Blog Research

Bengal Fan Rock Magnetism

Sediments eroded from the Himalayas during their 50 million years of uplift have largely been carried out to sea and deposited in great cone-shaped piles to the east and west of the Indian subcontinent. The Bengal Fan, the deposit fed by the modern Ganges and Brahmaputra rivers, is by far the larger of these sediment accumulations. In the winter of 2015, I sailed on International Ocean Drilling Program Expedition 354 to sample these sediments. We collected cores from a transect across the middle of the Bengal Fan to track the flow of detritus from the rising Himalayas to its ultimate resting place in the Bay of Bengal. Questions that the scientists on Expedition 354 are interested in answering include:

  • How has the rate of sediment flow changed through geologic time?
  • Has the source of the sediment changed as the mountains rose?
  • How has the sedimentation changed in response to changes in climate? And how has the region’s climate changed in response to the uplift of the mountains? (And: how have changes in climate affected the uplift of the mountains themselves?)
  • How do sediments spread themselves out over the massive fan deposit? 
  • Do rapid sedimentation events (turbidites) indicate storms? Earthquakes?

Over the next two to three years, I will be looking for student partners to help me address some of the questions above using the magnetic mineralogy and magnetic properties of Expedition 354 sediment samples. Magnetic minerals, for example, can help distinguish between sources of sediments. Because they are sensitive to chemical weathering, magnetic minerals may allow us to examine changes in climate over geological time. Magnetic properties of sediments can even allow us to study the physics of turbidity currents that carry sediment down the fan. As a student involved in this project, I expect you to become familiar enough with marine geology and rock magnetism to carve out part of this project that you can do yourself. Ultimately, you will gain a deep enough understanding of rock magnetic techniques to be able to collect, analyze, and interpret your own data, and to present your own findings on part of this project. High-quality research will contribute to a peer-reviewed publication and presentations at national conferences (meaning that you may have a chance to participate in a conference, and you will be a co-author on the presentation).

For Students
Timeline: Summer 2015 to 2018(?)
Courses Required: Physical Geology w/Lab (TESC 117); Sedimentology and/or Physics 2 (Electricity and Magnetism) are helpful
Fieldwork: No
Laboratory Analyses: Bulk magnetic properties, size-fraction magnetic properties, magnetic hysteresis and components, magnetic anisotropy, reflected light microscopy, UV-vis spectroscopy
Data Analysis: Factor analysis and related techniques, least squares fitting
Software: Excel, Mathematica, R, Python
Conference Opportunity: Yes, in 2016/17
Required Reading
Burbank DW. 2005. Earth science: Cracking the Himalaya. Nature 434:963–964.

France-Lanord C, Schwenk T, Klaus A. 2014. Bengal Fan: Neogene and late Paleogene record of Himalayan orogeny and climate: a transect across the Middle Bengal Fan. College Station, TX: International Ocean Discovery Program IODP Sci. Prosp. 354.

Molnar P. 1997. The rise of the Tibetan plateau: From mantle dynamics to the Indian monsoon. Astron. Geophys. 38:10–15.

Schwenk T, Spiess V. 2009. Architecture and stratigraphy of the Bengal Fan as response to tectonic and climate revealed from high-resolution seismic data. In: Kneller BC, Martinsen OJ, McCaffrey B, SEPM (Society for Sedimentary Geology), Geological Society of London, editors. External controls on deep-water depositional systems (SEPM special publication). Tulsa, Okla: SEPM (Society for Sedimentary Geology). p. 107–131.


Layered Intrusions

Layered intrusions are the solid remains of ancient underground magma systems. Unlike granites or other relatively silica-rich rocks, layered intrusions are often rich in iron and magnesium. The squishing and squashing as magma is injected into the crust, the density contrasts between magmas and crystals, and the chemical changes in magma, crystals, and hydrothermal fluids produce a range of unusual rock textures – patterns in size, shape, and orientation of crystals in the rock. Some of these same processes are also responsible for economically valuable deposits of metal ores (for example, in the Bushveld Intrusion in South Africa or the Stillwater Complex in Montana). Ultimately, we want to know what these textural patterns mean: We use the magnetic properties of these layered intrusions to figure out how mineral crystals are oriented within the rock. We use that information to determine the history and spatial pattern of stretching, squashing, squeezing, shearing, settling, etc. in the mushy mix of crystals and melt that eventually becomes the layered intrusion.

Layered intrusions can also retain a record of Earth’s magnetic field for long intervals of geological time. We have used rocks from the Stillwater Complex in Montana to examine characteristics of Earth’s magnetic field 2.7 billion years ago, when those rocks cooled to a low enough temperature to record ancient magnetic fields.