CTgeo

Connecticut Rocks

Background:

Geologists have divided rocks into three large groups, based on the way the rocks form. They are:

  1. Sedimentary rocks: Rocks that form from the cemented sediments of weathered rocks (called clastic sedimentary rocks) and rocks that form from sediments that precipitated out of water (called chemical sedimentary rocks).
  2. Igneous rocks: Rocks that form when melted rock cools and solidifies.
  3. Metamorphic rocks: Rocks in which all or some of the minerals have recrystallized but not melted, as a result of being deeply buried and subjected to higher heat and pressures than the minerals could withstand. Any kind of rock can become any other kind of rock. The Earth recycles.

Rock Cycle Chart

 

 

 To see larger photos, click on thumbnails

Objectives:

1.  You will learn about the three different types of rocks.

2.   In the lab you will make examples of some of the various rock types. 

3.   Hardness (resistance to abrasion) of various rocks will be determined by experiment in the lab.

 4.  You will examine and learn to identify some of the rocks found in Connecticut.

  Materials: 

  •             4 small aluminum pie tins for each group

  •             Sand

  •             Salt

  •             Sand and gravel mixture

  •             Fine soil, preferably containing clay

  •             White craft glue

  •             Clear jars with lids, at least one for each group

  •             Waxed paper or plastic wrap

  •             Small state bedrock map

  •             Large state bedrock map

  •             Set of rocks containing igneous, sedimentary and metamorphic rocks found in Connecticut

  •             Two coffee cans for each group

  •             Balance scale

  •             Samples of granite and marble

 

Procedure:

 Sedimentary Rocks

 

In this activity you will make models of sedimentary rocks in order to understand better how they form. (The first part of this activity has been modified from EarthComm, Understanding your Environment, 2000, It’s About Time Press.)

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Figure 1. Arkose (sandstone) on north side of Sleeping Giant in Hamden. Called brownstone in the building trade.

 

Figure 2. Locations of sedimentary rocks in Connecticut

 

 

 

 

 

 

Follow the directions below to simulate the formation of four different kinds of sedimentary rocks. The fifth activity is designed to help you understand how materials settle when they are deposited by water.

 

  1. Mudstone (or shale, if made with clay)
  • Spread some wet mud thinly in a pan.
  • Set the mud out in the sun undisturbed until all the moisture has evaporated from the mud. 

 

2.   Rock Salt

  1.  
    • Add salt to a container of warm water. Dissolve the salt by stirring, and continue to add more salt, until the water contains so much dissolved salt that it begins to collect on the bottom of the container. This is called a saturated solution.
    • Pour a few millimeters of the saturated salt water into a shallow plate, dish or pan.
    • Let the water evaporate overnight. Do not disturb the setup until all the water has evaporated. 

 

3.   Sandstone 

  1.  
    • Make a mixture of half water and half white craft glue. (Elmers Glue works well.)
    • Combine this mixture with a handful of dry sand in a small container. Pour off any excess liquid.
    • Line a small bowl with waxed paper or plastic wrap and pour in the sandy mixture.
    • Let this mixture stand undisturbed until all the water has evaporated, which may take several days.

 

4.   Conglomerate

  1.  
    • Make a mixture of half water and half white craft glue.
    • Combine this mixture with a handful of sand, gravel, and clay in a small container. Pour off any excess liquid.
    • Proceed as for sandstone

 

5.   Sediment deposition

  1.  
    • Pour a mixture of soil, sand, and gravel into a clear container. Add water to fill about three-fourths of the container.
    • Close and shake the container; then set it aside where it will not be moved or disturbed in any way.
    • Observe the container over the next several days.

a)     Describe what you observe immediately, by the end of class, and over the next several days.

 

Observations and Comparisons: 

  1. Examine the "rocks" that you made. These samples are models of sedimentary rocks.
    1. Remember that clastic sedimentary rocks form when particles of weathered materials settle and are cemented together. Which of the rocks you made are clastic rocks?
  1.  
    1. Are any of the rocks that you made chemical sedimentary rocks? If so, which one(s) and why do you think so?
  1.  
    1. Did any features appear in any of the rocks as a part of the drying process? If so, describe them. 
  1.  
    1. Do you think it is possible that the features you described in the previous question could be found in real rocks? Explain.
  1.  
    1. Draw a labeled diagram of each sedimentary rock that you made.
  1. A collection of real sedimentary rocks have been numbered and set out for you to observe. Compare these real rocks with the model rocks you made. Are they similar? Try to identify the real rocks by pairing them up with the rocks you made. Write the numbers of these rocks next to the rock’s name:
  • Shale or Mudstone:

