Walking around Manhattan’s Central Park, if you have any interest in geology, you can’t miss the outcrops. They’re everywhere.

Most are schists – the product of sediment scraped off subducting ocean floor, shoved beneath the emerging continent of Laurentia, which today forms the heart of North America, and pressure cooked to produce a layered texture and glittery minerals. According to a guide from by The American Museum of Natural History, which has lots of useful information and field descriptions of various outcrops, chemical analysis of garnets in the schist suggests the original sedimentary rocks were heated to about 600 degrees C and squeezed to pressures of 5,000 atmospheres, or about 5 times the pressure found at the most extreme ocean depths. That means that the rocks were buried as deep as 15 km below the surface.

It all happened between 480 and 440 million years ago (known geologically as the Ordovician era), when modern-day Manhattan lay at the eastern edge of Laurentia. Dense rock at the base of the Laurentia plate subducted to the south below a north-traveling oceanic plate. Just like in today’s subduction zones, the down-plunging plate generated magma, which fed volcanic islands. Meanwhile another subcontinent, called Baltica and now the center of northwestern Eurasia, approached from the east. The offshore volcanic islands and Baltica eventually collided with Laurentia, forming a large mountain chain in eastern Canada and US (see figure, though Baltica is not shown). Those mountains were long since swept away by erosion, but their roots persist in the Piedmont plateau in the Appalachia Highlands.

Hundreds of millions of years later, Baltica and Laurentia would split apart again to eventually form today’s landmasses.

As the intervening ocean closed between Laurentia and both the volcanic arc and Baltica, erosion from all 3 shores dumped sediment into the marine waters. Sediment from Laurentia formed a continental slope, and megathrust earthquakes along the subduction zone roiled the sediments, creating landslides that eventually formed distinctive sand and mud formations called turbidites. Other sediments would join them from the volcanic arc and Baltica as the landmasses approached one another, while reefs growing in the tropical waters that became limestones. The island arc and Baltica plowed these deposits forward until some met the downward trench of the subduction zone, which dragged some into heated and pressurized underworld. After metamorphosis at 15 km depth, erosion of the mountains and tectonic forces led to uplift and surface exposure of the schist. Today in Central Park its platforms and rock walls provide climbing structures for kids and lounging spots for residents and tourists.

What is the source of the sediments that became Central Park’s schists? Were they from Laurentia, or the island arc, or Baltica, or a mix of all three? One recent paper (https://www.mdpi.com/2076-3263/14/7/190) suggests the Jutland Klippe sequence in New Jersey as the source of the sediment, and therefore a Laurentian source.

The United States Geological Survey says that the Jutland rocks occur in 8 fragments made up of multi-colored shale and sandstone, with some limestone, dolomite, and embedded pebbles. They were folded and thrust over other formations during the Taconic orogeny, then later deformed during the formation of the Appalachian Mountains between 325 and 260 million years ago, and then once more around 200 million years ago when eastern North America and Europe rifted apart during the breakup of Pangea.

There are two formations that chemically match the Manhattan schists: The Jutland Klippe, a mix of quartz sandstone, shale, limestone, and quartz pebble conglomerate in the New Jersey highlands dating to 497-459 million years ago, and the Normanskill formation, a mixture of shale, sandstone, and turbidites found throughout eastern New York that Natural Atlas says is 471-459 million year old. The authors of the paper peg it as more recent, around 450-445 million years old, which they believe is after subduction had ended, making it impossible that the Normanskill sediments could have gone down the subduction trench.

Only the Jutland sequence matches the time frame of subduction, and its chemical similarity to the Manhattan schist strengthens the argument.

The magma chambers beneath volcanoes are a seething, pressurized cauldron occasionally roiled by injection of more magma from the deep places of the Earth.

Such injections can trigger an eruption, like an inflated balloon that’s close to bursting: A bit more air pumped in could cause it to pop. In a magma chamber, the air pump is upwelling magma from the Earth’s mantle. Rather than bursting everywhere like a balloon, magma may accommodate an overburden of pressure by forcing its way through pre-existing cracks and faults – the plumbing channel leading to the vents and craters of a volcano.

Modern techniques can detect earthquakes from fresh magma entering a volcano’s underground chamber, but does that mean an eruption is coming? If so, how much warning do emergency planners have?

