CSUGlobal Radiometric Dating Techniques & Tectonic Plates Lab Report GEOLOGIC TIME AND ROCK & MINERAL IDENTIFICATION GUIDE ADAPTED FROM HOME SCIENCE TOOLS

CSUGlobal Radiometric Dating Techniques & Tectonic Plates Lab Report GEOLOGIC TIME AND ROCK & MINERAL IDENTIFICATION GUIDE ADAPTED FROM HOME SCIENCE TOOLS (2008B)

Geologic Time (Radiometric Dating Techniques): Answer the questions below.

What unstable isotope would be best to refine the date of bones found in a cave hearth built by humans between 30,000 and 50,000 years ago?
We find samples of an igneous rock demonstrate it has been through 4 half-lives. The test element has a half-life of 150 million years. How old is the rock?
Argue for or against the following: A stone tool fashioned from a chunk of obsidian yields a date of 5,000,000 years old, therefore, the tool was made by a human 5,000,000 years ago.
You are trying to figure out the age of what is thought to be a very old fossil with a volcanic ash layer immediately above the fossil. We know the fossil is at least more than 250 million years old. Should we use carbon 14 to date the fossil, or uranium 238 to date the volcanic ash layer, and why?
If the parent isotope starts with 100 grams, but your samples yield only 12.5 grams of the parent isotope, how many half-lives have passed?

Rock & Mineral Identification

Minerals are naturally-occurring, solid substances composed of chemical elements. This means that minerals, ranging from salt to rubies, are made up of compounds of elements that appear on the periodic table. Each type of mineral has a specific chemical composition and consistent physical properties. They are inorganic, not living or made up of living things. Minerals form a crystalline structure which gives rocks their “rough” texture.

Rocks are mixtures, or aggregates, of different minerals. Some rocks, like limestone, are composed mostly of one mineral, but the majority of rocks are made up of several major minerals. Rocks are divided into three categories based on how they are formed: igneous, sedimentary, and metamorphic.

Igneous rocks form when hot magma from beneath the Earth’s surface cools rapidly. Igneous rocks that cool beneath the surface (perhaps by hitting underground air pockets) are called intrusive or plutonic igneous rocks. Granite is an example. When the magma cools on the Earth’s surface by flowing from the mouth of a volcano as lava, the resulting rock is called extrusive or volcanic rock. Basalt and obsidian are common examples of volcanic igneous rocks.

Sedimentary rocks are formed by layers of sediment accumulating and being compressed together for extended periods of time. Most sedimentary rocks have layers, and they often contain fossils, as living material was buried in the sediment before it was compressed into rock. Common examples of these rocks are limestone, sandstone, and shale.

Metamorphic rocks are rocks that have been changed by high pressure or heat. The crystal structure is changed, the texture often becomes coarser, and sometimes new minerals are formed in the process. Metamorphic rocks are the most complex group of rocks. Schist, slate, and gneiss (pronounced like “nice”) are common examples.

Identifying Rocks

This kit includes 15 common rocks with examples from each category. Observe them closely with the included magnifying lens.

Color. As a general rule, darker rocks are made of minerals with iron and magnesium, such as magnetite or biotite. Lighter-colored rocks may have lots of quartz, calcite gypsum, or halite in them.

Texture. Is it coarse-grained or glassy-smooth? Is it dense with very small particles? Are minerals visible to the naked eye? The texture of a rock depends on what is it made of. For example, igneous rocks go by basic crystal size, and sedimentary rocks will have a texture of clastic, chemical, or biogenic, depending on how they formed. Metamorphic rocks will be identified by whether they are foliated or not foliated.

Structure. Look for layers, which are often an indication of sedimentary rocks. Some volcanic igneous rock will have a sponge-like structure – pumice is an example of this. It is less dense than water, so it floats! Sedimentary rocks may have layers in them, but this is more common to shales. They can also have fossils, or banding.

Minerals. Look at individual grains with the magnifying lens and see if you can identify any of the composite minerals. With larger grains, you may be able to identify what they are since you are also learning about mineral identification this week.

Acid Test. Limestone contains a carbonate compound that dissolves in acid, producing bubbles. Test for bubbles with a few drops of vinegar.

Igneous Rock Identification

↓Texture/composition→ Felsic Intermediate Mafic
Phaneritic (coarse) Granite Diorite Gabbro
Aphanitic (fine) Rhyolite Andesite Basalt
Glassy Obsidian
Vesicular Pumice

Sedimentary Rock Identification

↓Composition/Texture→ Clastic Chemical Biogenic
Grain size >2mm (coarse) Conglomerate
Grain size 2 mm – 0.062 mm
(medium) Sandstone
Grain size