This semester of Intro to Geology has been a very trying course. There’ve been many days where I’ve left class confused, disgruntled, or just downright frustrated with my not being able to grasp what processes are making our earth GO. Now that we’ve reached the end, however, I wanted to use this blog post to take the time to look back on all the information (both new and old) I’ve learned and recovered.
I now know the following facts about our Earth’s geologic makeup…
Igneous rocks are divided into three classes, depending on their silica content: felsic, intermediate, and mafic. Felsic rocks have a high silica content, low density, high viscosity, lower temperature, and tend to be continental. Intermediate denotes that they contain qualities in between felsic and mafic. Mafic rocks have a low silica content, high density, low viscosity, higher temperature, and tend to be oceanic. A felsic composition tends to lead to a more violent volcanic eruption while a mafic composition tends to lead to more quiet eruptions.
Moving right along with the topic of volcanoes, there are certain types that are very important. Scoria (Cinder Cone) volcanoes tend to be composed more of felsic material–specifically basalt–and tend to be mostly ash ejections. The Cascades and Andes mountain ranges are good examples of this volcanic type. Shield volcanoes are the largest volcanoes, spanning great distances across a landscape. They too are composed mostly of felsic material, and eject tephra–mostly lava–from the center. The chain islands in Hawaii such as Kilaeau and Mauna Loa are good examples of shield volcanoes. Composite volcanoes tend to consist of an intermediate mixture of rock–mostly andesite–and can be some of your most violent eruptions. The deadliest part of a composite volcano is the pyroclastic flow that hugs the earth as it travels at whopping speeds down the sides of the volcano. Five great examples of this volcanic type include Mt. St. Helens, Vesuvius, Mt. Fuji, Mt. Kilimanjaro, and Mt. Etna. The last type of volcanoes is calderas, which are basically craters/remnants of ancient supervolcanoes that carved a basin out of a landscape. Calderas form in four basic steps: venting, bulging, erupting, and then the crater that is left gets filled in. Yellowstone, Santorini, and Crater Lake are fantastic examples.
When it comes to types of rocks, there are three: igneous, sedimentary, and metamorphic. In the igneous category, there is rhyolite (felsic), granite (felsic), diorite (intermediate), andesite (intermediate), basalt (mafic), gabbro (mafic), and obsidian. In the sedimentary category, there is sandstone (clastic), siltstone (clastic), shale (clastic), breccia (clastic), conglomerate (clastic), graywacke (clastic), coal (nonclastic), limestone (nonclastic), and gypsum (nonclastic). Lastly, in the metamorphic class, there is hornfel (contact), skarn (contact), marble (contact/regional), gneiss (regional), slate (contact), quartzite, phyllite, amphibolite, serpintinite, schist, greenstone, and green/blueschist.
The last thing I really want to talk about is that of the six principles used to determine the relative ages of rocks. First, there is the Principle of Superposition–says that older rock is situated/layered underneath younger rock. Second, there’s the Principle of Horizontality–rock layers are arranged horizontally. Third, there’s the Principle of Inclusion–layers of younger rock can contain pieces/remnants of older rock. Fourth, there’s the Principle of Lateral Continuity–rock layers cut straight across a landscape in bands. Fifth, there’s the Principle of Cross-Cutting Relationships–younger layers of rock can cross-cut layers of older rock, but are still younger in age. Lastly, there’s the Principle of Faunal Succession–where fossils are used to help determine the surrounding rock’s relative age.
Although there’s a lot more I could say, Intro to Geology has been an enlightening, fascinating class that I’ve enjoyed. Thanks to Dr. Pillar for a great semester—I’ll miss ya in the spring!