  • Sandstone:

  • Conglomerate:

  • Rock salt:

 

  1. Look at the simplified bedrock map of Connecticut. 
  1.  
    1. Does Connecticut have any sedimentary rocks?
  1.  
    1. If so, where are the sedimentary rocks located?
  1.  
    1. Name the sedimentary rocks:
  1. Observe the jar of sediments that you prepared.
  1.  
    1. What has happened to the mixture of materials you shook in the jar?
  1.  
    1. How are the sediments arranged by size?
  1. Sediments are also sorted by running water in rivers. The size of particles that a river is able to carry and deposit depends on its speed. Study the table below that indicates the speed that is needed to move different sizes of particles. Then use the table to answer the following questions:

a.      What kind of environment might have produced the sandstones (arkose) found in Connecticut? 

b.     What kind of environment might have produced the shales and mudstones? 

c.      What kind of environment might have produced the conglomerates?

Velocity (m/sec) Grain size moved
1.2 Fist-sized or larger
0.9 Between gravel and fist sized
0.6 Gravel
0.3 Sand 
0.2 Silt
0.15 Clay

 

6. Which of the "rocks" you made doesn't occur in Connecticut? Try to figure out a possible reason why this rock is not found in Connecticut.

Igneous Rocks

Observing Connecticut's Igneous Rocks:

All igneous rocks form from hot, molten rock. How deep the molten rock is when it cools determines how large its crystals grow. Some igneous rocks form when the molten rock called magma flows out onto the ground. On reaching Earth’s surface many gases escape from the magma producing a changed liquid that is renamed lava. Lava, which is a very hot 1000o to 1200o C, is cooled quickly by the much cooler ground and air that surrounds it. As it cools, it becomes a fine-grained rock made of tiny crystals that may be difficult to see.  

Sometimes lava cools so quickly that all its atoms remain jumbled and no crystals form. The texture of such lava rock is called a glass.  

Igneous rocks that form when magma cools deep in Earth’s crust cool very slowly and have large enough crystals to identify easily. Such rocks are called coarse-grained. Igneous rocks with the same composition (made of the same kinds of atoms), but with different textures, do not look the same and are given different names. 

 

Connecticut’s Igneous Rocks

 

 

 

Figure 3. The igneous rocks of Connecticut. Red are Mesozoic-age basalts, brown are Ordovician and Permian granites.

 

 

 

 

About 220 million years ago, the land that eventually became Connecticut was part of the only continent that existed on Earth at that time. This huge supercontinent, called Pangaea, began to break apart causing several large cracks to form in central Connecticut. About 200 million years ago some of these cracks became deep enough that magma squeezed into them from deep in the Earth’s crust and made its way to the surface. The lavas that squirted out over Connecticut at that time produced fissure eruptions that did not form volcanic mountains. Rather, the very fluid lava flowed out over all of the land in Connecticut, Rhode Island and Massachusetts. In fact, many fissure eruptions occurred at that time all along what is now the east coast of the United States as far south as Georgia. The dark basaltic magma came from deep in the Earth, probably from as far down as the upper mantle. 

 The dark gray, fine-grained rock that formed from this lava is called basalt by geologists and is commercially called traprock. Basalt is quarried at several places in central and western Connecticut. The larger quarries are in the towns of North Branford, Wallingford, Meriden, Plainville and Southbury where the basalt is crushed into various sizes to make asphalt, concrete, riprap and driveway stone. (If your driveway is made of dark gray crushed stone, it is very likely basalt.)

                                    

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Figure 4. Diabase of Sleeping Giant in old quarry in giant's head, near Mill River. Sleeping Giant is a sill. The basalt was intruded between layers of sedimentary rock and did not flow out on the surface like most of the basalt in Connecticut.

 

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Figure 5. Basalt dike running from lower left toward upper right through arkose.

 The cracks that formed as Pangaea broke up into several continents did not always extend to the surface. Sometimes, on its way to the surface, the magma in a fissure forced its way between two horizontal layers of rocks and then flowed horizontally for a while before cooling. An intrusion that forms in this way is called a sill.  Such intrusions were near enough to the surface that the magma still cooled quickly, but not as quickly as the lava that forms basalt.  

