Download Make Your Own Fossils!

Anna Cajiga
SME 301
Geology Summary Corrections
April 12, 2005
Title
Make Your Own Fossils!
Benchmark
Area: Earth Science
Category: Geosphere
Benchmark: Explain how rocks are formed (Middle School).
Misconception
Fossils are actual preserved animal or plant parts.
Background
There is a common misconception that fossils are all preserved animal or plant parts.
However, this activity illustrates that many fossils are in fact molds or casts of the original plant
or animal. Although some fossils are preserved animal or plant parts, such as petrified wood,
there are also other types of fossils. When a living organism falls into mud and leaves an
impression, the sediments in the mud can become compacted and cemented, forming a
sedimentary rock that still has the impression of the organism in it. This is known as a mold
fossil. Sometimes, this impression can be filled into with sediments that eventually become
compacted and cemented to form a sedimentary rock that has the same shape as the living
organism. This is called a cast fossil. In both cases, the fossil that remains is not the preserved
plant or animal, but instead a cast or mold of it.
Materials
An example of a real mold fossil
An example of a real cast fossil
Clay
Shell
Two paper cups
Plaster of Paris
Scissors
Water
Directions
1. Flatten a piece of clay until it looks like a thick pancake.
2. Press the shell into the clay and then remove it.
3. Observe the clay and compare it to the example of a real mold fossil.
4. Cut the bottom off of a small paper cup to make a collar.
5. Put the collar around the mold fossil you just created.
Pour about half an inch of water into another paper cup. Slowly add plaster of Paris and stir until
it has the consistency of thick soup.
6. Pour the plaster of Paris into the collar on the mold fossil. Allow it to dry for 24 hours, then
remove the dry plaster of Paris cast from the mold.
7. Observe the cast and compare it to the example of a real cast fossil.
Results
At the conclusion of this experiment, students will have one mold “fossil” where they can
clearly observe the imprint of the shell in the clay. They will also have one cast “fossil” in which
they can see that the plaster of Paris has formed a shape that is identical to the original shell.
Discussion
There are three types of rocks in existence – sedimentary, igneous and metamorphic.
Sedimentary rocks form when weathering of rocks and landforms creates sediments on the
surface of the earth. When sediments pile up on top of each other, the pressure from the
sediments on top presses the sediments on the bottom together. Minerals, usually calcium
carbonate, dissolve in the water and become like cement that “glues” the sediments together.
This is called lithification. When sand sediments are compacted and cemented, sandstone is
formed. Silt and clay sediments can go through lithification and become shale. When shells, or
calcium carbonate, are cemented and compacted, limestone is formed. You can often see layers
in sedimentary rocks since they are formed from different sediments building up over time.
Fossils are also found in sedimentary rocks because sedimentary rocks form at the earth’s
surface, which is where living organisms are found. Mold fossils and cast fossils are two types of
fossils. Mold fossils are formed when a living organism falls into mud and leaves an impression
in it. In this activity, mold fossils were illustrated by pressing a shell, which represents a living
organism, into the clay, which represents mud. In the real world, after an organism falls into
mud, the mud can harden into rock over time so the impression of the animal is left in the solid
rock. A cast fossil is formed when a mold fossil is filled in with sediments over time. These
sediments can then be compacted and cemented to form a sedimentary rock that has the exact
same shape as the original living organism. In this activity, the impression of the shell in the clay
represents the mold fossil, and the plaster of Paris represents the cast fossil that was created as
sediments built up over time and became compacted into a sedimentary rock. Sedimentary rocks
are not the only types of rocks. After a sedimentary rock has formed, it can either be weathered
back into sediments, or it can sink below ground through cracks in the earth’s surface. Once
underground, one of two things can happen to the rock. Heat and pressure can be applied to the
rock until a chemical change takes place and a new material is formed. This new material would
be a metamorphic rock, which are formed when pre-existing rocks undergo pressure and heat.
This process is called metamorphism. When sandstone becomes a metamorphic rock, it is called
quartzite. Shale becomes slate when it is put under intense pressure and heat, and limestone
becomes marble. After their creation, metamorphic rocks can undergo weathering and turn into
sediments. The other option that both metamorphic rocks and sedimentary rocks can undergo is
sinking underground through cracks in the earth’s crust and eventually melting into magma in
the hot mantle of the earth. This will result in the creation of igneous rocks. Like the formation of
metamorphic rocks, the creation of igneous rocks occurs underground. If rocks melt into magma,
they will eventually cool again, forming igneous rocks. Igneous rocks can either cool slowly
underground, called intrusive igneous rocks, or quickly above ground, called extrusive igneous
rocks. When the rocks cool quickly, the crystals in them are too small to be seen with the naked
eye. When they cool slowly, they have time to form large crystals that can be seen with the
naked eye. Granite, basalt, pumice and obsidian are all examples of igneous rocks. Granite has
large crystals and basalt has small crystals. When pumice forms, gasses get trapped in it and
form holes. Igneous rocks can then be melted again and undergo metamorphism to change into a
metamorphic rock. Igneous rocks can also be weathered into sediments and then undergo
lithification to be formed into a sedimentary rock. This whole process of one type of rock having
the ability to become any other type of rock is known as the rock cycle. It becomes clear that, as
the Law of Conservation of Matter states, the matter on earth is never created or destroyed, just
changed. One type of rock can eventually become another type of rock. All sediments on the
earth will also eventually be turned into new rocks.
