Author: Ajay Limaye

Rifts are important to all epic stories. The story of the North American continent and its predecessors is no exception.

Episodically, forces from Earth’s interior started to stretch the crust apart, threatening to split the continents and open up a new ocean basins. Our first example, the Midcontinent Rift, started 1.1 billion years ago and eventually spanned an arc from modern-day Nebraska north to Minnesota, and back down to Michigan.

Our second example, the Reelfoot Rift, began around 600 million years ago. It includes portions of modern Arkansas, Tennessee, Missouri, and Kentucky. “Successful” rifts eventually open up ocean basins, but neither the Mid-Continent Rift nor the Reelfoot Rift made it that far. Geologists are biased for action, so these “failed” rifts get a special name: aulacogens.

So what can we learn from these buried schisms from hundreds of millions of years ago?

The forces of separation may be inevitable, driven from deep within. Yet what fills the space of our separation has lasting consequences. In the case of the Midcontinent Rift, tectonic forces threatened to break the continent apart — but lavas helped to fill the gap. These lavas persist today, 1.1 billion years later, along the shores of Lake Superior. The waters circulating through the volcanic rock concentrated copper ore that has supported trade for thousands of years. Whereas weaker sedimentary rocks were scoured away, the stronger volcanic rocks stood up to attack from ice sheets. So despite being surrounded by one of the flattest regions in the US, those ancient rocks support  towering cliffs along Lake Superior. That lake, cradled by low land above the rift, stores a whopping ten percent of the global of fresh surface water supply.  

On the other hand, the legacy of the Reelfoot Rift suggests a more ominous set of consequences for deep divisions. Take a look at this forecast of earthquake damage risk for 2017, produced by the US Geological Survey:

As you might expect, California has some of the most widespread elevated risk of a damaging earthquake, given its tangle of active faults. The elevated risks in Oklahoma and Kansas are a recent addition to this map due to induced earthquakes from hydraulic fracturing (aka “fracking”). But you’ll find another zone of surprisingly high earthquake risk in the east-central US, right in the neighborhood of the crust weakened by the Reelfoot Rift.

Some of the largest earthquakes ever to strike the continental US occurred in this same region. From December 1811 to February 1812, three enormous earthquakes struck near New Madrid, Missouri. Shifting ground beneath the Mississippi River threw it into instant turmoil, even causing the river to temporarily flow backwards by some accounts. Spouts of liquefied sand burst forth from the ground. Look around in Google Earth, and you might see the relict sand deposits lying on the surface.

The region was sparsely populated at the time of the earthquakes, but now hosts 11 to 12 million people in the vicinity of Memphis and St. Louis. So could a similar earthquake sequence strike again? The available geologic evidence, largely from dating sand eruptions, indicates that similar events occurred in the past, at intervals separated by 200 to 800 years.

Just as cultural observers are digging to unearth the fault lines hidden beneath the surface of American society, geologists are searching for new clues about the rifts in the literal bedrock of America. Many citizens are disoriented by the suddenly uncertain contours of our society.

If there’s one way that geology can guide us, it’s this: the consequences of division can last long a lot longer than you might think. What will fill our political gaps: a society strengthened by renewed civic engagement, or one weakened by fake news? Either way, Earth history has its eyes on you.

For geoscientists, lacking direct experience is a common quandary (and it should be noted, a happy one!) One great workaround: the thought experiment. Back in 1909, the eminent geographer William Morris Davis used his powers of imagination to visualize how the Connecticut River might have carved its valley and left terraces. Here’s a beautiful sketch from his writings:

River terraces in Massachusetts, from W. M. Davis’ Geographical Essays.

Davis surmised that the Connecticut River, migrating to and fro about its valley, left terraces as it gradually cut downward.  So was his guess plausible?

Nowadays Davis is better known for his ideas on whole-landscape evolution, decades before the plate tectonics revolution. In contrast, Davis’ hypothesis for “accidental” terrace formation popped up only now and then in the intervening decades and largely went untested. In the meantime, a different explanation gained popularity.

River terraces  — those remnant valley bottoms perched along river valleys all over the world — are eroded into old river sediments or bedrock, and are often interpreted as the signature of a river’s response to climate change.  The argument goes like this: if tectonic uplift pushes the landscape up at a steady rate, then a river steadily cuts downward to keep pace. In this quasi-equilibrium state, the river never pauses its downward carving. But if an outside force disturbs the river from equilibrium — for instance, by changing the supply of water and sediment–then the river may temporarily pause its downward journey before catching up with a rapid pulse of downcutting. Terrace surfaces would be eroded during the pauses, and the terraces would be stranded when the river cut downward.