Central Washington University professor Hannah Shamloo says it could be weeks or months, at least for one volcano in the Pacific Northwest. She analyzes crystals that initially form in the deepest part of the magma chamber as the upwelling magma gradually cools. The magma eventually rises high enough to be injected into the main chamber of the volcano, where it encounters a new chemical environment that promotes the rapid growth of an outer layer on the crystal, a bit like a tree ring added during the warm summer months.

Later pulses of magma bring new changes to the chemistry, promoting a new, rapidly adhered outer layer to the crystal, which can acquire many outer layers over time.

After each additional layer, the crystal drifts around in the hot, pressurized magma, and atoms within the crystal slowly diffuse from one layer to the other. What begins as a sharp boundary between two different chemical profiles gradually blends at the margin (see figure above). The more time spent in the thermal soup of the chamber, the blurrier the boundary. The blurring rates for various elements in different minerals, and in different environmental conditions, have been revealed through carefully controlled laboratory experiments.

Sooner or later, the crystal gets ejected during an eruption. Once it cools amidst the lava or tephra, the mobile atoms within the crystal become fixed in place. Researchers then thinly slice the crystal to examine the distribution of atoms across the layers. The known diffusion rates allows researchers to calculate how long the outer layer was exposed to magma. In short, it reveals the time between the most recent change in magma chemistry and the ensuing eruption.

Early results from Mount Baker, located in northwest Washington State, suggest rapid progression. “In some scenarios, injection to eruption time is as short as weeks, but more [often] on the order of months, which is really impressive and much shorter than we thought,” said Dr. Shamloo.

Her team, which includes professors Kristina Walowski and Susan DeBari at Western Washington University, is creating a long-term data bank of the volcano’s activity. “We’ve really just scratched the surface at Mount Baker by looking at four eruptions. Mount Baker [volcanic field] has a 1-million-year-old history,” said Dr. Shamloo.

Inner Workings

Researchers can draw on a wide range of crystals, including olivine, plagioclase, sanidine, ilmenite, pyroxenes, zircons, and apatite. They can track the movement of iron, magnesium, nickel, calcium, manganese, strontium, barium, sodium, potassium, titanium, aluminum, chromium, hydrogen, chlorine, and lithium, depending on the constituents of the crystal and the time frame they are interested in.

When searching for samples, researchers like to find layers of airy tephra, the product of viscous, explosive eruptions. “When it erupts explosively, those tiny particles cool rapidly, so it locks in those pre-eruptive chemical fingerprints, whereas when you have stuff erupting effusively, like [flowing] lava, you can have this secondary thermal history that can interfere with the diffusion time,” said Dr. Shamloo.

Another complication is that a new outer zone in a crystal can be attributable to factors other than magma injection. Changes in pressure, or degassing of magma, may also lead to chemical changes in the magma and a new outer ring on a crystal. “It’s our job to interrogate every part of the chemical system to figure out [the cause of the new layer],” said Dr. Shamloo.

One complementary technique is analysis of melt inclusions, which can be glassy blobs of magma or other constituents trapped within a crystal. Chemical proportions within the inclusion suggest the temperature and pressure in the magma when the inclusion entered the growing crystal, which in turn suggests potential causes for growth of the outer crystal layer.

Kenda Lynn is a research geologist at the US Geological Survey Hawaiian Volcano Observatory who uses diffusion chronometry and other methods to monitor the Big Island’s iconic volcanoes. Her group uses inclusion barometry to examine trapped elements like carbon dioxide that are a fluid because of the high-pressure environment of their formation. “By measuring these tiny fluid inclusions, you can convert that to a pressure, which equates to a depth underneath the surface of the volcano,” said Dr. Lynn.

Old and New

Mount Baker hasn’t been active in historic times. Seismic rumbling and steam emissions raised concerns in 1975, but the magma beneath the volcano stayed underground, and its last major eruption was 6,700 years ago, and as a result Mount Baker is not heavily monitored. “It’s not like Mount Saint Helens, where we have monitoring data and material from a recent eruption. We need diffusion chronometry to understand pre-eruptive timescales for these volcanoes that haven’t erupted in our lifetimes. Otherwise, we have no way of knowing how they behave,” said Dr. Shamloo.

Along with Mount Baker, she and Drs. Walowski and DeBari are investigating Washington State’s Goat Rocks formation, which is the rocky remnant of an ancient volcano that was active between 3 million and 600,000 years ago. Recent glaciation exposed volcanic layers that stretch from the beginning to the end of the volcano’s life, granting geologists an unusual window into the past. “That’s really unique for a Cascade system, because typically the older units are buried or eroded,” said Dr. Shamloo.

