciences (a problem in high school), both in support of Earth systems literacy. Quasi-experimental evaluation of the impact of this program on participating educators is being used to document project outcomes and indicate the potential of this integrated approach as a model.

Near Surface Studies
What is hidden beneath our feet?

Geophysical studies are conducted at Earth's surface as well. Future projects will focus on Virginia geology and archaeology. One potential near-surface geology project involves looking at regional crust and mantle structure (pending instrumentation). Geoarchaeology projecs are available in collaboration with research staff at James Madison's Montpelier and can use a variety of techniques. In particular, ground penetrating radar uses technology similar to the ScS reverberation seismic reflection studies described above, and can be used to investigate near surface features of historical or geologic interest.

Water in the Mantle
How much water is in the mantle?

Each of the major mantle minerals has the ability to incorporate some amount of water into its structure, but some minerals have a much greater storage capacity than others. Because the assemblages of mantle minerals change across mantle discontinuities, this may lead to layers that are unusually water-rich or water-poor. Changes in discontinuity depth, seismic velocity, and rock density are related to temperature and chemistry of the mantle rocks.  Recordings of seismic waves made by seismometers peppered across the surface of the globe allow us to access depths in the planet's interior that cannot be reached any other way.  Properly analyzed, these seismic waves yield a clearer picture of where water resides in the mantle. 

My research focuses on two specific signals as indicators of water. First, a strong 520-km discontinuity results from increased water content of the mineral wadsleyite. A corridor beneath the eastern United States analyzed with ScS reverberations has very strong 520-km discontinuities relative to the impedance contrasts of the corresponding 410- and 660-km discontinuities. These observations may be indicative of a water-rich transition zone beneath the eastern United States and Gulf of Mexico. Second, increased water content in this region of the mantle may induce melting if conditions are right. A low-velocity layer near transition zone depths may be indicative of melting in the deep upper mantle. Much of my research focuses on detecting these layers, determining their geographic extent, and analyzing possible origins and fates of these melts, in collaboration with geodyanmicists who use numerical modeling approaches.

Canadian Northwest Experiment (CANOE)
How are continents formed?

CANOE is a Y-shaped array of nearly 60 broadband seismometers located in northern British Columbia and Alberta and southern Yukon and Northwest Territories. Most stations are spaced 35-50 km apart, with a densified region along the eastern arm of the array that has station spacings of 10-12 km. The instruments were installed during the summers of 2003 and 2004 and recorded data until late September 2005. The CANOE array traverses a wide variety of continental settings, allowing the study of mantle discontinuity variability associated with continental assembly. The accretional progression spans nearly 4 Ga of geologic history, beginning at the eastern end of the array with the Archean Slave Province, continuing west to the Wopmay and Racklan orogens, and finally to the Northern Cordillera, which extends to relatively recent times. The close spacing of instruments in the CANOE array will provide a detailed view of the mantle across these transitions. A combination of this study with the first proposed study allows comparisons between continental and oceanic regions for the largest span of ages available on earth.

I have taken a two-fold approach for this study. First, receiver functions across the CANOE seismic study are used to analyze the structure of the transition zone. Ps conversions from the transition zone discontinuities at 410-km, 520-km, and 660-km are analyzed to study variations across the region. Discontinuity properties are generally consistent across the study area and seem undisturbed by the tectosphere or overlying features. The only major discontinuity variability is observed at stations at the western end of the array in the form of a split 660-km discontinuity, potentially related to lower temperature or increased aluminum content reflected as phase changes in the garnet component of the mantle beneath the Northern Cordillera.  Second, analysis of splitting of the shear phases SKS, SKKS, and sSKS is used to study anisotropy beneath the region. Splitting times derived from multi-event station averages average ~1.4 s, and fast directions are coherent yet suggestive of strong variability of mantle anisotropy across the region. Additional variations in the fast directions and delay times are suggestive of complexity in the region and stem from differences in regional tectonic settings and the potential influence of a second layer of anisotropy beneath a portion of the array.