Rock Slide—Palisades Cliffs May 12, 2012 Images and “Why it Happened”

Location of rock slideLamont Doherty

I found the top image online (not sure of the exact date—but obviously before May 12). Even though it was taken at a different angle than the one showing the May 12 slide, I circled parts of the cliff that are easily matched. Notice the huge chunk of rock in the top image (right side) that is no longer present after the slide. That must be what fell. Also, note the number of trees that must be buried by this slide. Before After

Notice people for scale

Again, notice people for scale

The rock slide was large enough to register on the seismometer at Lamont Doherty (Columbia’s geology research facility located on top of the cliffs not far from the location of the rock slide). Each line on the seismogram is showing ground vibration over time. When the ground vibrates, a needle (attached to a very sensitive spring) will record that vibration on a piece of paper. The ground is constantly vibrating (for example, trucks driving by) which results in the small “wiggles” seen on the seismogram. When something causes the ground to shake in an unusual way, it clearly stands out on the seismogram. The numbers on the left side of the seismogram represent the time of day (military time—Greenwich Mean Time)

Why did this happen? View of Palisades from above View of Palisades cliffs—close up The Palisades were formed about 200 mya when Pangaea began to split. As the plates split apart, magma intruded into the surrounding rocks and solidified close to the earth’s surface. The Palisades are made of a rock called diabase, which is a type of basalt. If basalt cools close to the surface, it tends fracture in vertical columns (see next slide). Basalt is also high in iron, which causes it to rust if exposed to the atmosphere. The intrusion of diabase (basalt) is clearly visible in the picture to the left and the rusting is clearly visible in the picture to the right. Notice that there are sections of the cliff that are black—which is the “pre-weathered” color of the rock.

Giant’s Causeway in Ireland—perhaps the best example of columnar jointing on the planet Why did this happen?

Due to the vertical fracturing from the columns, the Palisades is prone to this type of “mass wasting.” Water will seep into the cracks—which increases weathering in two ways. 1. During the colder months, repeated freezing and thawing causes the cracks to expand (water expands when it freezes) 2. The water will also react with the iron in the rock, causing it to rust—which further weakens the rock. Eventually the column is weakened enough that it collapses—often in spectacular events like this one. Notice that the “scar” on the cliffs follows the shape of the columns (the rock slide is essentially a collapsed column). Also notice how rusted the scar is, indicating extensive weathering.

Why did this happen? Collapses like this are rather common along the Palisades. Note the columns to the right of the picture below. These are likely candidates for a future rock slide.

Why did this happen? In the pictures below you can see evidence of slides that happened in the past. Notice the scar in the cliff and the rock pile near the river (left side of the picture). If you look carefully, the trees directly above the rock pile are a lighter green than the ones to the right and left of it. The lighter colored trees represent younger ones that grew recently on the exposed rocks from the slide. Also notice, when outlined, the shape of the deposit is similar to the current rock slide (see picture insert at bottom)—which is typical of such a feature (like a river, the sediments “fan” out away from the source). The circled features on the right most likely formed from a rock slide as well. Most recent slide Where rock slide occurred

Do NOT attempt to walk along the rocks near the river. It takes a long time for rocks to settle after a rock slide. Many of these rocks can still move. Keep in mind that most of these boulders are larger than a car. Walking on them would be very dangerous. I’m not 100% sure where along the cliff line this is, however, based on Google Earth and comparing the before and after pictures, I’m pretty sure it’s right below the State Line Lookout. I’m not sure if they have closed that lookout due to the slide, but if not, that’s probably a good spot to see it close up (from the TOP). You can also hike down to the river just north of the Women’s Federation Monument (B/W trail). You can then take the W trail along the river (north) until you reach the slide. Again: Do NOT attempt to walk anywhere near the slide. You also may want to check with the New York/New Jersey Trail Conference (who manages these trails) to get updated trail information ( A.pdf version (much higher res) of the map below can be found at (I also saved a copy on the eboard) Getting Closer