When a fissure eruption stopped, the magma in the vertical crack cooled about as quickly as that in a sill. These vertical intrusions are called dikes. The magma that cools as sills and dikes solidifies into an equivalent of basalt called diabase that has somewhat larger crystals.  Diabase has the same chemistry as basalt but has a different texture because of its slower formation. Even so, both basalt and diabase are classified as fine-grained rocks by geologists.

Observing Connecticut’s Igneous Rocks:  

  1. Study the bedrock map of Connecticut. 
  • Describe the places in Connecticut where you might expect to find basalt or diabase.
  • Is it possible to collect either of these rocks where you live or where you go to school? 
  • What rock(s) do you see next to the basalts and diabases shown on the map?
  • What other igneous rocks are found in Connecticut?  
  1. A cross-section is a drawing of an imaginary vertical slice through the Earth so that the layers of rock below the surface can be visualized from the side, as you would see them in a steep road cut. Think of slicing into a layer cake. Only when you cut into the cake can you see how many layers there are, how thick each one is, whether there is a fruit filling, etc. Geologists draw cross-section models based on observations of rocks made in the field.

Figure 6.  Hypothetical cross section of folded rocks. Notice how the different units (colors) are found on both sides of the fold on the ground surface.  

      On the top of both the large and small Connecticut Bedrock Maps there is a cross-section. Notice that the sedimentary and igneous rocks in the center of the state are tilted toward the east.  This is because long faults (cracks where the rock moved) formed along the edges of the central part of the state as North America was pulling apart from Africa about 180 million years ago. This allowed the central part of Connecticut to drop down lower than the land to the east and west. The eastern side dropped more, causing the whole central area to tilt toward the east. 

All of this geologic activity fractured rock in Connecticut. But because basalt is formed of interlocking crystals, it survived the tension more intact than the clastic sedimentary rocks that are loose pieces of sediments cemented together. Basalt also doesn’t fall apart as easily when it is exposed to rain, snow, and alternating freezing and thawing. The basalts and diabases have endured and today they form the tops of ridges, called traprock ridges, in central Connecticut. The sedimentary rocks, on the other hand, have worn down and form the valleys in central Connecticut. 

      a.     Look at the part of the large geologic map that contains the sedimentary and igneous rock.  Name all the traprock ridges you can find. 

                  b.  Name the traprock ridge closest to your school. 

                  c.   Have you hiked on any traprock ridges of Connecticut? Which one(s)?

Sometimes magma of basalt-like composition forms where it can't move toward the surface. Eventually, over a very long period of time, this magma deep in the Earth cools and crystallizes into a dark gray rock called gabbro. On the small bedrock map you will find gabbro listed in the lower left corner under "Selected Plutonic Rocks". Plutonic rocks (named after the god of the underworld, Pluto) are those coarse-grained igneous rocks such as granite (described next) and gabbro, which cooled very slowly deep below the Earth's surface. The gabbro in Connecticut is actually now a metamorphic rock called "metagabbro" because it was changed after it formed. You will read about metamorphism when you read about metamorphic rocks. 

At subduction zones, where two tectonic plates are coming together, one of the plates is usually pushed down under the other. The crustal rocks that end up deep in the Earth get hot enough to melt. Since materials expand when they melt, this magma is less dense than the surrounding rock and slo-o-o-o-owly works its way to the surface as a big blob, like a lava lamp, only much more slowly. This magma contains much more silica than a basaltic magma, which makes it thicker and unable to flow easily. If the magma is under pressure and encounters a weak zone, it can crack rock and work its way to the surface to erupt as a volcano.  Many silica-rich magmas do not make their way close to the surface. They cool slowly at depth, where the surrounding rocks are hot, and produce larger crystals, forming a light colored, coarse-grained rock called granite, with randomly oriented grains. The mineral crystals in granite are large enough to be identified easily. One granite in Connecticut is the Narragansett Pier Granite, which is pink, light gray and black. The crystals are all locked together, with no spaces between the grains. It is a very hard rock that is quarried for use as building stone and riprap. You can see granite at many Long Island Sound beaches, as it is frequently used to make sea walls and jetties (long, narrow piles of rocks sticking out into the water). When it is used like this it is called riprap. 

