Sunday, April 7, 2024

New Jersey Earthquake (Part II)

In Part I of this post, I introduced the recent (April 5) New Jersey earthquake felt here in the Hudson Valley. Let's now talk a little about the geology.

The east coast of the U.S. has a long history stretching back almost 1.5 billion years (compared to the 4.5 billion year history of the Earth as a whole). During that time, our area experienced four major mountain-building events (which geologists call orogenies from the Greek work for mountain - oros). These mountain building events were due to collisions of crust from plate tectonic movement. Most of those mountains have since weathered away leaving remnants of rock that formed miles below the surface. There were also two major rifting events in that time period as blocks of crust pulled away again after those collisions (sometimes they also stuck around!).

All of these collisions and rifting events left folded rocks and faults. Many people are under the mistaken impression that faults only occur out west but there are faults you can see in rock outcrops on the side of the road anywhere in the Hudson Valley. But, unlike those currently active faults out west (like the infamous San Andreas), most of the faults here in our area formed hundreds of millions of years ago and have been inactive for a long time. But, as we'll see, sometimes they can be reactivated.

Most active faults are found near tectonic plate boundaries. The San Andreas, for example, is basically the plate boundary between the North American Plate and the Pacific Plate.

Here on the east coast, however, we're a couple thousand miles from the nearest plate boundary which is in the middle of the Atlantic Ocean at the Mid-Atlantic Ridge which separates the North American Plate from the Eurasian and African Plates. However, as tectonics plates jostle each other at the plate boundaries, stresses are transmitted into the interior of the rigid plates. These stresses can sometimes trigger preexisting fault movement to form what are called intraplate earthquakes (earthquakes in the middle of a tectonic plate).

Typically, these intraplate earthquakes aren't very large, but occasionally moderate ones can occur and very occasionally large ones (look up the 1811/1812 New Madrid, MO or the 1886 Charleston, SC quakes).

OK, so now let's look at New Jersey. New Jersey is made up of several physiographic provinces.

As you can see, the boundary between the Highlands and the Piedmont regions is a fault called the Ramapo Fault.

The NJ Highlands region extends northeastward in the Hudson Highlands of New York and represents rocks that formed miles below an ancient Himalayan-scale mountain belt formed during an event called the Grenville Orogeny over a billion years ago.

The Piedmont region is underlain by sedimentary rock intruded by igneous lava flows forming linear ridges. These formed when the supercontinent of Pangaea began rifting apart around 200 million years ago. This formed what's called the Newark Basin or Newark Rift (which extends upwards into Rockland County, NY).

The boundary between the Highlands and the Newark Basin (Piedmont) is a major fault which formed at that time - the Ramapo Fault. In the diagram below, the grays, yellows, and reds represent sedimentary rocks which filled the Newark Basin as it was pulling apart and rifting while the black colors represent the igneous intrusions of magma and lava flows (the most famous being the Palisades Sill along the Hudson River). The NJ Highlands are off to the left (west) of this diagram in orange (the same rocks also underlie the basin).

I will pick this up in Part III in a day or two.

Saturday, April 6, 2024

New Jersey Earthquake (Part I)

So I was in my office yesterday morning (Friday, April 5), engrossed in proofreading an important document going out to our college's accrediting agency, when I vaguely felt something. My colleague, in the adjacent office said "Was that an earthquake?" and I first thought maybe it was someone working on the roof (which they've been doing a lot lately) or it was a strong wind gust (but looking out the window showed everything calm). I pulled up the National Earthquake Information Center and, low and behold, a magnitude 4.8 earthquake in northwestern NJ popped up. So I guess I did feel it, but didn't recognize it as an earthquake at the moment. Same is true for a lot of other people I talked to at the college.

As a geologist, a moderate earthquake in New Jersey doesn't overly surprise me. While not common, they do occur. A few aftershocks have also occurred, the strongest a 4.0.

Here's a picture you can find in a lot of geology textbooks. It illustrates, using the Virginia 5.8 earthquake in 2011 (also felt in the Hudson Valley) how east coast earthquakes are felt over a much larger area than west coast earthquakes of the same magnitude.

This difference is due to the fact that rocks in the east are older, colder, and more intact than rocks in the west. Out west, the rocks are much more fragmented by faults and these faults attenuate (weaken) the energy from earthquakes. Here's a map of felt shaking from the New Jersey quake.

So how often do earthquakes occur in New Jersey? Here's a nice map.

I will continue this discussion in a second post in a day or two...

Sunday, March 3, 2024

Giant Beavers!

Did you know there were once giant beavers roaming the Hudson Valley region?