Davis’ hypothesis is that rivers make terraces without pausing their downcutting, but instead by the unsteady lateral shifting of the river during downcutting. This distinction gets to the heart of reading the terrace record: if rivers in equilibrium also make terraces, then terraces may say nothing about regional changes in climate or tectonics. Instead, terraces would simply be the accidental remnants of wandering rivers.

So how might we distinguish between these competing arguments, given that most of the world’s terraces formed long before today?

One way is to see if the numbers add up: for example, how fast would a meandering river need to move in order to make an “accidental” terrace? To test this, my work unfolded in two steps. First, I translated Davis’ thought experiment to a simplified mathematical model, and then to computer code. With a flurry of for-loops and deluge of data structures, a Caltech supercomputer named for a turn-of-the century exploration ship tested hundreds of scenarios for valley evolution. Here’s a movie of the model in action:

A movie of valley evolution from our new paper in JGR-Earth Surface.

The top part shows a meandering river (in blue) flowing from left to right, viewed from above. Initially, the river buts up against bedrock walls: the topographic cross section in the lower part shows shows bedrock (black) and sediment (white), with the channel showing up as a narrow notch in the center of the cross section. As the clock zooms forward through 25,000 years, the meander bends migrate freely through the sediment and deform as they meet resistant bedrock. The uneven pattern of lateral erosion, coupled with vertical erosion, makes a set of terraces. The terraces are identified automatically during the simulation, and in the top section are colored according to their height above the channel. And just like that,107 years after Davis’ original hypothesis, we have new data to start testing it.

Based on these simulations, we built a predictive framework for what “accidental” terraces formed under steady climate should look like and how often they should form. In the second step of the analysis, I compared the model predictions to a selection of carefully studied valleys in the central and western US, including the Wind River valley from my last post. Here’s the summary figure:

Model predictions for terrace age and geometry and estimate erosion rates for natural river valleys from our new paper in JGR-Earth Surface.

For the geomorphologists: we found that key terrace attributes commonly attributed to climate change–such as the occurrence of pairs of terraces on either side of the valley, their length along the valley, and their separation in time by thousands of years–can form without climate change. These predictions hold for particular conditions: rivers must erode laterally relatively quickly (i.e., > 10 mm/yr) and cut down relatively slowly (i.e., < 1 mm/yr). By comparing to the natural river valleys, we see that the meandering river model with steady vertical incision explains key terrace attributes for the Colorado River valley (Texas) better than the Wind River valley.

Long story short: with an idea of how meandering rivers might make terraces under steady conditions, we have a better shot at identifying the terraces that really tell us something about climate change.

Particularly in the mountains, many of the flat surfaces you see are flat because a river used to run there. So give a glance around on your next trip through a river valley: you might be walking in the footsteps of the river itself. Terraces are a hot topic with lots of active research. For more new work on decoding the terrace record, see papers on the roles of landslides [Scherler et al., 2016], bedrock [Schanz and Montgomery, 2016], and climate change [Langston et al., 2015].

I had a new paper come out this month, entitled “Numerical model predictions of autogenic fluvial terraces and comparison to climate change expectations.” The paper and supplemental movies are up now at Journal of Geophysical Research – Earth Surface. This work developed from the last chapter of my thesis with my adviser and co-author Mike Lamb.  The question: how do we read geologic history from the shape of a river valley?

First, a little background. Most of us have some experience with rivers, which are hubs of human life worldwide. Close to home, the St. Anthony Falls on the Mississippi River were the nucleation point for the city of Minneapolis. The river has been central to the regional economy as an engine for flour mills and electricity, a highway for commercial shipping, and even the source of most of the world’s buttons. In nature, rivers also serve as hubs of activity for evolving landscapes. Rivers are conduits for the movement of sediment, water, and nutrients across continents to the ocean. In the life of a landscape, however, rivers are more than conveyor belts: rivers are also great integrators. Tectonics, climate, vegetation, and human disturbance all influence rivers, and rivers in turn shape the landscape around them. And lucky for us, the Earth’s surface and sediments hold a “fossil” record a river’s work over geologic time.

One of the most common remnants of a river’s activity is a river terrace. As its name implies, a river terrace is a flat area–in fact, it usually looks like a valley bottom. But there’s a twist: river terraces are former valley bottoms, abandoned so far above the active river that they are rarely (if ever) inundated in floods. Terraces often occur in a series of steps, and in an architectural mixed metaphor, a set of terrace steps that descend toward the modern river is called a “flight.” Here’s a spectacular flight of terraces from the Wind River valley, Wyoming from Hancock et al. [1999]: 

Between the Snake River in the foreground and the jagged Teton Mountains in the background, you’ll note several broad river terraces.

