When Rocks Roll: How Sediment Transport Shapes Planetary Surfaces

On Wednesday, March 19, Professor of Geology Michael Lamb will describe how flowing water and grains of sand create Earth's dramatic landscapes. Mars and Saturn's moon Titan show signs of similar processes. Lamb's work on the mechanics of landscape evolution may change how we think about debris flows in the San Gabriel Mountains, the effects of wildfires on erosion, and water on Mars.

The talk begins at 8:00 p.m. in Caltech's Beckman Auditorium. Admission is free.

Q: What do you do?

A: I'm a geologist. I study the basic mechanics of how sediment moves—how rivers erode rock and form canyons, how sediment builds deltas like the Mississippi's, how landslides work, and so on.

In the U.S., my field dates back to John Wesley Powell, who led a survey party by boat down the Grand Canyon a few years after the Civil War. For decades, geomorphology consisted of classifying landforms—identifying different types of mountain ranges, or hills, or canyons. Now, we're quantifying the processes that operate at Earth's surface to shape those landforms.

One challenge, which is true of much of the Earth sciences, is to link these process studies with the evolution of Earth's surface over geologic time. So in addition to field work, my research group has a lab where we make indoor rivers and landslides, and 'speed up time' to observe their dynamics. For example, the Mississippi River jumps to a new course from time to time, and each jump builds a new lobe of the delta. These jumps happen about once every 1,000 years, but by scaling things properly we can speed up the clock and build deltas much faster in the laboratory. We also study rare, catastrophic events, such as megafloods, which would be difficult or dangerous to measure in nature.

It's an exciting time to be a geomorphologist because we have the opportunity to apply lessons learned on Earth to Mars and Titan—two other planetary bodies with river networks. Titan's surface appears to be active, with rainfall feeding into lakes and rivers with enough force to move gravel and even cobbles, which are roughly tennis- to basketball-sized. Titan is so cold that the "rain" is liquid methane and the "rocks" are water ice, but otherwise the system seems very similar to the water cycle on Earth. On Mars, the river canyons are now dry, probably long dry. In many ways, Mars represents the ultimate inversion problem. With little to go on except for images and topographic data, what can we deduce about the water flows and climate on Mars that led to the formation of these ancient features?

Q: How did you get into this line of work?

A: I started college at the University of Minnesota in civil engineering. I always was fond of mechanics, and thought that I would likely become an engineer. But I also was fond of the outdoors—natural landscapes and national parks—so I had a desire to connect both interests. I took elective classes in geology and found them to be much more interesting than my engineering classes, so I switched majors. I also had an opportunity as an undergrad to work in the St. Anthony Falls Hydraulics Laboratory at the University of Minnesota, which introduced me to the indoor-analogue river experiments I now do.

Flume experiments have been used in civil engineering for many decades—for example, if you dam a river, what will the effects be? In fact, what we're doing today is similar in some ways to techniques that were pioneered here at Caltech by Professors Vito Vanoni [BS '26, MS '32, PhD '40], Norm Brooks [PhD '54], and others—first in the Sediment Lab, which was built in 1936, and later in the W. M. Keck Laboratory of Hydraulics and Water Resources. But now we're addressing how the natural world works over geologic time, rather than how the engineered world works over human time.

Q: In light of our recent mudslides, do you have any take-home lessons for the general public?

A: I have been working on the connection between the wildfires, floods, and debris flows that tend to plague not only Los Angeles but other areas in the southwestern United States, and I will focus my lecture on this topic. For now I'll just say that what you hear in the news about hillsides giving way and causing landslides after wildfires is probably not accurate in many landscapes. Our work shows that there might be a different way to think about the fire-flood problem, especially in very steep and rapidly eroding landscapes like the San Gabriel Mountains.

Named for the late Caltech professor Earnest C. Watson, who founded the series in 1922, the Watson Lectures present Caltech and JPL researchers describing their work to the public. Many past Watson Lectures are available online at Caltech's iTunes U site.