Bear-sized giant beavers (Castoroides ohioensis) lived during the Pleistocene Epoch (the time of the most recent ice age) and grew to over 7 feet in length with a weight of over 200 pounds. As their scientific name implies, they were first discovered in Ohio (1837) but extended from the upper Midwest and Canada over to the east coast (another species - Castoroides dilophidus - existed in the southeastern U.S.).

Giant beavers did exist in the Hudson Valley. One, for example, was discovered (along with caribou and flat-headed peccary fossils) at the Dutchess Quarry Cave site near Middletown in Orange County (near where a lot of mastodons have also been discovered). At this site, the beaver radiocarbon dated to 11,670 +/-70 years before present.

Castoroides ohioensis skeleton in the Field Museum, Chicago

Here's an artist's conception by the famous early 20th century illustrator Charles R. Knight.

The giant beaver differed from modern beavers in that they probably had rounded tails and not the flattened ones seen today and also in their teeth. Their teeth were much longer (up to 6 inches) and were ridged unlike the smooth teeth of modern beavers. These ridges would have strengthened their teeth and their jaw structure suggests that they would have had a strong biting force. It's hard to say if they constructed dams and lodges as beavers do today and stable isotope evidence from their bones indicates a mostly aquatic plant diet. Their large teeth were likely better adapted to digging in pond muck than in cutting trees.

Giant beavers first appear in the fossil record near the start of the Pleistocene ice age almost 2 million years ago and died out around 11,000 years ago as the North American climate was warming and glaciers had retreated north. The end of their time also overlapped with the migration of paleo-Indians into eastern North America. Did they die off due to climate change or did humans kill them off? This is the perennial question of all the Pleistocene megafauna which vanished around the same time.

While there is no direct evidence these early Native Americans hunted giant beavers (although it seems unlikely they would not have utilized all food source animals) some of the tribes have stories and legends that seem to be about them (it's amazing to think about oral traditions lasting thousands of years). They feature in an eastern Cree creation story and in tales by the Chippewa, Seneca, and other tribes (see, for example,

The Hudson Valley was a very different place just a few thousand years ago!

Monday, February 26, 2024

Learning from the past

There was a recent paper in Science Advances that’s gotten a lot of press lately. It discusses the possible collapse of the Atlantic meridional overturning circulation (AMOC) due to warming from climate change. This is not a new idea, here’s a July 2023 paper in Nature Communications saying the same thing (and forecasting a timeline between 2025 and the end of the century).

Basically, colder and more saline ocean water is more dense than warmer and less saline ocean water and these density differences lead to the development of areas of sinking water which drive deep currents (called thermohaline currents). These thermohaline currents are effectively mixing oceanic waters around the globe and hugely important in global climate.

In the AMOC, the Gulf Stream and North Atlantic Current bring warmer waters from the equatorial regions northward. At higher latitudes, this now colder water sinks down to drive deep thermohaline currents circulating water back south. If global temperatures continue to warm, the melting Greenland ice cap will dump enough freshwater into the North Atlantic to potentially disrupt the AMOC. This will have profound climatic implications for all of us.

Geologists, who have a longer view of things, know that the AMOC hasn’t always existed and there have been times during the last ice age when it has repeatedly collapsed and restarted. One of those times it collapsed may be tied to events right here in the Hudson Valley.

Some papers have suggested that a time known as the Intra-Allerød cold period that began around 13,360 years before present (B.P.) was due to temporary collapse of the AMOC. As the mighty continental glaciers melted back into Canada around 13,400 B.P., large amounts of meltwater were flowing south down through the Hudson Valley. Glacial moraines (ridges of glacial sediment called till) formed dams down by the Hudson Highlands formed a large freshwater lake called Lake Albany. Lake Albany was over 150 miles long and over 200 feet deep in places.


Also dammed up was what’s now Lake Ontario. It was much larger at this time and called Lake Iroquois (misspelled in the diagram below). Eventually Lake Iroquois broke through the dam and massive amounts of meltwater raced down the Hudson Valley and into the Atlantic Ocean (the whole story is a bit more complicated but you get the picture). There is evidence for this event both in the sediments of the Hudson Valley as well as features on the continental shelf seafloor out from the mouth of the Hudson River.

 The hypothesis is that this massive influx of freshwater disrupted the AMOC and led to a cooling period known in paleoclimatology as the Intra-Allerød cold period. It’s not fully accepted due to difficulties in getting exact dates and correlations for events occurring thousands of year ago, but certainly an intriguing hypothesis and perhaps indicative of events going on in the present day with the Greenland ice cap.

 It also shows how the study of geology can help us understand events occurring in the modern day.