Research Interests

Most of my research efforts focus on magmatic processes, from mantle melting and melt transport to magma evolution and crustal formation. See below for some of my recent and ongoing projects.

AUV Sentry
AUV Sentry (photo: B. Cushman)

Studying the Seafloor

Deep underwater environments are challenging to work in, requiring special tools to deal with the immense pressure, low visibility, and other issues associated with submarine settings. Despite covering ~70% of our planet, the ocean floor is less well-mapped than the surface of Mars. Although satellite gravity data has been used to produce low-resolution bathymetric maps, only 10-15% of the seafloor has been mapped in any detail by shipboard surveys, and only a small fraction of that has been observed visually thanks to deep submergence vehicles and towed camera systems. As a result, there is much we still don’t know about the deep ocean, its geologic processes, and diverse ecosystems.
 

Magmatic processes and submarine volcanism

With recent advances in deep-sea technology, it is now possible to study the seafloor in much better detail. Using a combination of autonomous underwater vehicles, remotely-operated vehicles controlled from the ship, and human-occupied submersibles (such as the famous Alvin submersible), we can now make geologic maps and collect samples of the seafloor much like a field geologist would on land. These data provide critical context for understanding seafloor volcanism and the underlying magmatic processes.

Most crustal formation today occurs along mid-ocean ridges and backarc spreading centers. To better understand these important volcanic systems and their magmatic processes, we use a wide range of methods to study these systems over a variety of different scales, including high-resolution sonar mapping, rock sample collection and geochemical analysis, seismic methods to image the subsurface, numerical modeling, as well as studies of analog systems on land. Current projects include synthesizing petrological and geophysical data with seismic imaging results from studies on the Mid-Atlantic Ridge and the Lau Backarc Basin. For more information, see the following:

Geologic mapping on the GSC

 


 

Formation and Evolution of Hawaiian Volcanoes

 

Ka‘ena, O‘ahu’s hidden volcano

Kaena Volcano

A team of us at UH recently showed that the island of O‘ahu was actually constructed by at least three volcanoes, rather than the two previously recognized. Through geologic mapping and sampling using the underwater remotely operated vehicle (ROV) Jason, geochemical analyses, and gravity measurements, we have shown that the submarine ridge offshore of Ka‘ena Point is actually a separate volcano (now called Ka‘ena Volcano). Previously thought to be an anomalously long rift zone of Wai‘anae Volcano, we now know that Ka‘ena actually had its own distinct magmatic system, and likely began forming before Wai‘anae. We are continuing our efforts to better understand Ka‘ena’s magmatic plumbing system and evolutionary history, and address related questions about the formation and distribution of Hawaiian volcanoes.

 

view from Kalapana Postshield volcanism

Hawaii volcanoes go through a series of evolutionary stages as they move over the mantle hotspot, from an early pre-shield stage, through their primary growth stage (shield stage) characterized by relatively high magma supply and shallow magma storage areas, to a postshield stage marked by a decline in magma supply and deepening magma storage. While much attention has been given to shield stage volcanism, with particularly valuable data from the active ongoing eruption at Kilauea, postshield volcanism is not as well characterized for many volcanoes. We have been working to better document and understand this late stage of Hawaiian volcanism, its magmatic processes, and related insights into mantle melting and source composition. A new paper on Moloka‘i is currently in press, and we hope to expand our efforts to include a systematic treatment of petrologic processes across all postshield units in the main Hawaiian Islands.

 


 

Mantle Melting & Melt Transport

 

Deglaciation and volcanism in Iceland

Normally when we think about the link between volcanic activity and climate, we think of the effects volcanic eruptions have on the atmosphere. However, there’s increasing evidence that major climate events can also influence volcanic activity. This is best documented in Iceland, where the last major deglaciation (~10,000-15,000 yrs ago) is correlated with a large increase in volcanic productivity (up to ~30 times normal levels). The proposed mechanism for this is enhanced mantle decompression due to glacial unloading: as the large ice sheet that covers Iceland during glacial periods melts away, the pressure in the underlying mantle decreases and causes mantle rock to melt at higher rates. This increase in mantle melting rates results in more volcanic activity at the surface during and after deglaciation. The increase in volcanic productivity correlates nicely with a change in chemical composition of the erupted lavas to more depleted compositions, due to the increased contribution of melts generated in the shallow, depleted mantle.
 

table mountain

To determine the timing and geochemical signature of this process, we’ve been studying Iceland’s table mountains, which erupted through the overlying ice. We’ve discovered that the geochemical signature of glacial unloading shows up at the surface even earlier than previously thought, before the ice sheet had finished melting. As we gather more data on the changes in volcanic productivity and chemical composition with time, we can use this rather unique forcing function as a window for examining melting and melt transport processes in the mantle. It also has present-day implications as we try to understand the complex feedbacks between the earth and atmosphere. It’s even been suggested that present-day glacial melting in Iceland might be sufficient to cause small increases in mantle melting many kilometers below the surface (Pagli & Sigmundsson, 2008; Schmidt et al., 2013).

 

Melt genesis and transport

Although we understand a good deal about melting and magmatic storage at mid-ocean ridges, constraining rates and timescales of magmatic processes remains quite challenging. I have been working in Iceland to try and exploit a rather unique event in its past in order to constrain melt migration rates. At the end of the last ice age, glacial unloading in Iceland led to enhanced decompression melting of the underlying mantle, producing a large increase in volcanic activity during this time period and some corresponding changes in the geochemical composition of the erupted lavas (see section above). For the past few years, I along with some colleagues at the Univ. of Hawaii, Woods Hole Oceanographic Institute, and the Nordic Volcanological Institute in Iceland have been working to improve our understanding of this process, and better constrain the perturbation in the melt system during this time. We hope to use this ever-improving dataset to gain insight into some fundamental parameters of the melt migration system (such as melt ascent rates and porosity, for example).