  1. Look at your lab set of rocks. Can you find a piece of granite? What different colors do you see in it? Each color is a different mineral. Minerals are the solid materials that make up rocks. Look at sheet two of the big Bedrock Geological Map of Connecticut to see which minerals make up the Narragansett Pier Granite (Pn on the list of symbols). The piece of "granite" in the rock set is probably a mixture of Narragansett Pier Granite and Stony Creek Granitic Gneiss. You will learn more about gneiss in the section on metamorphic rocks.

There is one more igneous rock found in Connecticut. It formed in a different way from the other igneous rocks. In the Metamorphic Rocks section you will learn how rocks change when they are subjected to much higher temperatures and pressures than those under which they formed. Most minerals in a rock slowly recrystallize, rearranging their atoms to form new minerals or different shapes of the same minerals. But some minerals melt. Each mineral has its own melting point. The mineral ice melts at 0o C, but most minerals melt at much higher temperatures. During metamorphism, some minerals may melt. This melt may then migrate upward through the surrounding hot rock, sometimes finding weaknesses it can flow along. It then cools very slowly, along with the rest of the metamorphic rock. This rock ends up with huge crystals, larger than one centimeter in diameter. Its name is pegmatite. The most common minerals in pegmatite are quartz and feldspar, but often pegmatite ends up with odd atoms in it that no other minerals wanted, so there are often rare minerals in pegmatite. Mineral collectors, who want to find unusual minerals, or large ones, frequently look in pegmatite.  

4.     Look at your set of lab rocks. Can you find the pegmatite? What colors do you see in it?  

5.     Look on the small bedrock map. Where do you find pegmatite? Why do you think it does or doesn’t occur on the map?

 

Cool Rocks

A Lab Exercise to Make "Igneous Rocks" (contributed by Beth Troeger)

Purpose 

Some igneous rocks form by the cooling of magma that remains beneath the Earth’s surface.  Other igneous rocks form when magma reaches the earth’s surface.  This activity will show you some differences in the structure of those two kinds of igneous rocks.  See if you can observe what those differences are and what conditions cause them.                        

Imagine that the molten substance you will be working with in this activity is magma. 

Materials

For each team of four students: 

  1. Matches

  2. 2 ice cubes (store in a cooler or freezer)       

  3. 2-4 hand lenses

  4. Paper towel

  5. 2 paper or plastic cups (2-3 oz.)

  6. 2 votive candles with holders (aluminum foil pans)

  7. metal spoons

  8. 2 lumps of modeling clay

  9. 4 pairs of goggles

  10. 1/4 teaspoon measuring spoon

  11. 1/8 teaspoon of salol (phenyl salicylate) crystals (available from chemical supply stores)  

 

Procedure
Part A 

1.     In each team of four, two pairs of students should place a small amount (less than 1/8 teaspoon) of salol on a metal spoon. 

2.  Melt the salol by holding the spoon more than an inch above a flame. 

3.  Remove the spoon from the flame. 

4.  Add a few grains of salol as “seed crystals”. 

5.  Prop up the spoon handle against a small lump of clay so the spoon stays level. 

6.  Observe the crystals with a hand lens and draw what you see. 

Part B

1.  In each team of four, remelt the crystals in one of the spoons. 

2.  Rest the bowl of this spoon on an ice cube, using clay to keep the spoon level. 

3.  Observe the crystals with a hand lens and draw what you see.

 Part C

Repeat parts A and B, recording the time it takes for crystals to completely form.

 Part D 

Put two or three drops of melted salol on a glass or plastic microscope slide, and watch crystal growth with a microscope. 

(Hint:  If you want to save the salol for reuse, put plastic wrap on the slide before using it.  After the salol crystallizes, it will peel off the plastic wrap easily.)  

Summary 

1.  Which “magma” sample completely crystallized faster? 

2.  Which one produced larger crystals? 

3.  Did the crystals you observed have angular, sharp-edged shapes, or were they more rounded and smooth? 

Remember that igneous rocks form when magma cools.  Observe some igneous rock samples in a classroom collection.  Why do you think some samples are made up of small crystals and others are made up of large crystals?  What conditions could lead to the formation of crystals of different sizes in the various kinds of igneous rocks?

