GS 106 - Introduction to Physical Geology

2. Identify one igneous rock in your sample box with a coarse-grained texture and one igneous rock in your sample box with a fine-grained and record them. Based on these different grain sizes, explain which sample formed above ground and which sample formed below ground. Explain why their textures differ.

Answer: Granite is a coarse-grained rock, sample #2, and Basalt and rhyolite are both fine-grained rock, sample #3 and #5 respectively. Granite forms underground and basalt and rhyolite form about ground. Granite forms underground where the rock has time to cool slowly and so larger minerals have time to form. Basalt and rhyolite form above ground and cool very quickly so larger minerals don’t have time to form and the rock stays very fine-grained.

3. Describe the texture of obsidian as well as a broken surface of the sample. What is the cooling history of obsidian? Why was the rock prized by natives for creating tools?

Answer: On a broken surface, obsidian looks fairly smooth, glassy, with almost a rounded break, this is called chonchoidal fracture. Its is also very sharp on certain broken surfaces. Obsidian cools very quickly and as a result does not have time to form minerals. This means that at the atomic level there is no crystalline lattice, and so obsidian is volcanic glass. The reason it has been used by Native Americans for tool production is that it is extremely sharp and easy to shape in comparison

4. Describe the texture of pumice. How does pumice get its texture? Are their any other igneous rocks that have a similar texture?

Answer: Pumice has a very vesiculated texture, or basically its covered in holes. It gets that texture as pressure is release on the magma chamber and gas is able to exsolve or release from the magma creating these gas bubbles (similar to opening up a shaken soda bottle). Another rock with a similar texture would be scoria. to other rock.

9. What is the name of a rock that is poorly sorted with angular fragments and in what sort of environment would it form?

Answer: The name of the rock that is poorly sorted with angular fragments is referred to as a breccia. Breccias form in an environment where deposition is quick with little to reworking of the deposit, which is what we find in landslide deposits.

10. Is coal made of minerals? If yes, what minerals? If no, what exactly is it made of and why is it still considered a rock?

Answer: Coal contains no minerals because it is an organic rock and minerals must be inorganic. It is instead made of carbon. Coal is still considered a rock because it forms like other rocks by adding heat and pressure to its contents until it is a solid natural substance.

11. What happens to the grain size of a sedimentary rock as water begin to move more slowly? Based on your answer, what sort of clastic rock should one see at the head waters of a river? On a beach? Shallow ocean?

Answer: As water slows the sediments that the water carries will be deposited based on size, heavier particles drop first, lighter particles drop later. At the headwaters of a river we should find rocks like conglomerate. On a beach we should find sandstones. In the Shallow ocean we should see shales.

14. What is the progression of rocks formed when fine-grained sedimentary rocks are metamorphosed, from low-grade to high-grade metamorphism? What is happening to the rock’s composition and size of minerals as heat and pressure increases?  

Answer: The progression of rocks when a fine-grained sedimentary rock is formed is slate, phyllite, schist, then finally gneiss as heat and pressure are added. As this progression proceeds the minerals grow bigger with every change in rock type and minerals will also change as heat and pressure are added. For instance, in slate, minerals cannot even be seen for identification. As slate become phyllite micas became visible giving the rock its sheen. As phyllite becomes schist the minerals are even more visible, muscovite, and biotite micas are clearly seen, and plagioclase and/or quartz become visible. Lastly, as schist becomes gneiss, banding of minerals begins to form and quartz and plagioclase (or orthoclase) separate into bands and hornblende forms and separates into bands with biotite.

15. Discuss two possible ways that metamorphic rocks could form based on where on or in Earth high heat and/or pressure can be found.

Answer: There are many ways you can get metamorphism, the following is a list of the most common examples. Contact metamorphism is when rocks come in contact with either a lava flow or magma in a magma chamber. This provides high heat only to create metamorphic rocks. Metamorphism can also occur due to tectonic plate collision in wither a subduction zone or a continental-continental collision. In the case of subduction metamorphism is due to great pressure but low heat because water is present in subduction zones with lowers the heat. In the case of continental-continental collision, there is high heat due to friction and high pressure, this creates large areas of highly metamorphosed rocks. You can also get metamorphism through hot water or hydrothermal metamorphism. This is where hot water moves through rocks and alters them, and it is considered low temperature and low pressure metamorphism.