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Rock Study
BACKGROUND INFORMATION
Beneath New York City is a layer of rock approximately 450,000,000 years old. This rock layer, although mostly
hidden, has had a great influence on the formation and development of the city we live in. While most visible traits of
this rock are long gone, Central Park gives us an opportunity to see the foundations of our city. In this unit, students will
learn about the rocks most common to our area and how they were formed, and changed, over the years.
A rock is a naturally occurring substance made up of one or more minerals. Rocks are formed by a variety of processes
that occur on or within the earth, or through changes that occur to existing rocks. Minerals are also naturally occurring
substances formed inside the earth. They are formed from a combination of chemicals that are found in the mantle, the
hot liquid layer beneath the earth’s crusts. Minerals are inorganic (i.e., non-living) and have the same chemical
composition throughout. Minerals form when portions of the liquid mantle cool and solidify, or when liquids evaporate.
A rock may be made exclusively from one mineral, or from a combination of minerals.
There are many different kinds of rocks found on the earth. Geologists classify rocks based on how they are formed.
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Igneous rocks are formed when molten magma cools and hardens. As the magma cools, the minerals crystallize.
Igneous rocks can be formed inside the earth (called intrusive) or outside (called extrusive) when a volcano erupts.
Intrusive igneous rocks are exposed when the rocks above them have worn away. Geologists classify igneous rocks
according to the size of the crystals and the mixture of minerals found in the rock. Granite is one of the most
common igneous rocks on earth.
Sedimentary rocks are formed when sediment (small pieces of rock broken off from larger pieces of rock) are
deposited, build up in layers, and harden into a new rock. Sedimentary rocks are often formed in aquatic
environments such as river beds and deltas, where water carries material and deposits it. These rocks are often
layered, and may contain fossils. Limestone and sandstone are two examples of sedimentary rocks.
Metamorphic rocks, as the name implies, are rocks that are changed from their original form. This change is brought
about by extreme heat and pressure within the earth, caused by various geologic processes. The heat and pressure are
enough to cause the rock’s minerals to recrystallize and change positions, but not quite great enough to cause the
rock to completely melt. Igneous, sedimentary, and even other metamorphic rocks often (but not always) have wavy
lines and patterns, or alternating bands of light and dark minerals that result from the pressure exerted on the rock.
Slate, schist, and marble are examples of metamorphic rocks.
Manhattan Mica Schist, a metamorphic rock, is one of the three kinds of rocks underlying New York City, and it is the
type most often seen in Central Park. The other two types of rocks in NYC are Inwood Marble and Fordham Gneiss.
Rocks are made up of one or more minerals. Manhattan Mica Schist is made up of the minerals feldspar, hornblende,
biotite (black mica), muscovite (white mica), and quartz. To identify the minerals in rocks, geologists study several
traits:
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Streak is the color a mineral leaves behind when scratched across a streak plate or unglazed porcelain. Many times,
minerals leave a streak that is a different color from the mineral itself.
Luster is the way a mineral reflects light. Quartz, diamonds, and other minerals have a glassy luster because they
shine like glass. Others, such as talc, have a dull luster. Still others, like gold, have a metallic luster.
Hardness is yet another trait of minerals. If you can scratch a mineral with your fingernail, it’s rated as very soft. If
you can scratch it with a penny, it’s soft. If you can scratch it with steel scissors, it’s medium. Finally, if steel
scissors or a file won’t scratch it, it’s hard.
Rock Study
VOCABULARY
Biotite: a typically brown to black rock-forming mineral present in at least some percentage in most igneous and both regional and
contact metamorphic rocks
Feldspar: the major constituent of many igneous and metamorphic rocks; they form at medium to high temperature and at some
depth; Ralph Waldo Emerson once wrote, "A man is like a bit of Labrador Spar, which has no luster as you turn it in your hand until
you come to a particular angle, then it shows deep and beautiful colors."
Fordham Gneiss: (pronounced nice ) a metamorphic rock; the oldest of the three New York City rock formations; characteristically a gray rock, frequently banded in appearance, and composed mostly of the minerals quartz, biotite (black mica), and several
kinds of feldspar
Garnet schist: metamorphic schist consisting of the less commonly found garnet mineral; of all minerals, garnet exists in the greatest variety of colors except blue
Geologist: a person who studies rocks and minerals that make up the earth's crust to learn more about the earth
Geology: the study of the earth, its rocks and minerals, and the changes it undergoes
Glacier: a body of ice, mainly consisting of recrystallized snow, flowing on a land surface; as the glacier that covered most of New
York 40,000 years ago advanced, it scoured and plucked out large amounts of rock and soil from the regions in its path; in this process of advancement rocks dragged under the glacier polished the bedrock and carved grooves and striations into it; when the ice
melted, a thin layer of clays, small pebbles, and small boulders were left behind
Hornblende Cleavable: a mineral of metamorphic and igneous rocks
Igneous: igneous rocks are formed when molten rock from deep within the earth comes to the surface, cools, and hardens.
