Dick Claesson, Bill Meurer, and Jeff Gee at the Middle Banded Series outcrop at Camp Lake, Stillwater Complex, MT

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.

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