4. Which of your igneous rock samples more closely resembles the salol cooled on ice? 

5. Which of your igneous rock samples more closely resembles the salol not cooled on ice?

 

Metamorphic Rocks

The Rock Cycle chart is a graphical demonstration of what can happen to any rock. A sedimentary rock can become buried by tectonic processes and heat up under pressure to the point where it recrystallizes without melting. It then becomes a metamorphic rock. If it got hot enough to melt, when it cooled and recrystallized it would become an igneous rock. It could even be broken up by weathering and become another sedimentary rock. The same could happen to an igneous or metamorphic rock.

   

Figure 7. Locations of metamorphic rocks in Connecticut. 

 

What do metamorphic rocks look like?

 Because metamorphic rocks form under pressure, and the atoms making up the minerals have time to move around to form the minerals, many metamorphic rocks have a distinct texture. This makes some of them easy to identify. One kind of metamorphic rock, called gneiss (rhymes with ice, silent "g"), tends to have stripes of light and dark minerals. The stripes may be straight, or folded, or broken up into short little segments. It's a little like a double hamburger - light bun, dark burger, light bun, dark burger, light bun.

  

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Figure 8. Gneiss is made up of alternating dark and light bands of different minerals.  

 

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Figure 9. Schist outcrop in Brett Woods Conservation Area in Fairfield, Connecticut.  

 Another rock, schist, has lots of shiny, flat mica flakes in it. These sometimes make schist break up easily and sometimes make it very shiny.  

1.   Which rock samples are schist and which are gneiss? Are there more than one sample of either in your rock set? If so, do they look the same? How do they differ? 

Sometimes the differences between schist and gneiss are not very obvious. Schist has more mica in it than gneiss, but gneiss can have some mica in it. Geologists made these artificial categories for rocks, but nature doesn't always follow the rules.  

Some metamorphic rocks are made up of only one mineral. Pure quartzite is one of those rocks. Because the quartz grains are about the same shape, size and color, it does not have any kind of distinguishing texture. The grains have no preferred orientation. The quartzite in A Connecticut Rock Set has some mica in it, as it is a "dirty" quartzite. It was made from sand with some mud mixed in. You have another metamorphic rock that is made up only of the mineral calcite. It is called marble and also has no distinguishing texture. Marble is blocky and white to light gray, depending on whether or not it is pure calcite. The sample you have is probably light gray, as there was some mud mixed in with the calcite as it precipitated out of water. Marble started out as a sedimentary rock called limestone. Some limestone has shells in it, which are also made out of calcite. But when limestone is subjected to enough heat and pressure to turn it into the metamorphic rock marble, the fossil shells are usually no longer recognizable.  

2.   Which two rock samples do you think are quartzite and marble? They can look very similar, but you can tell them apart by their hardness. Quartz is much harder than calcite.  

Another Connecticut metamorphic rock with no preferred orientation (a non-foliated metamorphic rock) is metagabbro. Two formations, the Preston Gabbro and the Lebanon Gabbro, both in eastern Connecticut, consist of this type of rock. The prefix "meta-" is generally given to metamorphic rocks that still look somewhat like their parent rock. Gabbro is an igneous rock with a composition similar to basalt, but instead of flowing out onto the ground and cooling quickly to form a fine-grained rock, the magma cooled very slowly far below the ground and thus gabbro is coarse-grained. Most gabbro formed at spreading centers, where tectonic plates were moving apart, allowing magma to move up from the upper mantle. Metagabbro is dark gray to black on a fresh surface. If you move it around in the light, you will see black, shiny flat surfaces on the plagioclase and augite grains. 

3.   Locate the metagabbro sample in your rock set. 

4.   Now that you know something about the three different kinds of rocks, and what some of them look like, it's time to figure out what rocks occur in your community. Find an outcrop (you have to be sure it isn't just a loose boulder which could have been moved there by humans or glacier ice) and bring in a piece of rock from it. Compare your outcrop rock to the rocks you have already seen in class. What kind of rock do you think it is? Get together with other students to compare your rock with ones they have brought in. Do your rocks all seem to belong to the same group of rocks, igneous, sedimentary or metamorphic?  

5.   Next look at a geologic map of your area, such as the Generalized Bedrock Geologic Map of Connecticut, or the Bedrock Geological Map of Connecticut, or a map of just your part of the state, such as a geologic quadrangle map. What kinds of rocks does the map show should be found in your area? Does this agree with what you and your classmates decided you had found? If not, why do you think there is a discrepancy? 

What rock is that?

