EARTH SCIENCE LAB Igneous Rock Identification: Virtual LAB Answer Sheet

EARTH SCIENCE LAB
Igneous Rock Identification: Virtual LAB Answer Sheet

Igneous Rocks

Igneous rocks are rocks which solidify from molten material (magma). Cooling of the magma can occur beneath the surface (plutonic) or on the surface (volcanic). Igneous rocks can be identified by the determination of the composition and texture of the rock. Once these two characteristics have been identified, the Igneous Rock Identification chart is used to identify the rock name.

Composition

Composition of igneous rocks is properly identified by determination of the rock's chemical composition. This, however, requires chemical equipment and apparatus that is unavailable in this lab. Fortunately determination of the exact chemical composition is not necessary. Color is often an indicator of the composition of a rock or mineral and can be effectively used to identify the composition of most igneous rocks. Light colors, including white, light gray, tan and pink, indicate a felsic composition. Felsic compositions are rich in silica (SiO2). Dark colors, such as black and dark brown, indicate a mafic or ultramafic composition. Mafic compositions are poor in silica, but rich in iron (Fe) and magnesium (Mg). Intermediate compositions have an intermediate color, often gray or consisting of equal parts of dark and light mineral . Beware that even though an igneous rock may have a felsic composition (light color), the rock can contain dark colored minerals. Mafic rocks may contain light colored minerals as well. As mentioned above, the composition of most igneous rocks can be identified using this system, formally known as the Color Index. However, there are exceptions. The two most notable are obsidian and dunite. Obsidian is volcanic glass which erupts as a lava flow. Most obsidian is felsic in composition, yet typically it will have a very dark color (dark brown to black). Dunite has an ultramafic composition yet is apple green to yellowish green in color. Dunite is composed almost entirely of the mineral olivine which usually contains both iron and magnesium.

Texture

The texture of an igneous rock does not refer to the roughness or smoothness of the surface. Textures are based primarily on crystal size. Pegmatitic texture is composed of very large crystals (larger than 2-3 cm). Phaneritic texture is composed of crystals which are large enough to see but smaller than pegmatitic texture, and the entire rock is composed of crystals. Aphanitic texture is a fine grained texture but the crystals are too small to see. Porphyritic texture is composed of crystals of two different sizes. Typically, the large crystals (phenocrysts) are visible while the smaller crystal are not (referred to as groundmass). Glassy texture is the most readily recognized. The rock is composed entirely of glass. Few, if any, crystals will be visible. Vesicular texture is formed when lava solidifies before gases are able to escape. The result is a "bubbly" appearance. Lastly, pyroclastic texture is composed of volcanic fragments. These fragments or clasts can be very fine (ash) or coarse (lapilli) or very coarse (bombs and blocks).

Sample Identification

On each of the following pages you will find an image of a rock and a Igneous Rock Identification Chart. Identify the composition by identifying the color and determine the texture by examining the crystal sizes that are present. Once this is done the rock name is easily determined using the Classification of Igneous Rock chart. Click each answer, then check to see if you have correctly identified the rock sample.

Igneous Rock Identification Chart

EARTH SCIENCE LAB
Sedimentary Rock Identification: Virtual LAB Answer Sheet

Sedimentary Rocks

Sedimentary rocks are rocks composed of sediment. Sediment is deposited in a number of environments of deposition, by both moving air and moving water. Sedimentary rock identification is primarily based on composition. Texture will still be used but in a different sense than for igneous rocks.

Texture

Texture of sedimentary rocks in this lab will be taken to indicate origin or type of sediment found in the rock. Three types of "texture" will be used - clastic, chemical, and biologic.

Clastic Rocks

Clastic sedimentary rocks contain clasts. These are fragments or pieces of rock or minerals. The composition of clastic sedimentary rocks is divided into three types - clay/silt, sand and gravel. Clay and silt are less than 1/16 mm. These are not visible to the unaided eye. Sand is clasts between 1/16 and 2 mm in size, and gravel is greater than 2 mm.

Chemical Rocks

Chemical sedimentary rocks are identified by identifying the mineral from which they are composed. In this lab there are four minerals that need to be identified - quartz, halite, gypsum and calcite. Quartz has a hardness of 7 and is very difficult to scratch, even with a good quality knife blade. Gypsum is relatively soft (Hardness =2) and can be scratched easily with a fingernail. Halite is common table salt and is most easily identified by taste. However, this is not a sensible practice in a large lab with many different people handling the samples. Halite has a hardness of 2.5 and cannot be scratched by a fingernail (unpolished fingernail). Calcite readily reacts with a small drop of HCl.

Biologic Rocks

Biologic sedimentary rocks are which form as the result of the accumulation of organic material or biologic activity. Coal is usually obvious to most students even though few people seem to have ever actually examined it up close. The dark brown to black color is the most obvious characteristic. Coquina and limestone are both composed of calcite . Coquina is composed almost entirely of shell or fossil fragments. Limestone may or may not contain fossils fragments. Both will react to HCl. Limestone containing fossils is referred to as fossiliferous limestone.

Sedimentary Rock Identification Chart

EARTH SCIENCE LAB
Metamorphic Rock Identification: Virtual LAB Answer Sheet

Metamorphic Rocks

Metamorphic rocks are rocks that have undergone a change from their original form due to changes in temperature, pressure or chemical alteration. The classification of metamorphic rocks is based on the minerals that are present and the temperature and pressure at which these minerals form. Determination of this information is not easily accomplished in this lab. Therefore, a simplified system is used based on texture and composition.

Texture

Texture is divided into two groups. Foliated textures show a distinct planar character. This means that the minerals in the rock are all aligned with each other. This planar character can be flat like a piece of slate or folded. Non-foliated textures have minerals that are not aligned. Essentially, the minerals are randomly oriented.

Foliation

Foliated textures show four types of foliation. Slaty cleavage is composed of platy minerals that are too small to see. Typically, these rocks split along parallel, planar surfaces. Phyllitic foliation is composed of platy minerals that are slightly larger than those found in slaty cleavage, but generally are still too small to see with the unaided eye. The larger size gives the foliation a slightly shiny appearance. Schistose foliation is composed of larger minerals which are visible to the unaided eye. Platy minerals tend to dominate. Gneissic banding is the easiest of the foliations to recognize. It is composed of alternating bands of dark and light minerals.

Non-Foliation

Non-foliated textures are identified by their lack of planar character. Further identification of non-foliated rocks is dependent on the composition of the minerals or components in the rock. Anthracite coal is similar to bituminous coal. Both are black in color , and is composed of carbon. Anthracite coal is generally shiny in appearance and breaks with a conchoidal fracture (broken glass also shows this type of fracture). Metaconglomerate is composed of pebbles and gravel that have been flattened due to directed pressure. Quartzite is composed of quartz sand grains. Quartz has a hardness of 7, which makes it difficult to scratch. Marble is composed of calcite and will readily react to a small drop of HCl.