She hopes that diffusion chronometry and other methods will shed light on the natural history the Cascade subduction zone volcanoes, including the effect of the basement rock that magma intrudes into and absorbs before it erupts. “I’m most excited about the earliest [history], because we typically don’t get that access. Using diffusion chronometry from the very beginning to the very end to build a true data set of behavior for these eruptions through time at a Cascades [style] eruptive system will be really cool,” said Dr. Shamloo.

Far and Wide

Diffusion chronometry isn’t restricted to sparsely monitored volcanoes like Mount Baker. Its use has exploded in recent years in places like Indonesia and Taiwan, Mount Aetna in Italy, and Mauna Loa in Hawaii.

Dr. Lynn notes that there are decades-long data sets of the seismologic activity and gaseous emissions at some of the world’s most studied volcanoes like Mauna Loa, but the magma chamber remains mysterious. Researchers use seismology, ground deformation, emitted gases, and lava composition to interpret what’s happening underground, but these measurements are indirect. The eruption of Mauna Loa in 2022 presented a unique opportunity to match advanced, real-time monitoring techniques to samples taken directly from the chamber. “Every time we’re able to make this connection between the mineral stories and our monitoring data sets, we can strengthen our confidence in our interpretations the next time we see earthquakes or tilts or other deformation at the surface. We have a better sense of what those data mean in real time, because we’ve done the ground truthing with the diffusion chronometry,” said Dr. Lynn.

Crystal analysis in Hawaiian volcanoes show a varied story, ranging from decades spent in magma chambers to hundreds of years. “These crystals sit around and are kind of recycled through the metaphorical washing machine as magmas come and go. Not every crystal makes it out in an eruption, and many are left behind to then record the next [magma injection] event,” said Dr. Lynn.

To query these different time frames, researchers turn to different atoms that move at strikingly different rates through the crystal. Atoms like hydrogen, which zip quickly through the lattice, reveal the life history of crystals that remain in the chamber for just a few hours or days after the final injection. Atoms like phosphorous move slowly and help researchers date crystals that spent years, or decades, or even centuries in the chamber. “You can leverage all these different elements to piece together the full lifespan of a crystal,” said Dr. Lynn.

The work at Mauna Loa and elsewhere is a good proving ground for diffusion chronometry, says Dr. Shamloo. The agreement between crystal stories and external monitoring in places like Hawaii emboldens her that diffusion chronometry is painting an accurate picture in the Mount Baker and Ghost Rocks volcanoes. “It gives me confidence that applying it to the Cascades, where we don’t have monitoring data, is actually worthwhile,” she said.

References:

Diffusion chronometry of volcanic rocks: looking backward and forward | Bulletin of Volcanology | Springer Nature Link

Hannah Shamloo interview with Nick Zentner about Goat Rocks: Hannah Shamloo: Goat Rocks

Smudged volcanic crystals offer clues to past eruptions | Science | AAAS

Some 17,000 years ago, the massive Cordilleran ice sheet looming over the Pacific Northwest began the retreat that would signal the end of the most recent glacial period. At its highest, it towered 4,200 feet above today’s Puget Sound and traveled as far south as Olympia before petering out. As it advanced, the glacier had plowed sand, gravel, pebbles, and boulders. It abandoned them in retreat, and meltwater from the glacier and other sources sorted them into deposits of sand and gravel.

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But glacial retreat wasn’t simple. After backtracking, they would temporarily readvance, plowing more debris. Sometimes ice dams created glacial freshwater lakes or trapped marine waters, and clay settled out. Then a new episode of ice retreat, and meltwater again sorted the detritus into a new layer of sand and gravel, which lay atop the clay. Glacial advance and retreats see-sawed, leaving alternating layers of clay and coarser sediment in a deposit that ranges from 50 to 200 meters thick.

When the ice left for good, sea levels rose, and marine waters infiltrated the deposits along meltwater channels that formed underneath the glaciers. Those deep channels now form the watery tendrils of Puget Sound. Seattle and its suburbs perch above on the remaining glacial deposit.

To the east, rivers draining the Cascade mountains carve sinuous paths into those layered deposits. Their floodplains erode away the clay and sand but leave streambanks and bluffs with sometimes visible bands of clay and sediment (figure 2e).

Unfortunately, the banks can be quite unstable. River bends undercut the deposits, causing bluff failure that carries mud, sand, and pebbles into the river plain. In the case of the Oso landslide of 2014 in the Stillaguamish River, a massive section slide flowed over a kilometer into and across the river channel, and buried a housing complex, killing 43 people (figures 1 and 3). There is also evidence of prehistoric landslides of various sizes up and down the major rivers.

