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EarthCache

Palisades Sill

A cache by njgeology
Hidden : 11/15/2006
In New Jersey, United States
Difficulty:
1 out of 5
Terrain:
1 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

What happens when tectonic forces start to rip a continent in two? At the Palisades sill you can see one chapter of this geologic story.


LOCATION: Palisades Interstate Park, New Jersey Section, Fort Lee, New Jersey (under the George Washington Bridge)

PARKING: The waymark will take you to the Parking Area.
ALTERNATIVE PARKING: Ross Dock N 40 51.62' W 73 57.37' Note: There is a $5.00 fee for parking May-Oct)

ACCESSING THE TRAIL: From the Parking Area, go around a bend to the Henry Hudson Drive (which does have the occasional car but is primarily used by bikers, joggers and pedestrians). If you park at Ross Dock, walk back up the road you drive in on, toward the George Washington Bridge.


THE PALISADES SILL

The Palisades sill is an igneous intrusion that formed approximately 200 million years ago in the Jurassic period. This 1000 foot thick intrusion “injected” itself between layers of sedimentary rock of the Lockatong formation as the supercontinent of Pangaea began to split apart (or "rift"), forming the North American plate and the African plate.

The Palisades formed when rising magma (molten rock) from the earth’s mantle pushed and stretched the overlying crust. As the magma rose, the crust thinned and begun to crack and pull apart from the tensional stress.

As the crust thinned and broke apart, cracks (or “faults”) formed. In some places, the magma exploited these cracks and continued to rise to the earth’s surface, forming new oceanic crust. (You can see an example of this at the Watchung basalt flows, west of the Palisades.)

In other places, the magma never reached the surface. Rather, the magma plume rose from areas of high-density rock to areas of lower density rock until it reached a location where the rock density was equal to that of the magma. Then the plume stopped rising, and instead flowed horizontally between existing layers of rock, forming intrusive sheet structures called sills. This is how the Palisades sill was formed.

If the rifting that led to the intrusion of the Palisades sill took place 200 million years ago, and if rifting leads to the formation of oceanic crust, then why isn’t the Palisades under water today? The reason is that the Palisades sill is at a failed rifting spot. While the continents were pulling apart at the Palisades and Watchung, something happened to cause the rifting to end at those locations and to begin at another location. The final rifting took place approximately 100 miles to the east of where the Palisades are today. As you would expect, the actual rift zone is now under water. The rifting is taking place at the mid-ocean ridge in the middle of the Atlantic Ocean.

The composition of the rock at the Palisades varies slightly across the formation, but in general, the rock type is diabase, also known as dolerite. Diabase is a basaltic rock (basaltic meaning a dark, dense igneous rock) that is composed mainly of the minerals feldspar (particularly plagioclase) and pyroxenes, along with other mafic minerals (mafic minerals are a group of dark-colored minerals, composed chiefly of magnesium and iron, that occur in igneous rocks). The diabase that makes up the Palisades sill has roughly the same composition as the basalt that forms the bottom of the ocean and the mid-ocean ridges.


Xenolith N 40 51.055’ W 73 57.680’

A xenolith is foreign piece of rock embedded in an igneous rock. This xenolith was ripped up from the surrounding Lockatong formation as the magma intruded into the Lockatong. This is a common process that occurs when a molten body of rock is injected into existing rock. As you examine this site you will notice that the edges of the xenolith seem disordered and mucky. This is due to the movement of water that moved parallel to the bedding of the xenolith. When water mixes with magma, it changes the chemical composition of the magma and causes it to behave differently.





Columnar Joints and Lower Contact N 40 51.254’ W 73 57.575’

Here you will see two different features: (1) columnar joints, and (2) the contact between the bottom of the Palisades sill and older Lockatong sedimentary rocks.


Columnar joints

As you walk along the Palisades you may notice that sections of rock look prismatic and are vertically stacked, like a group of pencils standing on end. At this stop, look to the right and you will see three of these structures hanging down from rocks. This is called columnar jointing, and is common in basaltic rocks. Columnar joints form when molten basalt cools and contracts as it hardens into solid rock.



Lower contact

At the contact between the igneous rock and the older sedimentary rock you will notice that the igneous rock is finer grained and textured differently. This is because magma that cools quickly forms smaller crystals than magma that cools slowly. The temperature of the intruding magma was roughly 1400 - 1100 degrees C and the surrounding country rock was about 25 degrees C. Thus, at this contact the magma cooled very quickly, creating a very fine grained crystalline rock. This is known as the chill zone.

Also, you will notice that below the contact, the sedimentary rock looks baked. Once again this is because of the temperature difference between the magma and the sedimentary rock. The heat from the magma altered (or “metamorphosed”) the sedimentary rock, transforming it into metamorphic rock called hornfels. Metamorphism that occurs due to the heat of an igneous intrusion is known as contact metamorphism.




To claim this cache: Answer at least one of the following questions, and post your answer in your log. Tell us how many people were in your group. (You don't have to wait for a confirmation from us to claim the cache. We trust you!)

1. Describe the xenolith. Why is it there? 2. What is the approximate height of the cliffs that make up the Palisades sill at this location? (Use your GPS to determine the altitude at the bottom and the top of the cliff.) To get to the top, use the stair access to the right of the cliff at the circle by Ross Dock.)


REFERENCES

Husch, J.M., 1992. Geochemistry and petrogenesis of the Early Jurassic diabase from the central Newark basin of New Jersey and Pennsylvania: Geological Society of America Special Paper, v. 268, p. 169-192.

Puffer, J.H., Husch, J.M., Benimoff, A.I., 1992. The Palisades Sill and Watchung Basalt Flows, Northern New Jersey and Southeastern New York: A Geological Summery and Field Guide: New Jersey Geological Survey Open-File Report OFR 92-1.

Walker, K.R., 1969. The Palisades Sill, New Jersey: A Reinvestigation: Geological Society of America Special Paper, v. 111, p. 1-166.


This Earthcache is brought to you by the
NEW JERSEY GEOLOGICAL SURVEY
an agency of the New Jersey Department of Environmental Protection.



Visit us at www.njgeology.org





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Current Time:
Last Updated: on 10/29/2014 1:43:18 PM Pacific Daylight Time (8:43 PM GMT)
Rendered From:Unknown
Coordinates are in the WGS84 datum