There are many ways for the sediments that become rocks to come about in the first
place. To understand this, we must first understand landforms and the various materials that
cover the earth’s surface. The earth’s surface does not look the same everywhere. One reason for
this is because of landforms. Landforms are the natural physical features of the earth’s surface.
For example, some areas are called plains. Plains are large areas of level land that is low in
elevation, meaning no higher than 1000 feet above sea level. Other areas are plateaus, or large
areas of level land that are high in elevation, meaning higher than 1000 feet above sea level.
Plateaus are surrounded in steep cliffs. Mountains are land that rises steeply and comes to very
narrow tops. Highlands are similar to mountains in that they are elevated land, but highlands are
not as high or steep as mountains. Basin and range is another landform. Basin and range is lowlying lands surrounded by higher ground on all sides. Landforms are not the only way that the
surface of the earth varies. The earth’s surface is also covered in many different kinds of
materials. One of these materials is soil, which is made of rock sediments, living and dead
organisms, water, and air. Some places on earth are just covered in sand, or small pieces of rock
sediments. Other kinds of rock sediments that can be found on the earth are, from smallest to
largest, clay, silt, pebbles, cobbles and boulders. All of these landforms and different materials
on the earth’s surface are connected to each other. The landforms can be broken down into
smaller pieces, and these smaller pieces are in turn broken down into even smaller pieces. This
process is called weathering, which is the breakdown of rocks and landforms into smaller pieces
called sediments. There are many ways that weathering can take place, through both physical
means, when just the appearance of the rock changes and the rock is made of the same materials
before, and chemical means, when chemical changes take place and the rock is no longer the
same material. Physical changes are caused by many factors. The force of moving water can
break down rocks into smaller pieces. This often happens in mountains where water can rush
down the sides of the landform. Eventually, this weathering can result in mountains becoming
highlands. Weathering from water also occurs in the floodplains of bodies of water, where rising
waters can wear away at rocks and landforms, leveling the land. Water can also fill tiny cracks in
rocks and then expand when it freezes, thus creating pressure that breaks the rocks into smaller
pieces. Large blocks of moving ice called glaciers also create a lot of force that has the potential
to break rocks apart. Glaciers can contribute to the creation of basin and range landforms. The
force created from rocks or sediments falling down a hill because of gravity can also weather
rocks. This frequently occurs on mountains and highlands because of the high elevations and
sloped sides. Mudslides are a great example of gravity causing weathering. Wind can remove
small sediments from rocks, and it can also carry sediments that hit other rocks and contribute to
them breaking apart. This factor plays a big role in weathering on plains and plateaus, where the
wind sweeps across the flat land, breaking apart rocks as it passes. In all landforms, animals
burying in the earth’s surface and plants growing in the soil both also contribute to weathering.
Another significant reason for weathering in all landforms is the actions of humans. Among
many other things, we build buildings and roads, dig mines, and plant food to eat, all of which
break apart and change the earth’s surface. Chemical changes can be caused when minerals in
rocks react with oxygen, called oxidation, or through minerals in rocks reacting with acid, which
often occurs because of acid rain. When earth materials are weathered and broken into smaller
pieces, these smaller pieces can then be transported to other locations, usually by water or wind.
This is called erosion. Together, weathering and erosion cause a leveling of the earth’s surface as
hills are worn down by weathering and valleys are filled in with sediments that have eroded.
While weathering and erosion work to level the earth’s surface, there are forces inside the
earth that work to build up the earth’s surface. Inside the earth, there are three layers. On the
outside, there is the outer crust, which is a brittle solid that is very thin, only about 10 to 25 miles
thick. Next is the mantle, which is about 2000 miles thick and is a moldable solid, similar to the
consistency of cornstarch and water. Lastly, in the center is the core, which is also about 2000
miles thick. There are two parts to the core, the outer core, which is a liquid layer, and the inner
core, which is solid. The outer crust is broken into 12 very large pieces that float on top of the
mantle, called tectonic plates. The continents and oceans sit on top of these plates. Plates holding
continents are called “continental plates” and plates holding oceans are called “oceanic plates”
Continental plates are made of granite and oceanic plates are made of basalt, which is denser
than granite. These plates do not sit still, they are actually always moving very slowly, a
movement that is called plate tectonics. Millions of years ago, all of the continents were one land
mass called Pangaea, but because of plate tectonics, they have slowly moved into their present
positions. The reason plate tectonics occur is that the mantle and the core are very hot because
radioactive decay of rocks heats the surrounding areas. Temperatures raise higher and higher
closer to the center of the earth. This allows convection currents to occur. As the mantle closer to
the center of the earth heats, some of the mantle melts and magma is created. The magma, or
molten rock, is very hot, so the molecules spread further apart and it becomes less dense than the
surrounding mantle and it begins to rise. As it rises towards the outer crust, temperatures being to
cool, so the molecules move closer together, it becomes denser, and begins to sink once again.
