The Attraction of Sedimentary Rocks

Kenneth Kodama knows that rocks retain a faithful record of the magnetic field, a fact that has had a tremendous variety of applications, including the reconstruction of continental positions hundreds of millions years ago.

The author of Paleomagnetism of Sedimentary Rocks: Process and Interpretation, Kodama’s work describes the paleomagnetism of sediments and sedimentary rocks, how sediments and sedimentary rocks become magnetized, and how the physical and chemical processes involved can affect the accuracy of paleomagnetism.

A professor of earth and environmental sciences, Kodama has focused his work primarily on sedimentary rocks. When these rocks form, they record the direction of the Earth’s magnetic field, and the tiny magnetic grains in the rock align with the Earth’s magnetic field lines. By examining this magnetism, scientists can determine the rock’s ancient position on Earth.

The Earth’s field lines are vertical at the poles, but horizontal at the equator. Their dip varies regularly with equator to pole so their dip records the ancient latitude.  But as sediment is buried under other layers of sediment, the rocks are subjected to compaction, rotating the magnetic grains toward the horizontal. Kodama’s work has shown that as the magnetism is compacted and rotated toward the horizontal, it appears as if the rocks were formed closer to the equator than they actually were.  His findings, once doubted, are now setting the standard for geologists everywhere.

Kodama’s latest project plays off his work in paleomagnetism, using the magnetism of sedimentary rocks to produce records of global climate cycles.

“Geological records show that global climate is forced at regular periods,” he says. “Scientists think that these climate cycles are driven by how much solar energy is hitting the top of the atmosphere.”

The changes in energy are caused by variations in Earth’s rotation axis, its tilt and the direction it points in space, and changes in the shape of Earth’s orbit. Kodama’s new work uses variations in the amount of the magnetic minerals in a rock to record the climate cycles. He has found magnetic records of climate in rocks 570 million years old.]

“We have found an easy and powerful way of recognizing these cycles with the magnetics,” he says, “The resulting records are like finding a metronome in the rocks that allows us to measure the passage of geological time.”