by Linda Sebastian
Northeastern School District
is actually four activities in one inquiry. In What Are Rocks and How Are
They Formed? students observe a variety of rocks and classify them based
on their observable properties and/or characteristics. In How Can Rock Properties
Help to Identify Rocks? students further classify rocks by identifying
and comparing their mass, dimensions, and volume. In How Do Rocks Cycle
on the Earth? students play a game to learn more about the three types
of rocks (igneous, metamorphic, and sedimentary) and how they cycle from one type
to the next over time. In My Life As a Rock students create stories
to show their knowledge of the rock cycle. The Family Page extends this learning
by inviting parents to facilitate home and community observations and discussion
related to rocks in the environment and their various uses in construction.
Related Topics: earth science,
fossils, dinosaurs, minerals, physical change, measuring volume
Ohio Academic Standards Alignment: Click here to view content standards alignment to Science for Ohio by grade level.
- The Three Types of Rocks.
Although there seem to be many different kinds of rocks, they all can be divided
into three main groups, according to the way in which they were formed.
- Igneous (meaning formed
from fire) rocks are formed from molten material (magma) in or below the
Earth's crust. The hot magma breaks through cracks in the Earth's surface
and forms rock called extrusive rock. Liquid magma that
reaches the Earth's surface is also called lava. Extrusive
rock is either glassy or made up of very fine crystals; because it cools
so quickly large crystals do not have an opportunity to form (examples:
basalt, pumice, obsidian). Rocks formed from magma that cooled below the
Earth's surface are called intrusive rocks and are coarser
and have much larger crystals because the magma cooled slowly (examples:
granite, quartz, feldspar, mica).
- Sedimentary rocks
were formed from different sediments that accumulated for several thousand
years and then were cemented tightly together. One kind of sediment that
forms sedimentary rock includes such materials as sand, clay, silt, pebbles,
and gravel. Streams and rivers carry these sedimentary materials to lakes
or oceans, where they settle to the bottom. As the sediment accumulates,
layers are formed, which then slowly change into rock. This change happens
because the weight of the top layers presses the sediment in the bottom
layers tightly together (examples: conglomerate, sandstone, shale, soft
coal). A second kind of sediment that forms sedimentary rock includes
the remains of many plants and animals that live in the ocean and take
calcium carbonate from the water to form shells or skeletons (examples:
limestone, chalk). A third kind of material that forms sedimentary rock
includes such chemicals as salt and calcium carbonate that are dissolved
in the seawater. Because of changes in ocean conditions, the water can
no longer hold these chemicals in solution and they deposit out, forming
accumulations that later harden into rock (examples: rock salt, very pure
limestone). Sedimentary rocks often contain the remains of early animals
and plants, called fossils, embedded in them.
- Metamorphic rocks
are igneous and sedimentary rocks that were buried deeply under other
rocks and then changed by heat and pressure. Metamorphic means "changed
in form." Some changes are physical in nature, where the original
materials in the rock were only rearranged. Other changes are chemical
in nature, where new minerals were formed (examples: gneiss from granite,
quartzite from sandstone, slate from shale, marble from limestone, hard
coal from soft coal, and graphite from hard coal).
- Looking for Clues.
By observing rocks individually, you can deduce much about them. Smooth,
round rocks have most likely been acted upon by running water. Flat rocks
are usually pieces of sedimentary rock. Pebbles and smaller rocks embedded
or cemented in the rock probably mean that the rock is a conglomerate.
Crystals in the rock usually mean that the rock is igneous.
- The Rock Cycle. See information contained in the A Rock Is Born
game (See Ready to Print.)
- Properties of Rocks and Minerals. Note: The properties listed below
are a sampling of terms commonly used by geologists.
- Color is determined by examining the powdery trail left on a
streak plate (piece of dull white tile). Note: Many minerals exist in
more than one color.
- Hardness can be measured formally using the Mohs' scale of hardness.
The higher the number on the scale, the harder the mineral. Hardness can
also be measured informally by rubbing two rocks or minerals against one
another. Whichever gets scratched is the softer rock or mineral.
- Luster describes the ability of a rock or mineral to reflect
light. The following words can be used to describe luster and are ordered
from most to least luster:
- sparkly (having embedded bits of crystals)
- earthy (no luster)
- Texture describes the surface of a rock or mineral.
- coarse: large grains or chunks visible with a hand lens
- fine: no large grains or chunks visible with a hand lens
- Transparency describes whether light can pass through a rock
- transparent (light passes through easily)
- translucent (light barely passes through)
- opaque (light does not pass through)
- Archimedes and the Bathtub. Archimedes was a Greek philosopher, mathematician,
physicist, and inventor. The best-known story about Archimedes concerns his
discovery of the principles of buoyancy and the law of density or specific
gravity. In ancient Greece, Hieron II (a prince) had been given a gold crown,
which he had sworn to dedicate to the gods when he became king. He asked Archimedes
to determine if the crown was made of pure gold. The problem was great since
Archimedes was not to damage the crown in any way. (Some versions say the
king was having a new crown made, but he did not trust his goldsmith, so he
ordered Archimedes to devise a way to see if the goldsmith used the exact
amount of gold. This version also says Archimedes would be put to death if
he failed.) One day, the answer came to Archimedes as he climbed into his
bathtub and noticed the overflow of water. He jumped out of the tub and ran
naked through the streets, shouting, "Eureka! Eureka!," which in Greek means,
"I have found it! I have found it!" It had occurred to him that, if the crown
were made of pure gold, it should take up the same amount of space as an equal
weight of pure gold. If the crown and the equal weight of pure gold were placed
in turn into a tub full of water, equal volumes of water should overflow.
When he found the overflow to be different, he knew that the king had been