Georgia Rocks!


Part I: Igneous Rocks

Several different minerals will often combine to form a rock. Three types of rocks make up the Earth. These three types are igneous, sedimentary, and metamorphic rocks. Igneous rocks form from liquid molten material (magma) ten times hotter than boiling water. Volcanoes like those at Hawaii spew magma onto the surface, where it cools to form igneous rocks. The entire surface of the planet Venus is probably covered with oceans of magma, and Earth may have once been similar. In time. Venus' magma oceans will cool to form an outer layer of igneous rocks much like those on the Earth's surface Igneous rocks form by the cooling and solidification of magma. This magma often erupts from volcanoes onto the surface, forming volcanic (extrusive) rocks. However, magma may also cool and solidify below ground, forming plutonic (intrusive) rocks. Stone Mountain is a plutonic igneous rock that formed several miles below ground. In this unit, you will study how igneous rocks form, what they look like, and where they are found in Georgia.

Difference Between Rocks and Minerals

From your ROCK KIT, remove sample numbers 13 through 17(5 rocks) . Each of these is an igneous rock. Although all formed from cooling magma, their appearances are quite different. The most obvious difference between these igneous rocks is their color The different colors in each rock are grains of different silicate minerals

1) Do you remember looking at the pink mineral, feldspar in your mineral kit, if not look at it now? . Find the grains of pink feldspar in Sample 13. Do these grains make up more than 25 % of the entire rock or  less than 25% of the entire rock?

2) Do you remember the black, sheet-like mineral mica from your mineral kit? It was number 12. Find the small black grains of biotite in rock sample 14 (use your loupe). Are these biotite grains larger or smaller than the pink grains of feldspar seen in Sample 13?

3) Sample 14 is the Stone Mountain Granite. Each different colored grain is a, different mineral. How many different minerals do you see in this rock?

4) Look for some black, blocky mineral grains in sample 15. These are pyroxene. In Sample 15 all of the black grains are pyroxene. Is there a little or a lot of pyroxene in this sample?

Causes of Grain Size

There are two kinds of igneous rock, intrusive and extrusive.  Grain or crystal sizes in rocks or minerals are affected by how fast or slowly they cool.  Intrusive igneous rock forms in the ground and extrusive igneous rock forms on top of the ground.

As magma cools slowly below ground, mineral grains (crystals) grow larger. Rocks composed, or made entirely of large grains, cooled slowly below ground. Rocks made mostly of small or microscopic mineral grains cooled rapidly, perhaps during a volcanic eruption.  Once magma erupts from a volcano, minerals cannot grow much larger. The mineral grains in some volcanic (extrusive) rocks are so small that they can only be seen with a microscope

5) Compare the size of the mineral grains in Sample 13 and Sample 14. Both of these rocks are granite, and both formed as magma cooled slowly below ground.   However, one of these rocks cooled slower than the other. Which rock cooled slower? Explain how you know.

6) Compare the size of the mineral grains in Sample13 and Sample 17.  Sample 17 is a volcanic (extrusive) rock, called rhyolite. It formed as magma erupted onto the surface during a volcanic eruption. A few mineral grains are visible in this rhyolite, but most are too small to see without a microscope. Why are grains in Sample 17 so Small?

7) Look at Sample 15 and Sample 16. These two rocks form from magmas with very similar compositions but very different cooling rates. One of these rocks has small gas bubbles (called vesicles) that form when magma erupts onto the surface from volcanoes. This sample also has smaller grains. Which sample formed from magma that erupted from a volcano?


8) Sample 16 is an igneous volcanic (extrusive) rock (called basalt), and it contains mineral grains that are too small to be seen without a microscope. With a microscope, you would be able to see pyroxene. What stopped these grains from growing larger?

Naming Igneous Rocks

The name of an igneous rock depends on its color (caused by the minerals it contains) and the size of its mineral grains. The color is dark if the rock contains mostly dark colored minerals such as pyroxene. The color is light if the rock contains mostly light colored minerals such as quartz and feldspar. Large visible mineral grains form in an igneous rock if the rock formed from slow cooling of magma below ground. Such large. visible mineral grains are found in plutonic (intrusive) igneous rocks. Stone Mountain is a pluton which means it was formed from magma that cooled underground. Stone Mountain formed about 9 miles underground over 300 million years ago. WOW!!!  

8a) What do you think happened so that we can see it today?

