Problem Set 1 Objectives To become familiar with various types of Geoscience research. To gain experience writing a short summary of a presentation.Assignment Problem set #1 is due 4th class day after the talk you attend. For example, if you attend a talk on a Thursday or Friday, the paper is due the next Thursday. After attending an approved lecture, write a short summary of the presentation. Your summary should be no longer than 150 words, which is approximately half a page double-spaced. Please follow the following format guidelines: 12 point Times Roman font, double-spaced, 1-inch margins. At the top of the page please include your name, the title of the lecture, the name of the presenter(s), and the time and date of the talk. In addition, there will be a sign-in sheet at each of the talks - make sure that you sign in at the talk you attend! If you fail to sign in you will not get credit for your summary.

Igneous Rock Classification

I.Igneous Textures—refers to the size and shape of the minerals in the rock A. Phaneritic: contain crystals large enough to see with unaided eye. When magma cools slowly over hundreds to thousands of years the minerals crystallize slowly and have ample time to grow large. Intrusive Rocks (Plutonic rocks) crystallized underground; only place to cool slowly enough to form large crystals Igneous Rock Classification

B. Aphanitic: crystals are so small they cannot be seen by the naked eye—or barely seen. Magma cools more quickly Extrusive Rocks (Volcanic Rocks): crystallized from lava that flowed out, or extruded, onto Earth’s surface

C. Porphyritic: rocks with large crystals surrounded by regions with much smaller crystals or areas with undistinguishable grains. Results from initial slow cooling (large crystals form) followed abruptly by rapid cooling (smaller surrounding crystals form) Classified as extrusive Phenocrysts Groundmass

D. Glassy: very rapid cooling (explosive volcano or flow into water), crystals don’t have time to form Ions frozen in place randomly: no internal structure. 1.Pumice: bubbling, highly gaseous, silica-rich lava cools very quickly 2. Obsidian: very silica-rich lavas, containing little gas, cool very quickly—they lack internal structure, so they fracture rather than have cleavage

II. Igneous Compositions Classified based on their silica content (silicon and oxygen) A.Felsic Rocks Felsic from feldspar and silica 1. Composition silica: >65% Al, K, Na some amphibole, mica, Na- plag, K-spar, quartz 2. Temperature when crystals start to form =  C (  F). Felsic Rocks felsic from feldspar and silica

3. Cools on surface = extrusive (volcanic) aphanitic = rhyolite, pumice & obsidian Cools underground = intrusive (plutonic) = phaneritic = granite. 4. Physical properties  Light color - pink and white with very few black specks  Least dense  Magma viscosity = high Note: Viscosity = resistance to flow (opposite of fluidity) Increases with decreasing temp— maple syrup example Increases with increasing silica content—we’ll come back to later Felsic Rocks felsic from feldspar and silica

B. Intermediate Rocks 1. Composition silica: 55-65% +Al, Ca, Na, Fe, Mg Pyroxene, amphibole, biotite, Ca- & Na-plag, sometimes quartz 2. Temperature when crystals start to form =  C (  F) 3. Extrusive (volcanic) aphanitic = andesite (2nd most abundant volc rock) Intrusive (plutonic) = phaneritic = diorite B. Intermediate Rocks

3.Extrusive (volcanic) aphanitic = andesite (2nd most abundant volc rock) Intrusive (plutonic) phaneritic = diorite 4. Physical properties lighter in color than mafic rocks—“salt and pepper” medium density magma viscosity = medium B. Intermediate Rocks  Viscosity = resistance to flow (opposite of fluidity)  Increases with decreasing temp., maple syrup example  Viscosity increases with higher silica content

C. Mafic Rocks: most abundant rocks of the Earth’s crust mafic = derived from magnesium and ferrum (Latin for iron) 1. Composition Silica: 45-55% Al, Ca, Fe, Mg Olivine, pyroxene, Ca- plagioclase 2. Temperature Crystals start to form =  C (  F) C. Mafic Rocks

C. Mafic Rocks: most abundant rocks of the Earth’s crust mafic = derived from magnesium and ferrum (Latin for iron) 3. Crystallize: surface = extrusive (volcanic) = aphanitic = basalt Intrusive (plutonic) = phaneritic = gabbro 4. Physical properties Dark in color Relatively dense Magma Viscosity = low C. Mafic Rocks

Back to Viscosity: Magmas with the most silica are most viscous - hardest time flowing And magmas with the least silica are the least viscous - flow easiest Viscosity = resistance to flow Resistance to flow (honey vs. water) Factors that affect viscosity: Composition ---> hi silica is more viscous

D. Ultramafic Rocks (The Mantle) 1.Composition Dominated by iron & magnesium silicate minerals Plus olivine and pyroxene very little silica <40% Mg, Fe, Al, Ca 2. Temperature: solid at temperatures = >1200  C (>2200  F)

D. Ultramafic Rocks (The Mantle) 3. Peridotite: contains % olivine - green rock 4. Properties Usually green in color b/c of olivine Hi density Dominated by iron & magnesium silicate minerals

Rock NameColorCompositionMineralsTemp Occurrence RhyoliteLight~70%wt SiO 2 quartz~800  Ccontinents (granite) low in Mg, Fe feldspar amphibole mica AndesiteSalt-pepper~60%wt SiO 2 quartz~1000  Ccontinent- (diorite) feldsparocean amphiboleboundary pyroxene Basaltdark~52%wt SiO 2 feldspar ~1200  Cocean- (gabbro) hi Mg, Feamphibole ocean pyroxene boundary

Same composition, but different texture. Rates of cooling! Crystallization of melt forming minerals.