Name Per. Pacific Northwest Rock Lab, Part II After you ve classified all your rocks, place their numbers next to the names and read the information about the rock. Check the classifications here (igneous, metamorphic, or sedimentary) with your guesses on the PNW Rock Lab, Part I. Feel free to look at the rocks again as you answer the questions in Part II. Igneous Rocks Pumice Mt. St. Helens Pumice, collected at Windy Gap, 6 mi. NE of the crater. Age = 35 years old. Dunite Twin Sisters, SW of Mt. Baker. Ultra basic (only 40% SiO2), rare because olivine is unaltered. Emplaced in Tertiary, 2.6-66 my (million years ago) 2 Basalt Near Black Lake, SW of Olympia. Age = Eocene (33.9-55.8 my) 16 Basalt Vantage, WA (east of Ellensburg). Age = Late Miocene (5.3-23 my). Andesite Porphyry Mt. Baker. Age = 10,000-500,000 years old. 6 Granite Near Squamish, BC, N of Vancouver, BC. Age = Tertiary, 2.6-66 my) 15 Granite Earthquake Point, along Columbia River, South of Chelan, WA. Age = Oligocene-Miocene (5.3-33.9) There are 7 igneous rocks in the collection. They are formed from the cooling of magma either above or below the earth s surface. If the magma is erupted out of a volcano, the rock is called volcanic or extrusive. Since the cooling of lava is fast above ground, the crystal sizes are small. If the magma solidified underground, the rock is called plutonic or intrusive. Since the magma is insulated by the surrounding rock, it cools slowly, and the crystals can grow very large. 1. Which of the 7 rocks are probably volcanic? 2. Which of the 7 rocks are probably plutonic? 3. Which 2 of these 7 rocks cooled so fast that holes remain where gas bubbles were trapped in the rock? 4. Which one probably contained the greatest amount of gas when it solidified? What is your evidence for this? 5. Which rock was possibly formed in the mantle?
Sedimentary Rocks Coal Chuckanut Formation, 4 mi S of Glacier, WA. High quality coal, light-weight, and will burn Age = Paleocene (~65 my) Shale Chuckanut Formation, S of Bellingham. Composed of clay minerals, soft. Age = Paleocene (about 65 my, based on fossils) Limestone Red Mountain, N of Kendall, WA (NE of Bellingham). Composed of calcite. Age = early Pennsylvanian (310-318 my, based on fossils) Sandstone Chuckanut Formation, S of Bellingham. Type of fossils indicate a somewhat warmer climate than present. Age = Paleocene (~65 my, based on fossils). Conglomerate Chuckanut Formation, Mt. Baker Hwy, NE of Bellingham, just S of Canadian border. Pebbles in rock rounded due to transportation in stream before cementation. Age = Paleocene (~65 my). The sedimentary rocks in this collection were all deposited in water. The amount of motion in the water determines what kind of sediment can be carried, and what kind of sediement will be deposited if the motion is decreased slightly. The faster the water, the larger the sediments that can be carried. When water slows down, the larger particles will be deposited first, and the finest particles will be deposited last. Limestone forms in quiet water by the slow accumulation of calcite from shells of living organisms. Coal forms by the accumulation of the organic remains of trees and other plants. In the swamps of still water. The material is converted to coal by the pressure of burial and/or folding. 6. Which of the rocks was probably deposited by the fastest moving water? How do you know? 7. Which 3 rocks above were probably deposited in still water? 8. Where do you think the sandstone was deposited, based on where you find large deposits of sand today? 9. Which of the rocks do you think are strong enough to resist weathering so that they commonly occur as cliffs? 10. Which of the rocks breaks the most easily? 11. Of all 18 rocks in this collection, which do you think were transported by water or ice prior to being collected? The roundedness of the rocks is the clue.