The size of an igneous rock's mineral grains are mostly too tiny  to see without a microscope if the rock formed from a lava erupted from a volcano. Such tiny, microscopic grains are found in volcanic or extrusive  igneous rocks.

9) Use the Igneous Rock Classification Chart to correctly name Samples 13-17.

Place each in the correct box on the classification chart (Hint: you have four choices and five samples; two samples have the same name.


Igneous Rock Classification Chart:


Plutonic (Intrusive Igneous) Rocks

Volcanic (Extrusive) Rocks







10) The gabbro and the basalt are the same color because they both contain what mineral?

Temperatures of Magmas

As magma cools, different minerals grow at different temperatures. Ice is a mineral, and it grows at 0°Celsius (32°Fahrenheit). Silicate minerals in igneous rocks grow at much higher temperatures—between 800 and 1200°C. The following temperature scale, called Bowen's Reaction Series, shows the temperatures at which different silicate minerals grow.

Temperature Dark Minerals Light Minerals Color of Igneous Rock 1200°C руroxene Ca-feldspar Dark

1000°C amphibole Ca- and Na-feldspar

800°C biotite Na- and K-feldspar Light & quartz

Rocks that contain mostly pyroxene form at high temperatures, so igneous rocks with mostly black pyroxene mineral grains form while magma is still extremely hot (1200° Celsius). Rocks that contain mostly quartz and K-feldspar (light colored, iron-poor minerals) form at lower temperatures, so igneous rocks with mostly quartz or K-feldspar mineral grains form when the magma has cooled to 800° Celsius.

11)Remember that the igneous rocks in your kit are Samples 13-17. Which two igneous rock samples formed from extremely hot magma (give sample #'s and rock names)?

PART II: Sedimentary Rocks

All rocks found in the southern half of Georgia are sedimentary rocks. All of the world's oil and coal reserves are in sedimentary rocks, and all fossils also. Sedimentary rocks form on the Earth's surface on beaches, in oceans, beneath glaciers, and along rivers. If solid particles of gravel, sand, or clay are compacted and cemented together, then clastic sedimentary rocks form. If plants or animals are involved in making minerals, then an organic sedimentary rock forms. If minerals grow in oceans, rivers, streams or groundwater, chemical sedimentary rocks form.

Clastic Sedimentary Rocks: Grain Size and Shape

A clastic sedimentary rock forms when older rocks are broken apart into sediment. These broken pieces of older rocks are transported by streams, rivers, oceans, and glaciers to a new location and then compacted or cemented into a new, clastic sedimentary rock.

Remove samples 2, 3, 4, 5, 19, and 20 from the ROCK KIT. These are clastic sedimentary rocks.

14) Look at the grains in Sample 2 and Sample 3. The grains in these rocks are broken pieces from older rocks. Are the grains in Sample 2 LARGER or SMALLER than the grains in Sample 32?

15) When broken pieces of rock are transported by streams, they become smaller and rounder. Which grains have been transported further, those in Sample 2 or Sample 3?

16) The grains that make up Sample 5 are even smaller than those in Sample 3. You can just barely see these individual grains without a microscope. Which grains have been transported further, those in Sample 3 or Sample 5?

17, Arrange Samples 2, 3, and 5 in order of DISTANCE their grains have been transported. Remember, longer distances make smaller grains.






18) Small, microscopic grains of sediment usually settle out of very still water (such as lakes). In addition, these sediments are often dark gray or black because they may contain alot of black organic material Look at Sample 3 and Sample 4 and decide which probably formed at the bottom of a lake.

Clastic Sedimentary Rocks: Compaction & Cementation

Broken pieces of old rock become smaller and rounder as they are transported further. These transported grains are called sediment. When these grains of sediment are compacted and cemented together, they become clastic sedimentary rock. For example, sand on a beach is sediment. When sand is compressed into a rock, it becomes sandstone.

19) Compare Sample 20 and Sample 3. The grains in Sample 20 are small and round The grains in Sample 3 are also small and round (use your hand lens). Which sample has grains that are compacted and cemented into a rock, Sample 20 or Sample 3?

20). Which sample has sand grains that have not yet been compacted and cemented into a rock, Sample 20 or Sample 3?