River terraces are usually subtle in profile, and hide in plain sight in river valleys across the globe. In some landscapes, terraces can be dated to hundreds of thousands of years old. These river terraces are some of the best available records of river response to environmental change. In fact, the most common explanation for why rivers abandon terraces in the first place is that climate change disturbs the balance of water and sediment delivered to the river, causing it to cut down further into its own valley. Thus, terraces hold key clues to climate in the past, and are vital to placing modern landscape change in context.

There’s a snag, however: rivers may make terraces of their own volition, without any changes in climate. Consider, for example, an experiment Les Hasbargen made here at St. Anthony Falls Lab in the late 1990s. In the experiment, a ~0.5 square meter patch of sediment served as the initial landscape, and was subjected to steady “rainfall” in the form of misted water droplets and steady “uplift” to build topography. A vivid landscape in microcosm burst into bloom, with famously shifting mountain ridgelines and yes, terraces. This sandbox-scale experiment showed that under simulated conditions with steady climate and tectonics, landscapes can develop features like river terraces that would otherwise seem to require climate change. Resolving terrace origin is difficult  using field observations alone, in part because river terraces are thought to form over millennial timescales.

So how can we reconstruct a valley’s history after arriving so late in the game? Stay tuned for Part 2.

It’s hard to imagine, but simultaneity is a thoroughly modern concept. Before the telegraph, long-distance messages were carried exclusively by hand. The time delay built into all forms of communication ensured that what happened in the city one day would be a historical account by the time it made it to the countryside. By effectively eliminating the transit time of information, the telegraph transformed human conception of time and invented a “now” that could be experienced across a state, a country — and the world.

The story of the telegraph is one focus of The Information, James Gleick’s tour of information theory circa 2010. The book traces its subject historically, then concludes with Gleick’s assessment of our data-inundated times as patrons of The Library of Babel. We meet many fascinating characters along the way, including the quixotic encyclopedia writers who sought to transcribe every fact in the known world; Charles Babbage, the designer or a machine whose sprawling mass of gears and levers presaged the computer, a century before transistors; and Claude Shannon, whose theoretical work at Bell Laboratories is the foundation of modern digital life. Gleick amiably translates complicated source material, like quantum computing, in understandable and engaging terms. If the book drags anywhere, it’s in the extended history of the telegraph. But then I got to the invention of simultaneity.

Earth scientists are exceptionally curious about the things that predate the present. Yet our field is being similarly revolutionized by awareness of the planet’s activities in near real-time.

For decades seismometers have listened to the pulse of the planet, capturing local tremors and teleseismic calamities. Since the 1980s, the Landsat satellites have been capturing the mother of all home movies: a record of Earth surface change replete with meandering rivers, advancing dunes, surging glaciers, flowing lava, and shrinking lakes:

Lake Urmia, Iran

(Here are more jaw-dropping time lapse movies via Google Earth Engine).

The uniformitarian mantra “the present is the key to the past” may limit one’s geological imagination — look no further than T-Rex and the Crater of Doom. Yet paradoxically, that dusty old tenet of pre-plate tectonics geology may hold more power today than it ever has. Our worldview has literally expanded to a view of the whole world, and now even an armchair geoscience buff can see the present unfold just about anywhere on Earth’s surface.

Considering this embarrassment of riches, the challenge of sifting through the data to reach the information is a formidable task. The development of information theory in the 1940’s came just in time for Watson and Crick’s discovery of DNA, and informed the analysts who ultimately cracked the genetic code. Circa 2016, there’s never been a better time to crack the code of Earth’s surface.

“Welcome to the lithosphere!” – Jason Saleeby, greeting newly erupted lava in Hawaii, March 2013.

So I’m starting a blog.

Towards the end of my PhD I started a log of sorts to track my research–incremental steps, breakthroughs, dead ends–and to maintain a space for taking down half-formed ideas and sorting out alternatives when I hit a roadblock. Now, a year plus into a postdoc, the log is a sprawling jumble of largely mundane details and non sequiturs. I’m probably never reading most of that again. Nonetheless, taking some time to put ideas on the page has helped me keep my bearings and (hopefully) see some of the bigger picture.

With any luck this occasional project, a digital scrapbook for my adventures at the St. Anthony Falls Lab, will yield something of broader interest. Posts may flow out in voluminous sheets, quixotically striving to blanket the digital plains. More likely, new thoughts will surface sporadically in energetic blobs. Either way, you’ll find them here. And so, without further ado…
Welcome to the blogosphere.

Blog coming January 2016.