Rocks are divided into three groups, depending on how they formed. As the rock cycle shows, rocks can change from one type to another, but this happens only over very long time periods. Most geologic events happen very slowly.  

Lets look first at igneous rocks. All igneous rocks were once liquid, called magma (although sometimes the liquid was very thick, even a crystal mush, like a partially melted snow cone). When the magma cooled below a certain temperature, crystals began to grow until eventually all that was left was a rock made up of interlocking crystals. Occasionally the magma flowed into water or was thrown up into the air as tiny droplets. In both cases the magma cooled so quickly there wasn’t time for the atoms to organize themselves into a crystal structure, so a volcanic glass resulted. Obsidian, scoria and pumice are examples of volcanic glass. 

Volcanic rocks are those igneous rocks that formed when magma flowed out onto the surface of the Earth, or at least made it so close to the surface that the surrounding rocks were cool. Magma flowing on the Earth's surface is called lava. Crystals then had time to form, but not to grow very large. These are referred to as fined-grained igneous rocks. Magma sometimes doesn’t make it to the surface. At depth the rocks are hot and magma may take thousand to millions of years to cool there. Thus the formation of coarse-grained igneous rocks. 

Igneous rocks are named by their color and grain-size. Color is determined by the chemistry of the original magma. Depending on its chemical composition, various minerals form in the rock. If the magma contains lots of iron and magnesium, dark minerals and thus dark rocks form. If richer in silica, lighter minerals and rocks form. You can identify igneous rocks by looking at the following table.

Grain size

Light color

Intermediate color

Dark color

Fine

Rhyolite

Andesite

Basalt, diabase (a slightly coarser-grained basalt)

Coarse

Granite, pegmatite

Diorite

Gabbro

Glassy

Pumice

Obsidian

Scoria

In Connecticut, we have only basalt (which is commonly called traprock), diabase (also called traprock), granite, pegmatite and gabbro. Both the granite and the gabbro are somewhat metamorphosed (see metamorphic rocks below). 

Sedimentary rocks are another group. They formed either by the cementing together of sediments (small pieces of broken up rocks) or by precipitating out of water, especially if the water evaporated.

 Clastic sedimentary rocks are made up of rock pieces cemented together. One could call these recycled rocks. Or in some cases they are recycled plant or animal remains. The finest grains are too small to see even with a magnifying glass. These fine sediments settled out of very quiet waters in lakes or swamps. The others were deposited by moving waters of various speeds, as you learned in the streams exercise. These rocks are named according to grain size and shape, as well as by mineralogy. 

Table of clastic inorganic rocks (made of rock particles)

Grain size

Rock Name

Too fine to see

Shale, if clay minerals

Siltstone, if fine quartz

Sand-sized

Sandstone

Arkose, if contains feldspar

Gravel-sized or larger

Breccia, if angular grains

Conglomerate, if rounded grains

 

 

 

 

 

 

 

Table of organic rocks (made of former living materials)

Grain size

Rock Name

Fine

Chalk (made of microscopic shells)

Bituminous coal (plant material)

Medium to coarse

Limestone (made of visible shells)

Peat (brown plant material)

Table of rocks that formed as precipitates from water

Diagnostic features

Rock name

Fizzes with dilute HCl

Limestone

Fizzes, when powdered, with dilute HCl

Dolomite

Fizzes with dilute HCl, generally banded

Travertine

Scratches glass, doesn't fizz with HCl, conchoidal fracture

Chert

Very soft, light colored

Rock gypsum

Feels damp, tastes like salt

Rock salt

In Connecticut we have only arkose (a type of sandstone), conglomerate, shale and siltstone. All of these are Triassic or Jurassic in age and are confined to the Central, Pomperaug and Cherry Brook Valleys. They are red to reddish-brown in color, except for the shale, which is dark gray to black.  

On to more recycling. Metamorphic rocks started as sedimentary or igneous rocks, or sometimes even other metamorphic rocks. Because of tectonic changes, such as continental collisions, or deep burial by very thick sediments, they reached a temperature and/or pressure where the minerals became unstable in their present configuration and either recrystallized into shapes which relieved some of their stress, or into completely different minerals comfortable under the new conditions. All of this happened over a very long period of time and without melting.  