These banks are unstable due to differences in permeability: Winter rainfall settles into upper layers of sand and pebbles, but stops at underlying layers that are less permeable, such as those with significant amounts of clay. The upper layers saturate with water, becoming heavy and dense. Meanwhile, at river bends, the water erodes the base of the bluff, weakening it until the overbearing mass becomes too great and the sand and gravel slide into the floodplain.

Floodplain Hazard

The landslides are extremely hazardous to those living in floodplains below unstable bluffs, but they may be predictable, according to USGS scientist Shelby Ahrendt. The largest and most devastating slides, she said in a 2024 online talk, occur where less permeable layers are deepest. Her group published a related paper in 2025.

Dr. Ahrendt is working with a smaller, more active section of a section of the Nooksack River to the north, in Whatcom County and close to the Canadian border. It runs through similar glacial deposits, and sections of the bluff tumble into the floodplain and the river itself every few years. She used 89 years of satellite imaging to plot the course of the river and timing of historic landslides along several kilometers of the Nooksack River, and found that landslides big enough to alter river flow can influence the location of future landslides (figure 4).

Her work paints a picture of landslides that displace the river, adjustment of the river channel to the landslide debris, and creation of a new upstream meander that erodes the unstable slope at a new location and eventually initiates another landslide. “[It has] a sort of slinky effect up and down the river valley. This idea can help us understand landslide hazards in river valleys, including the Oso landslide region,” said Dr. Ahrendt (figure 5).

Like other Washington rivers, the Stillaguamish River has generated many historic and prehistoric landslides, including the 500-year-old Rowan landslide about 3 kilometers to the west of Oso, which pushed the course of the Stillaguamish River about 600 meters to the south. Native Americans living in the area must have witnessed it.

Similar to the landslide cycle taking place on the Nooksack River, the Rowan landslide on the Stillaguamish may have triggered the Oso landslide, said Ahrendt. It straightened the course of the river, and that led to a new riverbend to form to the east – the site of the Oso disaster. Over hundreds of years, periodic flooding could have led to undercutting of the bank, and along with other factors resulted in the catastrophic failure in 2014.

“A landslide creates an upstream meander, which destabilizes a new region of erosion and landsliding. This model all seems to fit here. It’s just operating on a much longer time scale [than the Nooksack River] because [the Stillaguamish] is a much larger river valley,” said Dr. Ahrendt.

I viewed the below during a recent outing, prompted by a field trip post from Bellingham geologist Dave Tucker.

Wouldn’t such faults be rare? What was the likelihood that you happen to run across such a major fault where rock was rubbing against? Why are slickensides common? Thanks to ’s Geologic Column, I found a great explanation.

The answer stems from the horizontal layers of sedimentary rock. Mud or sand deposit on a parallel surface, layer following layer. You can split shale along the layered planes. When tectonic forces uplift a thick rock, the formation is usually tilted. As it continues to rise, the pressure is uneven, and the layers start to bend and eventually break – often along the lines of bedding planes or cross beds. Once broken, vertical forces push the broken pieces past one another. They grind together and voila! Polished slickensides.

Slickensides can also form in igneous and metamorphic rock, but I would assume they wouldn’t break so tidily, except in cases like schists where planarity has been preserved or created.

They’re still produced by faults, but much smaller and more numerous faults than the big ones I had naively envisioned.

Where I live, the bedrock is mostly the 50-million-year-old Chuckanut formation — sandstone and shale deposited in a vast flood plain. In some places the formation is 9 km thick — it must have been subsiding as more mud and sand deposited over millions of years.

From Wetland to Slickenside

As the basin filled, more sand, mud, and pebbles buried the earlier deposits. Overriding pressure turned those deposits into sandstone and shale. Between 45 and 25 million years ago, subduction of an oceanic plate led to regional uplift, folding some of the deeper, more pliable rock. Upper, cooler layers of the formation remained brittle. Uplift shoved the brittle rock upwards, bending and then snapping some of it to create rock with slickensides.

On a large scale, the formation folded from compression, split up from strike-slip faults, and broke. Uplift turned them into some of the foothills and a few of the westernmost mountains of today’s Cascades, which were themselves formed much more recently by other tectonic forces. Similar strata of the same age can be found east of the Cascades (the Swauk formation) and out into the San Juan Islands and northward into Canada (some members of the Huntingdon Formation).

