A Teacher’s Guide to Staten Island Geology

Staten Island provides teachers with a good opportunity to introduce students to some of the basic principles of geology. All three basic rock types can be found in a relatively small area, and there are still places where teachers and students alike can do their own rock collecting. 

Categories of Rocks

Igneous: Rocks that have been formed from the cooling and crystallizing of molten rock (either magma or lava). Basalt and granite are two common igneous rocks. Igneous rocks usually have what is known as a crystalline texture made of interlocking minerals grains. The grains fit together, like the pieces of a puzzle, with no spaces in between.

Sedimentary: These are rocks that have been formed from the eroded and weathered particles of other rocks. The individual grains that make up the rock have usually been cemented together by other minerals, often quartz or iron oxides. Igneous, metamorphic and sedimentary rocks can all be broken down by physical and chemical weathering, and their grains recombined to make a sedimentary rock. Sandstone and shale are common types of sedimentary rocks.

Metamorphic:   These are rocks that have been exposed to heat, pressure and chemically active fluids (superheated water), and have changed to become a different kind of rock. For example, sandstone (a sedimentary rock) exposed to heat and pressure, becomes quartzite (a metamorphic rock). Likewise, limestone changes into marble. This change takes place in the solid state; in other words, the rock does not melt and then recrystallize (then you would have an igneous rock).
 

Staten Island Rocks:  the New York City Group

This is a group of related rocks, formed around the same time, about 550 million years ago. These are the so-called basement rocks of Staten Island, and they do not outcrop anyplace, although you can see them on the surface in the Bronx and Manhattan. Typical kinds are Fordham gneiss, Manhattan schist and Inwood marble (all metamorphic), and granite pegmatite (igneous).

Where to find them
Although there are no surface outcrops, during the 1960s a water tunnel was dug between Brooklyn and Staten Island. The boring went down about 1,000 feet, and went through the New York City Group rocks. Much of the waste rock was dumped on vacant land at the end of Forest Avenue, and provided a fertile hunting ground for local rockhounds for many years.  

The rock dump area has been reduced in size in recent years, but is still available for collecting. It is located just outside the Home Depot parking lot at the end of Forest Avenue. Between the parking lot and Forest Avenue there is an open weedy area. This is what remains of the old rock dump. There are several piles that have been excavated, where specimens may be collected fairly easily.

What you will find
Gneiss: This rock has a banded appearance, with alternating layers of light and dark minerals. The light minerals are quartz and/or feldspar. The dark minerals are biotite mica and/or hornblende.

Schist: Some of the rocks will be composed almost entirely of sparkly mica grains. The ones composed of silvery grains are muscovite schist; the one composed of black grains are biotite schist. Both can also contain small masses or thin layers of quartz, and small red garnet crystals. Schists typically show a foliated, or layered appearance, from the presence of so many flattened mica grains.

Marble: This is a fairly plain-looking white rock, sometimes containing small grains of a light brown mica (phlogopite). It will fizz when exposed to dilute hydrochloric acid (but probably not with vinegar).

Granite pegmatite: The granite here has fairly large grains, so that it is classified as a pegmatitic granite, indicating that it took a long time to cool and crystallize.  Look for a light-colored, mottled rock, containing grayish quartz, white to pink feldspar, and silvery muscovite.

Handy tips
Winter is a good time to go collecting, as the vegetation is down, and it is easier to see the rocks. Avoid summer, as numerous bees and wasps find the flowering weeds growing in the lot attractive. There are also bound to be ticks there, from spring through fall.

Digging tools really aren’t necessary, as there are probably enough rocks on the surface. A geologist’s pick/hammer is useful if you want to break down the pieces into student sizes. Don’t use a regular carpenter’s hammer; they are not made for hitting rocks and may break.

Contributed by Edward W. Johnson, Staten Island Museum