Metamorphic rocks are named according to their textures and mineral composition. The two textures are foliated and nonfoliated. Think of leaves, also called foliage. Leaves are wide in two dimensions and thin in the third (their thickness). Foliated rocks have flat layers that are much thinner in one dimension than the other two. They are either striped or in thin layers. Nonfoliated rocks aren't. They are generally made up of the same mineral that is equidimensional (like a ball or cube, not like a leaf). When you look at them you see no preferred orientation of the minerals.  

Table of foliated metamorphic rocks

Description of rock

Rock name

Thin layers of micas (thin, flat minerals), may break easily along layers

Schist

Banded, light and dark layers of minerals

Gneiss

Black, made of tiny needle-like grains

Amphibolite

Fine-grained micas, usually shiny, often crinkled look to surface

Phyllite

Very fine-grained, breaks into flat layers

Slate

Table of nonfoliated metamorphic rocks

Description of rock

Rock name

Hard, scratches glass, made of quartz, sometimes contains minor other minerals

Quartzite

Soft, can scratch with knife, fizzes in dilute HCl, (dolomite fizzes weakly)

Marble

Dolomitic marble

Medium to coarse grained, dark colored

Metagabbro

Black, shiny, light weight, conchoidal fracture

Anthracite coal

Connecticut has no slate or anthracite, although lots of slate is found in Vermont and a little coal occurs in Rhode Island.

 

Identify the rocks in your lab collection on the following sheet, including a description of each one that explains why you think it is what you named it.

 

Rock Identification Sheet

Number

Description

Rock name

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Mechanical Weathering

Teacher Sheet

 

Background

 Weathering is the process by which rocks are broken down by natural conditions at the Earth’s surface.  Because rocks form at depth below the Earth’s surface, the minerals that make up the rocks are not in equilibrium with the conditions of lower temperatures and pressures on the surface.  This makes them unstable.  Rocks are constantly breaking down, but it generally is a slow process, so in the length of a human lifetime, the rocks seem to be unchanged.   

Mechanical weathering breaks rocks up into smaller pieces while not altering the chemistry of the minerals.  However, breaking a rock into smaller pieces exposes more surface area to chemical weathering, so the two types of weathering frequently work together.  In temperate climates such as in Connecticut, frost wedging is a major cause of mechanical weathering.  When temperatures are above freezing during the day in winter, water gets into cracks in rocks.  At night when the water freezes, it expands by 9%, forcing the cracks to widen.  Over time, the rock literally falls apart. 

There are several experiments which students can perform in class to see the effects of weathering.  The first two mechanical weathering experiments are examples of frost wedging and root wedging.  Tree roots work their way into cracks in rock in search of water and nutrients.  As they grow they are able to force the cracks to widen.  The third mechanical weather experiment demonstrates differential weathering by abrading different rocks to see the differences in the amount of material removed from the different samples. 

For the frost wedging experiment, students will examine the process of water expansion as it turns to ice.  They will put water in paper cups to a level marked with a waterproof pen on the outside of the cup.  These will then be frozen and examined again to see that the top of the ice is higher than the water line mark.  Sometimes the paper cup will even be torn.  You can explain that this is what happens to water that is allowed to freeze in water pipes, resulting in leaking, broken pipes. 

Root wedging is demonstrated by planting bean seeds in wet plaster of Paris.  Most of the bean seeds will germinate and split the plaster apart as they grow.  The students may realize that plaster isn’t as strong as rock, but this is still the same process.  To improve the percent of seeds that germinate, soak the seeds overnight before they are put into the plaster. Also, soupier plaster of Paris seems to work better than a drier mix. 

As an example of both abrasion and differential weathering, students will shake up samples of three different rock types to see the differences in how much they break up.  The abrading of rocks happens mostly in streams and on the beach, but also as a result of wind-blown silt. 

Chemical weathering is also important in breaking down rocks. Feldspars gradually break down into clay minerals, allowing the rock to fall apart. The marble of western Connecticut is soluble in acidic water, thus marble occurs now in valleys and also contains small caves.