The Chuckanut formation is also the source of many plant fossils that reveal a diverse tropical landscape that includes tree ferns, conifers, and many flowering plants, but few vertebrates, with the exception of a couple of turtle shells, various animal tracks, and, in 2023, a single fish.

Earthquake monitoring technology is relatively new. Many seismometers dotting the planet were emplaced to monitor nuclear testing in the mid-20^th century, then found use in the measure of earthquakes. Networks like the USGS Seismographic Network expanded that capacity and focus on natural earth movements.

But the subduction zones where earthquakes cluster have been around far longer, and the dynamic processes that govern them remain mysterious. To better understand these complex systems, geologists need to look further back in time. They’d like to know the intensity of the megathrust earthquakes that occur at “stuck” subduction zones, their epicenters, and how they may propagate through connected fault zones. That information can pinpoint movement along faults over time and could open the door to predicting future events.

The trouble is that past earthquakes don’t leave much physical evidence. Strike-slip faults can leave recognizable off-set patterns, such as the disrupted streams and fence lines from the movement along the San Andreas Fault in historic times, and displacements of rock formations by as much as 130 miles.

It is a different challenge to find evidence of ancient earthquakes along subduction zones, where movement takes place deep underground as an ocean plate sinks below continental rock, and leaves little or no trace on the surface.

One option is to look at turbidity zones along continental shelves, where sediment has deposited from land erosion. It gets disturbed by massive earthquakes, causing them to slide towards deep oceanic troughs and leave telltale patterns. Such deposits can be sampled, but not easily. They are most useful in the study of deep time, after they’ve lithified into rocks called turbidites and been uplifted to the surface, where geologists can read their evidence of ancient subduction zone earthquakes. But turbidites aren’t plentiful and can be challenging to date. They also don’t tell much about recent history.

Some megaquakes generate tsunamis, which in turn dump vast amounts of sediment where they come ashore. Such deposits are useful when visible, but many are long-since eroded away, or remain buried beyond the view of geologists.

Other coastal evidence of past earthquakes includes sequences of tidal and wetland environments, produced by the sudden drop and gradual lift of the land surface after large subduction earthquakes. This happens because the jammed slab puts pressure on the overlying plate, buckling it upward at the margin and raising the land surface by a few feet. When the next slip occurs, the vice releases and the land drops back to its “natural” elevation nearly instantly. Tides or tsunamis then wash in, drowning the plant life and depositing a layer of intertidal mud.

This process leads to recognizable sequences of deposits: Peat from coastal wetlands or forest soil is overlain by a layer of tidal mud as the land drops in elevation, and beach ridges erode away. “Ghost forests” like this one on the coast of Washington State may occur where trees killed by inrushing saltwater leave stumps behind. When the diving plate gets stuck again, the land slowly uplifts, and another layer of peat or forest soil accumulates until the next slip of the subduction zone, and another layer of tidal mud arrives. The repeating cycle leaves a distinct pattern of sedimentary layers.

Archeology can also reveal widespread abandonment of coastal villages due to an earthquake or tsunami.

All of this evidence is useful, but it’s somewhat scattershot.

Bed Time Stories

In a new paper in Earth-Science Reviews, geologists studying the Alaska-Aleutian subduction zone (AASZ), where the Pacific plate dives below the North American plate, outline another source of earthquake records: the sediment beds of Eklutna Lake in the Chugach Mountain Range and Skilak Lake on the Kenai Peninsula.

The AASZ is about 4,000 km long, stretching from the Alaska Range of mountains in southcentral Alaska to Russia’s Kamchatka Peninsula. The subduction generates magma that drives 40 active volcanoes in the AASZ. Like some other subduction zones, it can also generate massive earthquakes when the diving plate, “stuck” against the overlying continental crust, suddenly slips. In 1964, the 9.2 magnitude Good Friday earthquake ruptured an 800 km section of the fault. The geologic activity and hazards associated with AASZ, including tsunamis, make it an important system to understand.

Lakes deposit seasonal sediment that can be stirred up by shaking, or experience underwater landslides along sloped bottoms.

Based on a combination of coastal evidence and analyses of the beds of the two lakes, the group came to a few conclusions. One is that the Good Friday earthquake was an exceptional event over the past 2,000 years, as it ruptured multiple sections of the fault zone. They also found evidence of several closely-timed earthquakes in the lakebeds, which can’t be seen using coastal evidence. They suggest that multiple areas of the AASZ can become stuck, and there have been cycles of ruptures in the eastern part of the zone, or “rupture cascades,” as the authors write. The biggest lock zone in the eastern AASZ is the Prince William Sound region.

