My Interests
The primary focus of my present research is on submarine
impact craters and their contribution to climate change and megatsunamis. This research grew out of my work on
the thermal history of the earth when Ann Isley and I discovered that mantle
plumes had the same periodicity as impact cratering events (Isley and Abbott,
2002, Journal of Geology, 110, 141-158).
We did a compilation of impact cratering events and found that the
record was woefully undersampled (Abbott and Isley, 2002. Earth and Planetary
Science Letters, 205, 53-62).
As a result, I started to look for impact craters on the ocean
floor. I found that the
Holocene age impact crater candidates could be located using a combination of
bathymetry derived from satellite altimetry and the directions to the source of
chevron dunes. I am now part of a
research group (the Holocene Impact Working Group) that is focusing on two
goals: looking at the effect of submarine impacts on climate and determining if
chevron dunes are megatsunami deposits. We have located a candidate crater set in the Gulf of
Carpentaria, Australia with an inferred age of AD 572±86 (Abbott et al., 2007). The craters have produced impact spherules of magnetite,
impact glass, and probable shocked quartz. The date of AD 572±86 is, within error, the same as the age
of the climate downturn at AD 536.
Ice core work is underway to see if the samples of the GISP2 ice core
dating to 536 A.D. contain impact ejecta from the Carpentaria craters. We are also working on samples from
chevron dunes in Madagascar to see if they contain impact ejecta.
Past Research Focus
My past research was on the thermal history of the earth,
and the manner in which heat transport through the crust and upper mantle
influenced geological processes, both ancient and present-day.
An important theme in my past research centers about my
theory that plate tectonics - meaning sea floor spreading and subduction - has
been active over nearly the entire span of earth history, that is, since the
early Archean era 4 billion years ago. The key difference between ancient and
present-day tectonics is due to the ever decreasing thickness of oceanic crust
caused by falling upper-mantle temperature. The most important consequence of
this decrease, which I investigated in my 1984 Abbott & Hoffman Tectonics
paper, has been a change in the style of subduction, from predominantly shallow-dipping
"buoyant" subduction in the Archean to predominantly steeply-dipping
"Andean" subduction today. The rarity of Archean andesites and
potassium-rich granites is thus explained. In my 1991 Geophys. Res. Lett. paper
I propose that buoyant subduction is also responsible for the formation of the
thick tectosphere beneath Archean cratons, because buoyant oceanic lithosphere
is carried beneath and underplates originally thinner continental lithosphere.
This underplating mechanism is able to explain the low degree of metamorphism
of most cratons, because the mantle is thichened without significant horizontal
compression of the crust.
Without independent information on the amount of cooling of
the upper mantle, and hence the thickness of oceanic crust through geologic
time, these ideas would be rather speculative. I have therefore concentrated my
recent efforts on deriving an upper mantle cooling history from petrological
data (my 1994 Abbott et al. J. Geophy. Res. paper). This work used measurements
of the iron-magnesium ratio for a suite of "MORB-like" rocks to infer
their temperature of formation. We are able to estimate both the distribution
of mantle temperatures at any one time in earth history and a clear decrease of
the mean temperature with time. These temperature data, together with a
Mackenzie & Bickle - style model for crustal production, allow us to
predict the time of transition between styles of subduction. The timing agrees
very well with appearance of rock suites associated with Andean subduction.