This cycle causes the upper mantle to move, and in turn, the plates riding on the mantle also
move. Different things happen when the plates move towards each other or away from each
other. When tectonic plates pull apart from each other, it is called divergence. When the plates
pull apart from each other, an opening appears, called a trench. Magma from the mantle then
comes up through this crack, and then cools, creating new, mountainous crust. This is happening
in the middle of the Atlantic Ocean. The chain of mountains and volcanoes in the Atlantic Ocean
is called the Mid-Atlantic Ridge. Tectonic plates can also move towards each other, called
convergence. Two things can happen when plates converge. If the plates have the same density,
they simply crumble and fold as they push together, creating mountains. If the plates are of
different densities, the plate with the higher density sinks below the plate with the lesser density.
In other words, oceanic plates will sink below continental plates because oceanic plates have a
higher density. This is called subduction. The plate that sinks melts in the mantle and becomes
magma because of the high temperatures here. This magma then comes back to the surface
through cracks in the crust and creates volcanoes. Lastly, tectonic plates can slide past each
other, which is called a transform fault. When this happens, the top part of the plates sticks
together but the lower part slides past each other. This causes strain to build up in the rock,
which can eventually be released and an earthquake results. Plate tectonics cause the surface of
the earth to constantly change, but it is important to remember that since the plates in the earth’s
crust are moving so incredibly slow, these changes take a very long time to take place.
Gaining more knowledge about geology helps us to explain many facts we may not have
understood before. For example, learning about plate tectonics explains why earthquakes and
volcanoes always tend to occur in certain places around the globe. This is because earthquakes
and volcanoes are caused by the movement of tectonic plates – earthquakes by plates that slide
past each other, and volcanoes by one plate that sinks below another plate. Since this is what
causes volcanoes and earthquakes, it makes sense that volcanoes and earthquakes would occur at
locations around the globe where two tectonic plates meet. Understanding plate tectonics also
explains why the continents appear as if they could fit together like puzzle pieces. Since tectonic
plates are always slowly moving as a result of the convection currents caused by the heating and
cooling of the earth’s mantle, it is possible for them to drastically change locations over time.
Because of this, the continental plates were, at one time, one large land mass, that has since then
broken apart into the separate continents. Weathering helps us understand how potholes in the
road form. When water melts, it can fill the cracks and holes in the roads. If the temperature then
drops, the water will freeze and form ice, which takes up more space than water does. This
applies pressure to the road and can contribute to it cracking more, and eventually crumbling.
When this happens repeatedly, large potholes will form. Lastly, learning about the rock cycle
allows us to see how various rocks are formed. This can help us explain why we can see crystals
in some rocks and we cannot see crystals in other rocks. When igneous rocks are formed from
the cooling of magma, the magma can either cool quickly above the surface of the earth, or
slowly below the surface of the earth. When magma cools slowly, it has time to form crystals
that we can see with the naked eye, but when it cools rapidly, it does not have time to form
crystals large enough to be seen with the naked eye.
Classroom Resources
1. Plate Tectonics with an Orange
(Found at <http://www.coaleducation.org/lessons/wim/10.htm>.)
In this activity, students are asked to peel an orange in as few pieces as possible and then use
toothpicks to re-secure the pieces of peel to the orange in order to gain a clear visual
representation of the earth’s crust, plate tectonics and fault lines.
2. Everybody Needs a Rock by Byrd Baylor
This picture book teaches children to explore the characteristics and qualities of rocks using their
senses, and even encourages them to start their own rock collection so they can further explore
aspects of geology.
3. “The Earth Field Trip” NOVA Video Set
This set of videos includes a detailed teacher’s guide and videos about volcanoes and
earthquakes with real life footage, allowing students to understand and visualize the effects of
plate tectonics and the forces within the earth.
Credits
I found this activity in the course pack for SME 301. I reworded the directions a bit in
some places, and I added steps for students to compare the “fossils” they created to real cast and
mold fossils, because I felt that this would help strengthen their understanding of the different
kinds of fossils and how they are created.