Metamorphic Rocks 12 Gneiss Northern Cascades, S of Ross Dam near Hwy 20. High-grade metamorphic. Age of parent rock maybe Jurassic (146-200 my), age of Metamorphism = Late Cretaceous (66-90 my) 17 Garnet Gneiss Viewpoint SW of Ross Dam, along Hwy 20. High-grade metamorphic rock. Age = same as #12. Marble Soda Springs Quarry, 7 mi west of Lake Wenatchee, W of Wenatchee in the N. Cascades. Age = Pre-Jurassic (older than 200 my) Quartzite Collected at Puget Sound beaches, but was transported by glaciers and originates in Canada. Greenstone Near Nooksack Falls, along Mt. Baker Hwy. Low-grade metamorphic rock. Consists of microscopic crystals of chlorite (green), epidote (green), and albite (white). Age = Middle Jurassic (160-180 my). Graphitic Phyllite Middle Fork of Nooksack River, E of Bellingham. Low-grade metamorphic rock. Age = pre-jurassic (older than 200 my). The 6 metamorphic rocks in this collection were originally either igneous or sedimentary before they were changed (metamorphosed) by heat and pressure. New minerals form by recrystallization, but the elements in the rocks stay the same. For example, limestone (CaCO 3 ) metamorphoses to make marble (CaCO 3 ). Often, with high temperatures, the size of the crystals becomes larger. Garnet (Mg 3 Al- 2(SiO 4 ) 3 ) can form from the metamorphosis of pyroxene (MgSiO 3 ) and feldspar (KAlSi 3 O 8 ). Metamorphic rocks are usually denser than their unmetamorphosed counterparts because of the high pressure they experience, and they often have uneven streaks or stripes from the flattening of layers as the rock is compressed. 12. The metamorphic rocks are described as either highgrade or low-grade metamorphic rocks. Examine the diagram to the right. What defines whether a rock is high-grade or low-grade metamorphic? Based on the description of metamorphic rocks above, if the low-grade rocks had undergone higher grade metamorphism, what sorts of changes would you expect to see in the rocks? List at least 2. 13. Which one of these rocks was probably a sandstone before it was metamorphosed? 14. Which of the rocks is probably metamorphosed granite? 15. Which rock looks like it might be metamorphosed shale? 16. Which rock looks like it might be metamorphosed basalt? 17. Which rock is metamorphosed limestone?
Use the map of Washington State below to plot the approximate locations of your 18 rock samples. Label each rock location by number and whether it is igneous (I), metamorphic (M), or sedimentary (S). Also consult the state geologic map of Washington, since the collection of PNW rocks is mostly from western Washington. Note that the age of rocks in each section are listed from youngest to oldest in the key. This map is located in the Rock Cycle and Plate Tectonics Documents section of my website, and at http://www.dnr.wa.gov/publications/ger_geol_map_washington_pagesize.pdf. Below is a simple cross-section of Washington State, based on our understanding of the plate tectonics in the region. Use the online geologic map, the map you made, the cross-section, and the information given about each rock to answer the following questions. 18. What two major regions of WA State are most volcanic rocks found? Of the two regions, which area, in general, contains the older rocks?
19. Where are most of the metamorphic rocks found in WA State? What does this suggest about the geologic history of this area? Hint: If metamorphic rocks are usually formed through the heat and pressure caused by deep burial, how are these rocks now found at the surface? 20. Where are most of the sedimentary rocks in the rock collection you studied located? What does this suggest about the geologic history of this area? 22. Where are most (largest area) of the youngest sedimentary rocks located? Read the key for these rocks. What does the key say about the origin of these sedimentary rocks? 23. What type of rocks make up the bulk of the Olympic Mountains? What do the lines that run approximately in a N-S direction across the Olympics represent? What does this suggest about how the Olympics formed? Where did these rocks probably originate, and how did they get to the top of the mountains? 24. What type of rock (igneous, metamorphic, or sedimentary) in the collection are generally the oldest? Why do you think this is the case? 25. Where are most of the oldest rocks in the state found? What does that imply about the geologic history of this area? 26. The cross-section of the western PNW give the plate tectonic activity and motion for this area. Which two regions in western WA state is the origin of the geologic features best explained by the cross-section? 27. Which rocks in the collection are not explained by the cross-section? Why do you think they are not explained by the picture?