Inwood Marble: a metamorphic rock; white to light gray rock
Magma: the molten rock within the earth's crust
Manhattan Mica Schist: a metamorphic rock; makes up the bedrock of most of the island of Manhattan; like Fordham Gneiss,
fairly resistant to erosion and therefore underlies higher ground; composed mainly of mica schist, a foliated dark gray rock composed
of the minerals muscovite (white mica), quartz, biotite (black mica), hornblende, and several kinds of feldspar
Metamorphic: metamorphic rocks are formed when sedimentary and igneous (and even metamorphic) rocks are subjected to extremely high temperatures and pressure
Mica Schist: metamorphic rock usually derived from fine-grained sedimentary rock such as shale; commonly contains the flat,
transparent mineral mica
Milky quartz: any quartz crystal or cluster that is white in color and cloudy; the cloudiness occurs due to fluid present during crystal
formation
Mineral: a substance that is inorganic, that occurs naturally in the earth, and that has the same chemical make up through and
through, and whose elements are bonded together in a specific pattern such as in crystalline form
Muscovite: a common rock-forming mineral found in igneous, metamorphic and detritus sedimentary rocks; an easily cleavable but
durable mineral occurring in shades from white and silver to green and brown
Rock: a naturally occurring substance which is composed of one or more minerals; rocks are divided on the basis of the mode of
origin into three broad classes — sedimentary, igneous, and metamorphic
Sedimentary: sedimentary rocks are formed when sediment accumulates in layers and solidifies
Weathering: the discoloration or breakdown of rocks and minerals due to rain, wind and water
Rock Study
LEARNING FROM OBSERVATION
Summary: Students will learn how to analyze rocks and rock formations in order to draw conclusions about mineral
composition, glacial scarring, evidence of weathering, and signs of life.
Materials: pieces of Manhattan Mica Schist, magnifying glasses, photograph of glacier, pencils and paper for taking
notes on nature hike
Activities:
Teacher will take class on a hike to explore rock formations around the Harlem Meer.
PRELIMINARY EXAMINATION OF SCHIST: Sitting in a circle, students will be given one piece of
Manhattan Mica Schist to closely examine with magnifying glasses (hand out “Geology Worksheet — Manhattan Mica
Schist”). At the end of this exercise, they will be challenged to recognize their particular rock amongst the others in the
group. Questions to consider include the following: Was every student able to recognize his or her rock? What did
students remember about their rocks that enabled them to distinguish them from the rest of the group? Is the rock hard,
bumpy, shiny, etc.? What composes the rock? Is it made up of different materials?
MINERALS: Walk to the rock formations on the south shore of the Harlem Meer. Stop at the first large boulder
you see. Its mineral bands are obvious. (Remember that students can walk around the rocks in this area but they may
not climb.) Look for the minerals in the Manhattan Mica Schist on nearby rocks. Instruct students to draw a close-up of
the mineral patterns.
GLACIERS: Walk to the next large rock formation which extends from the top of the hill to the path. Explain
that 40,000 years ago a thick sheet of moving ice covered our area. The rock they are looking at was formed a long time
before the glacier arrived. The glacier came from the northwest and moved right over this rock. What do they think a
moving river of ice would do to the rock underneath it?
As you take a look at this rock, notice the southeast side of the rock formation, which is rather jagged. The
glacier “plucked” out pieces of rock as it moved over this side of the outcropping (this effect is actually called
“plucking”). The glacier smoothed the northwest side of the rock. It is easy to notice the difference in the two sides of
this rock formation as you walk from one side to the other along the path.
As the glacier moved, it carried small and large pieces of rock and sand with it. What effect do the students
think this would have on the rock underneath it? Instruct the students to examine the rock carefully for scratches in the
rock. These are called glacier scratches and grooves. They were formed by the rocks and boulders the ice carried with
it. Show the children pictures of present-day glaciers to help them understand what a continental glacier looked like.
WEATHERING: Explain that rocks are always in a constant state of change. Can students think of any ways
that rocks can be changed? How do these changes occur? Instruct students to find a rock that has been changed in some
way and to record their findings in their journals. Discuss how rocks can be changed by water, ice and freezing
temperatures, the growth of plants and trees, chemicals, and the effect of humans. In addition, lichens, which eat or flake
away pieces of rock to make soil, are dangerous to rocks.
SIGNS OF LIFE: Allow students to examine rocks for signs of life such as small plants and lichens, mosses,
insects, or other wildlife. Explain that rocks are the main support for both man and nature. Rocks hold moisture and
offer warmth or coolness, and they also shelter plants and animals. Discuss places or things made out of rocks, minerals,
or soil.