21) Which is probably younger, Sample 20 or Sample 3? Explain your answer

22) The name of a clastic sedimentary rock depends on the size and shape of its grains. If the grains are large and angular, the rock is a breccia. If the grains are large and round, the rock is a conglomerate. If the grains are sand-sized and round, the rock is a sandstone. Mudstones are made of even smaller grains. Shale is a type of mudstone that breaks in thin, flat layers. Place Samples 2, 3, 4, and 5 in the correct boxes on the rock classification chart.

Clastic Sedimentary Rock Classification Chart



large, round grains


small, sand sized grains


tiny, almost microscopic grains


grains too small to see,

breaks into thin layers

may contain plant fossils



Chemical Sedimentary Rocks

Like clastic sedimentary rocks, chemical sedimentary rocks also form at or near the Earth's surface. However, while clastic rocks are formed from broken pieces of older rocks, chemical rocks are formed from minerals that grow in water.

Pure water has a chemical formula of H2O. In nature, water is never pure H2O. It always contains small amounts of dissolved elements such as calcium (Ca), iron (Fe), and silicon (Si). If water contains large amounts of Si, then microscopic crystals of the mineral quartz (SiO2) may grow from the water. Numerous microscopic crystals of quartz may grow together to form the chemical sedimentary rock, chert.

23) Remember the physical properties of the mineral quartz. It has a high hardness (will scratch glass), uneven fracture, and usually a light color.  Chert is composed of microscopic crystals of quartz. Look at Samples 21, 22 & 23 and decide which one of these is chert.

Two other chemical sedimentary rocks form by the weathering of the mineral K-feldspar (potassium feldspar). Remember that K-feldspar has medium hardness, good cleavage, and light color (white or pink). Water can break apart this mineral, pull the K (potassium) from its crystal structure, and replace it with water molecules. When this happens, the mineral is transformed into kaolinite, a soft, white mineral. The chemical sedimentary rock kaolin is made entirely of the mineral kaolinite. South Georgia has huge deposits of kaolin.  

If more extreme weathering removes both potassium (K) and silicon (Si) from

feldspar, then bauxite forms. Both kaolinite and bauxite are mined in Georgia for aluminum (Al). Bauxite is a white to reddish material that is characterized by small, round clumps. Although it may appear similar to kaolin, the small round aggregates are diagnostic.

24) Compare Samples 21 and 22. One of these is white and extremely soft. The other is not quite as soft and contains small spheres. Which Sample Number is kaolinite?

25). Which is bauxite, Sample 21 or Sample 22?

Organic Sedimentary Rocks

Organic sedimentary rocks also formator near the Earth's surface, however they require plants or animals for their formation. Most seashells are made of the mineral calcite. When marine animals die, their calcite shells may accumulate on the seafloor. These shells may be compacted and cemented into solid rock, forming organic sedimentary rocks called limestone

26). Look at Samples 1, 6, 23, and 24. Two of these samples are composed of the mineral calcite, so they are the organic sedimentary rocks called limestone. Use a drop of dilute hydrochloric acid to determine which of these two samples are limestones.

27). Both of these two limestones you just identified contain fossils. Study these carefully. Are the fossils in Sample 6 and Sample 24 fossilized seashells or fossilized plants?

Plants that grow in swampy areas may accumulate into thick layers when they die. Because water in stagnant, swampy environments is usually low in oxygen, these thick accumulations of dead organic matter do not completely decay. Instead, the increased pressure and heat from the overlying layers may transform lower layers into a black organic rock called coal.

28). Compare Sample 1 and Sample 16. Both are black. One is coal, and the other is

basalt (a volcanic igneous rock). Coal is less dense than basalt Which sample is


Classifying Organic & Chemical Sedimentary Rocks

Sedimentary rocks can be divided into three general groups: clastic, chemical, and organic You have already identified specific clastic sedimentary rocks (such as sandstone, conglomerate and shale). Here, practice identifying chemical and organic sedimentary rocks.

29) Place Samples 1, 6, 21, 22, 23, and 24 in the proper boxes on the Rock Classification Chart below (Hint: two of these samples are limestone).

Organic Sedimentary Rocks

Fossilized seashells.

Composed of calcite.


Black, altered plant material.



Chemical Sedimentary Rocks

Angular fracture

composed of microscopic quartz grains


White, extremely soft!


White to reddish, soft, round, aggregates.