The researchers combined observations from sites throughout the region, using radiocarbon dating of organic material like stems, leaves, tree trunks or peat to link the evidence of specific events at multiple locations. They identified 7 megathrust earthquakes in the past 4,000 years, and 10 big quakes in the past 6,000 years. Before 1964, the most recent comparable temblors occurred 850 years ago and 1,450 years ago.

Glaciers feed both lakes annual layers of dark, coarse-grained clay in the spring and summer, followed by light-colored clay in the winter. Extreme floods regularly disrupt the beds, whether due to intense snow melt, autumn storms, or glacial lake outburst floods. The sediment patterns reflect glacier expansion during the Little Ice Age between 1600 and 1900 AD and other periods.

By comparing the lakebed layers to recent earthquakes, the researchers could identify minimum shaking intensities required to leave various imprints in each lake, whether in the form of a turbidite, landslide, or sediment liquefaction. Such “event deposits” then get covered by background sediment until the next earthquake powerful enough to affect the location. Unlike other earthquake deposits, the lakebed record is continuous. This allows researchers to develop high-resolution age estimates using techniques like 210Pb-137Cs, carbon-14 dating, and by simply counting individual layers. They can achieve nearly annual resolution when sedimentation rates are high.

The two lakes, Eklutna and Skilak, are far enough apart to experience different tectonic conditions and slight differences in their exposure to megathrust earthquakes. Lake Skilak is near the transition between locked and creeping sections of the subsiding Pacific plate, making it valuable for the study of the transition zone where these two modes of subduction intersect. There are also numerous volcanoes near both lakes, which produce datable ash layers.

Researchers use statistical methods to identify unusually thick lakebed layers suggestive of turbidites or landslides. Skilak Lake deposits revealed 19 earthquakes in the past 1,500 years, while Eklutna Lake revealed 21 earthquakes in the past 2,300 years.

In comparison, coastal evidence has revealed just 4 megathrust earthquakes in the past 2,500 years in the Prince William Sound region, suggesting that many of the earthquakes recorded by the two lakes had a different source, perhaps including smaller megathrust earthquakes or earthquakes originating from within the crust or subducting slab instead of the subduction zone itself.

The researchers described several conclusions:

· The coastal and lake records reinforce the belief that the 1964 earthquake was an unusually strong event compared to those of the past 2,000 years. It was also the only such quake to simultaneously rupture the Prince William Sound, Kenai, Barren Islands, and Kodiak sections of the AASZ. Some lake sediments preserve evidence of other, similarly wide-ranging ruptures, while other sediment profiles showed smaller ruptures of the Prince William Sound section.

· Boundaries between sections of the AASZ in south-central Alaska appear somewhat fluid, with the different sections rupturing at different times.

· The Kodiak and Semidi sections of AASZ have ruptured simultaneously in the past, and could do so again, leading to a large earthquake involving the entire eastern AASZ.

· Between large simultaneous ruptures, partial-rupture earthquakes occur, potentially reducing stress in the fault zone.

· The researchers believe that the Prince William Sound region of AASZ may be the most ‘locked’ part of the zone, in part because it did not rupture along with neighboring sections during an 800-year interseismic period, along with its tendency to rupture with some periodicity over the past 4,000 years.

To learn more, head on over to Earth-Science Reviews to read the rest of the paper.

On day 1, I drove from my home in Bellingham, through Seattle and Auburn, east on highway 410, and eventually to Sunrise Visitor Center overlooking the northeast flank of Mt. Rainier.

Along the road to Sunrise, I spotted these near horizontal columns of what appear to be andesite. Why are they horizontal? The thought is that eruptions flowed along the margins of glaciated valleys, and the columns formed where the magma made contact with the ice. Lava contracts as it cools, leading to surface cracks that spread perpendicularly into the cooling lava — in this case, along a horizontal plane between the ridgetop where the lava flowed and the edge of a mountain glacier.

The same thing happened during eruptions of the Columbia River Basalt Group (CRBG) that erupted from wide-ranging vents between 16.7 and 5.5 million years ago. In those eruptions, the basalt flows cooled on level surfaces, producing dramatic vertical columns as cooling initiated from the top of the flow where it met the air.

Atop Sunrise I was met with a mix of threatening rain and wildfire smoke that concealed Mt. Rainier’s slopes but led to some interesting lighting in the high meadows.