Mechanical Weathering

 Student Sheet

 

Rocks at the Earth’s surface are constantly being broken down in a process known as weathering.  Mechanical weathering breaks rocks into smaller pieces.  It occurs from various processes.  One of the major types of weathering is frost wedging.  Rocks always contain cracks and water gets into these cracks from the rain.  Some of these cracks are so small they only occur between grains of the rock, others extend for several feet of more.  If the temperature drops below freezing, the water turns to ice.  When water turns to ice it expands by 9%.  This means if the volume of water is 100 ml, after it freezes it occupies 109 ml of volume.  As water changes to ice it has enough energy to push the rock apart, thus widening the crack.  Over time, these cracks continue to enlarge, actually breaking the rock into smaller pieces.  If you ever visit the top of Mount Washington, in New Hampshire, you will find there is no solid bedrock on top, only thousands of loose pieces of rock.  Frost wedging has broken up all of the surface rock on top of the mountain, where temperatures drop below freezing at night most of the year.  Let’s look at how water expands when it freezes. 

Materials 

  • Two 6 oz paper cups

  • Permanent marker

  • Plaster of Paris

  • 250 ml beaker

  • 50 ml graduated cylinder

  • Bean seed

  • 3 pieces of granite

  • 3 pieces of marble

  • 3 pieces of sandstone

  • Coffee can with lid

 

Procedure 

1. Take a small paper cup and put a permanent mark on it 1/2 inch below the top.  Carefully fill the cup just to the line.  Set the cup on a flat surface and look at the water level to be sure it is right at your mark.  Now put the cup in a freezer.  Check it tomorrow to see where the top of the ice is.  How does it relate to the original water level?

2. What do you thing would happen to the pipes in your house if the water froze in them?  Remember, the pipes are completely full of water, so there is no room for expansion.

 3.  Another type of mechanical weathering is root wedging.  When a seed falls into a crack in a rock, if there is a little water in there, it may germinate.  If it find any nutrients, the little seedling will continue to grow.  As it grows, its roots expand and widen the crack.  As an example of how that works, measure out 40 ml of dry plaster of Paris and put it in a 250 ml beaker.  Add 20 ml water and stir until all the lumps are gone.  Now put half of the plaster mix into a 3 oz paper cup.  Drop a bean seed, which was soaked in water overnight onto the center of the wet plaster.  Do not push it in.  Cover with the rest of the plaster mix.  Set this in a place where it will not be disturbed.  Examine the cup after one week.  What do you observe?

One week___________________________________________________________________

8 days__________________________________________________________________

9 days__________________________________________________________________

10 days_________________________________________________________________

2 weeks_________________________________________________________________

 

4. Abrasion is a third type of mechanical weathering.  As small stones are moved around in a stream, they bump into or grind against each other, breaking off small pieces.  Points and edges are especially susceptible to abrasion because they are attacked from two or three sides.  The result is that stones in streams become smaller and rounder over time.  Stones on the land are also abraded by dust in the air on a windy day.  This is a very slow process, but where there is lots of wind and the air contains lots of dust, much weathering occurs over time.  Weathering rates of various rock types are not all the same, depending on mineral content, grain size, rock type and climate.  This results in differential weathering where two or more rock types occur in contact with each other.  For this next exercise you will become the agent of mechanical weathering by abrasion. 

Carefully weigh three samples of granite together.  Record their total weight on the Mechanical Weathering Data Sheet (A).  Now put the pieces in a one pound coffee can and put on the plastic cover.  Shake the can vigorously for three minutes, being sure to hold the plastic lid in place as you shake.  Pour out the rock pieces onto a piece of white paper.  How has the rock changed?

4. Pick up all of the rock pieces larger than this dot ‘.’ and put them onto another piece of paper.  Take these to the scale to weigh them.  Record the weight on the Data Sheet (B).  Carefully clean out the coffee can with a paper towel over a trash can.  Repeat steps 3 and 4 with samples of marble and sandstone. 

5. When you have abraded all of the samples, finish filling out the Data Sheet by subtracting the weight B from weight A.  Record that value (C) for each rock type.  Divide C by the initial weight A, then multiply by 100.  Record that as D.  This is the percent of weathering that occurred to each sample.

 6. Did all of the samples weather at the same rate? 

7. Rank them from most weathered to least weathered

Most weathered                       ________________________

Second most weathered           ________________________

Least weathered                      ________________________

 

8. Which of these rock types would be most likely to form hills or mountains? 

9. Which rock type would make the best tombstones, especially in an area with frequent wind and dust? 

10.  Both the sandstone and the granite contain large amounts of quartz, which is very hard?  Should the quartz be resistant to weathering? 

11.  Why do you think there were differences in the weathering of the quartz-containing sandstone and  granite. 

Mechanical Weathering Data Sheet

  

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