Nonfoliated Metamorphic Rocks

Sedimentary and igneous rocks may be heated deep beneath the Earth's surface and transformed into new rocks without melting or weathering the material. These are called metamorphic rocks. If these rocks are heated and transformed while under extreme pressure, then layering of minerals develops, producing foliated metamorphic rocks. When magma from within the Earth rises and comes into contact with sedimentary rocks, with no directed pressure, the sedimentary rocks are transformed into nonfoliated metamorphic rocks. Metamorphism will destroy many features of sedimentary and igneous rocks, and new or larger mineral grains will form. For example, fossils may be present in sedimentary rocks, but metamorphism will destroy them. Also, gas bubbles (vesicles) may be present in volcanic igneous rocks, but metamorphism will destroy them. Metamorphic rocks are plentiful in the northern half of Georgia. Many metamorphic rocks, such as marble, are valuable economic resources in Georgia.

30). Look at Sample 24 and Sample 8. Sample 24 is a sedimentary rock, a fossil-rich limestone. Sample 8 was once a limestone, but it is now a contact metamorphic rock called marble. Do you see fossils in Sample 8? What happened to them?

31) Get Mr.s B to pllace a drop of dilute hydrochloric acid on Sample 6 (limestone, a sedimentary rock) and on Sample 9 (marble, a metamorphic rock). Both fizz. They fizz because metamorphism has not changed the mineral composition. What mineral is in both the sedimentary rock (limestone, Sample 6) and the metamorphic rock (marble, Sample 9)?

32) You know that Sample 6 is a sedimentary rock (limestone) and Sample 9 is a metamorphic rock (marble). You also know that both are made of the same mineral. Which sample has large, shiny grains of this mineral. Why?

33) Look at Sample 3 and Sample Z. Sample 3 is a sedimentary rock, a sandstone Notice the round, sand-size quartz grains (use your hand lens). Sample 7 was once a sandstone, but it is now a metamorphic rock called quartzite. Describe how the size and shape of the quartz grains in Sample 7 are different from those in Sample 3.

Foliated Metamorphic Rocks

If sedimentary or igneous rocks are pushed deep beneath the Earth during collisions of continents, then tremendous directed pressure will be placed on these rocks. This pressure (like the pressure from a steamroller flattening asphalt), along with heat, will cause the rocks to become layered, or foliated. The rocks will be transformed into foliated metamorphic rocks. Such directed pressures produced the Appalachian Mountains that extend into the northern part of Georgia.

34). Look at Sample 16 and Sample 10 Sample 16 is a volcanic igneous rock, basalt. The tiny gas bubbles (vesicles- use your hand lens) formed in this as lava erupted onto the surface. Sample 10 may have once been a basalt with gas bubbles, but it has been metamorphosed into a layered metamorphic rock, called schist Do you see vesicles in Sample 10? What happened to them?

35). Which sample has been subjected to intense pressure, Sample 10 or Sample 16?

36). Look at Sample 4 and Sample 11. Sample 4 is a sedimentary rock (shale), and it may

have some plant fossils. Sample 11 is a foliated metamorphic rock, called schist. It

may once have contained plant fossils, but metamorphism has destroyed them. Do

you see large crystals in the shale (sedimentary rock Sample 4)?

40) Place Samples 7-12 & Sample 18 in the proper boxes on the Metamorphic Rock Classification Chart (Hint you will have two samples of one rock type and two samples of another).




No layers

white or pink,

large calcite crystals


No layers

light colors,

interlocking quartz crystals





Large visible crystals


Black and white bands

Large visible crystals


41) You have just completed learning about metamorphic rocks.  Now complete the following charts as you review what you have learned about igneous and sedimentary rocks.  Place the correct rocks on the spaces in the chart.  We will begin with igneous.  Remember, samples 13-17 were igneous.

IGNEOUS Rock Classification Chart


PLUTONIC (Intrusive Igneous Rocks)

LARGE crystals or grains

VOLCANIC (Extrusive Igneous Rocks)

small or tiny crystals/grains












(samples 1-6 and 19-24)

Clastic Sedimentary Rocks

Large, round grains


Small, sand-sized grains


Tiny, nearly microscopic grains


Microscopic grains, breaks into thin layers.


Organic Sedimentary Rocks

Fossilized marine (sea) shells

contains calcite-fizzes in acid


Black, altered plant material



Angular fracture, composed of microscopic grains of quartz


White, extremely soft


White to reddish.  

Soft, round aggregates.



On a sheet of drawing paper, draw the rock cycle and label the steps.  Now place two or three samples of each type of rock onto the appropriate spaces.  When you have finished, call me over to admire your work.