After a bit of wandering, I drove back down to the highway and soon found my plan to drive south along road 123 on the east flank of Mt. Rainier thwarted due to road construction. I had to detour eastward towards Natches along highway 410, then southward and west again on highway 12. It added about 2 hours to my trip but it was a spectacular 2 hours.

The CRBG and their dikes made a dramatic appearance, but I didn’t stop to investigate further until I doubled back the next day, so more on them below.

I drove along highway 12, past Rimrock Lake in the darkness and to my motel in Randall, Washington. After a night’s sleep and breakfast at the Blue Stone Cafe, I attempted to drive to the east side of Mt. St. Helen’s and the Spirit Lake overlook, with the intent of hiking down to the lake. Again I was thwarted by road construction, which sent me on a detour of forest roads with no signage. I drove an hour or so up the mountain slopes, but with no map and no directions, I decided that the journey was more important than the destination and turned back towards Randall. Instead, I would retrace my route along Rimrock Lake, Natches, and the CRBG formations.

But first I stopped at various outcrops on St. Helens’ slopes, including these freshly eroded andesite boulders. You can see the scar above where the boulders broke away from the bedrock.

I was driving the road back to Randal when I spotted a faded sign pointed me to a forest road turnoff towards Layser Cave, which was new to me. I expected a geology interpretive site, maybe a lava tube, but what I found instead was a low-lying cave that Native Americans occupied for thousands of years as a seasonal encampment and butchering site. A forest service worker discovered the cave in 1982. I highly recommend checking out this interesting spot if you find yourself in the area. According to Macrostat, the cave formed out of basaltic andesite.

I returned to Randall and then retraced my route from the previous day, stopping along Rimrock Lake to explore its retreating shore. Immediately apparent were the miniature terraces marking recent shorelines.

These lines, though no doubt destined to be washed away soon when the lake level rises, illustrate the geologic principle of uniformitarianism — that today’s natural processes are no different than those of the past. The temporary shorelines look much like the exposed shorelines of glacial lakes on hillsides around Missoula, Montana and elsewhere.

Next, I drove northeastward on highway 12, then northwest on 410, and the massive dikes of the Columbia River Basalt Group Grande Ronde flow came into view. That jagged pillar on the left is the eroded remnant of one of the flows. My guess is that what we’re seeing in the photo is the eroded remains of a much larger subsurface dike that underwent uplift. To the lower right is what looks like a landslide scar. On the right side of the scar are columns of a presumably older CRBG flow that the dike intruded through.

Some older CRBG flows, or possibly Grande Ronde columns that subsided through normal faulting, form the banks of the Naches River in the foreground.

Lots of geology in action here. Macrostat shows two faults that define the course of this section of the Naches River, but says that the faults have unknown offsets.

Satisfied with my journeys, I continued on to Seattle, where I attended the American Society of Bone and Mineral Research annual meeting in as a medical writer. If you’re interested, you can read my news stories from the meeting.

The head of the family paused for a moment, and his bare feet sunk slightly in the soft ground of the marsh. The grass protruding from the mud had been trampled by the passage of large animals.

The others stopped as well, grateful for a brief respite while he examined the tracks. “Mammoth for sure. One is injured.”

He called his teenaged daughter to him, and she stepped forward gingerly through the muck.

“Here. The smaller tracks are uneven. Injured or exhausted. It won’t be long now.” She nodded in understanding and returned to her place among the others. The leader motioned to the rest. “After we set up camp, we’ll hunt for it.”

The group continued their journey. Not long after, a group of wolves, trailing the hunters, stopped and sniffed, then plodded on in anticipation. They knew that human packs foretold a kill that the wolves could feast on after the humans had had their fill.

This scene, or one like it, played out near modern-day White Sands, New Mexico. The Sacramento mountains rose on the eastern horizon. To the west lay a large, shallow lake. The hunter-gatherers walked through flood plain, tracking game like mammoth, ground sloth, and camels, or gathering tubers or seeds, or both.

We know this because a large team of researchers described the footprints, pressed into clay, and their surrounding geology in a 2021 article in the journal Science.

What is less clear is exactly how long ago it occurred. The researchers dated the footprints to between 21,000 and 23,000 years ago, when the last glaciation across North America was around its peak. That’s a significant date because archeologists have long thought that the earliest humans in North American came on the scene around 13,000 years ago, after the glaciers had receded. They were the Clovis people, who left behind distinct, fluted spearpoints. For decades, researchers have unearthed other signs of human habitation at places like Cooper’s Ferry in Idaho (15-16,000 YA), Paisley Caves in Oregon (14-15,000 YA), Meadowcroft Rock Shelter in Pennsylvania (up to 16,000 YA), and Bluefish Caves in The Yukon (24,000 YA), but the dates were controversial and some in the field continue to believe that the Clovis people were the earliest humans to reach the Americas.

If confirmed, the new date would extend human history in North America by 10,000 years. It also suggests that humans arrived before the last glacial maximum, which occurred between 26,000 and 20,000 years ago, because the ice would have blocked potential migration routes from Asia.

The initial date rested on evidence from an aquatic plant called Ruppia cirrhosa (or ditch grass) – specifically, its seeds, which are tough enough to last for many thousands of years and dot the surface of the clay that harbors the footprints. The researchers used carbon-14 dating to determine that the seeds were between 22,860 and 21,130 years old.

In the grand tradition of science, debate immediately ensued. Shortly after the publication, another group of researchers published a later comment that the Ruppia dates are unreliable because the plants can readily absorb carbon dioxide from ground water, which could skew the carbon-14 results and make the seeds seem older than they really are. The first group provided circumstantial evidence that such absorption was unlikely, but the commenters were not convinced. Instead, they suggest that the footprints may be 4,000 to 5,000 younger – still an important date, but likely after the glaciers began their retreat. Another possibility is that the seeds grew elsewhere at an earlier time, and were later eroded away and deposited by wind, which would make the deposit younger than the seeds. It’s also possible that the lake would have been too deep for Ruppia to have grown there. “There are reasonable alternatives to the plausible explanations Bennett et al. provide…. conflicting evidence suggests that the reliability of the Ruppia seed radiocarbon ages must be independently confirmed using other chronometric dating methods,” the commenting authors wrote.

In 2023, the researchers dated pollen associated with the footprints, which supported the original date. They also published a remarkable photo of the footprints. Check it out, then come back and keep reading.

Now, the researchers have responded with a new volley of evidence that supports the Ruppia date. Those humans strode along a lake, and in a June paper in Science Advances, they describe more studies of the lake. They traced the footprint sediments other layers from the ancient lakebed and used additional dating methods to arrive at similar dates.

Now and Then

After the last retreat of the glaciers, the climate became warmer and dryer. Today, the former lakebed is a salt flat, and the former marshlands to the east are now sand dunes.

The lakebed itself is poorly understood because after it dried, much of its sediment was eroded by wind, some of it winding up as the dunes to the east. The wind erosion left an escarpment ringing the borders of the modern-day salt flat, and the footprints were discovered in one of the exposed layers along the escarpment.

The new study includes a survey of sediment layers at 3 sites along a 3-kilometer segment of the escarpment. All 3 sites are some distance from the footprints, but the researchers traced the layers along the landscape to show that they are the same deposits that harbor the footprints. Stratum 1 lies below the footprints and is black to dark gray in color, suggesting a lakeshore wetland. Stratum 2 lies on top of stratum 1, and is therefore younger. Stratum 2 includesthe footprints at the archeological site. It has sublayers that range from black, to red-brown, gray, and white, which are likely due to the margins of the lake and wetland changing over time: Sometimes an area was covered by lake water and, in dryer times, it was a marsh or floodplain. Each environment leads to deposition of different particles with different organic content, giving each layer a distinct color and granularity.

The suggestion that the lake only occasionally intruded into the wetlands implies that the water level of the lake was likely low enough to accommodate local growth of Ruppia, according to the authors. That counters criticism that the Ruppia seeds had to have blown in from elsewhere.

The researchers obtained radiocarbon dates from Ruppia seeds and mud samples from stratum 1 and stratum 2, and found dates ranging from 23,600 to 17,000 years ago. Specifically, the sublayer encasing the footprints dates to between 22,400 and 20,700 years.

The results suggest that the eastern edge of that long-ago lake supported wetlands until at least 17,000 years ago, and the wider basin would have supported a wide variety of plants and animals that could have supported a human population. “The data presented here support that scenario for the lake margin but also show that wetlands were nearby. Critics of the initial date of the tracks agree that the tracks are extremely important but require independent age control. Those independent data are presented here and agree with the dating of Ruppia and pollen,” the authors wrote.

Is this the final word on ancient peopling of the Americas and the timing of arrival of humans? Certainly not. But the new data strengthens the case of pre-Clovis archeological sites.

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