_06_SierraNevadaNotes_w12

_06_SierraNevadaNotes_w12 - GEOLOGY 20 - LECTURE 6 - Sierra...

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1 GEOLOGY 20 - LECTURE 6 - Sierra Nevada (Ch. 8, 2, 18 - read selectively!) Topography and Structure of the Modern Sierra Nevada The highest peak in the Sierra, Mt. Whitney, is 14,495’ high and is the highest in the contiguous 48 states. The “High Sierra” includes the highest part of the range and extends from north of Yosemite to south of Mt. Whitney. 12 peaks are > 14,000’ high. The range is lowest in the north toward the Feather River where the elevations are around 6500’. The Sierra are wedge-shaped, with gradual western slopes (including a broad foothill belt) and a steep escarpment along the eastern flank - extensive normal fault system along base of eastern escarpment, related to Basin & Range extension (i.e., the Sierra Nevada are a tilted fault -block mountain range) - uplift of the modern Sierra Nevada was accompanied by westward tilting - uplift began only about 5 m.y.a. - very young mountain range - western slopes catch most of the moisture from storm fronts and are much greener than the eastern slopes in the rain shadow - numerous rivers drain the western slope, spilling into the Central Valley and eventually into “the Delta” (more on this later) Lake Tahoe - downdropped fault-block valley – part of the Basin & Range perched on the eastern margin of the Sierra - Lake Tahoe fault-block is bounded by uplifted Carson Range on Nevada side and the Sierra crest to the west - steep-walled and flat-bottomed – lake-water fills in the downdropped valley between normal faults - water is 1650’ deep (500 m), the second deepest lake in N.A. - if interested, browse bathymetric images of Lake Tahoe at http://tahoe.usgs.gov/bathimages.html Remember: Mountains are composed of the rocks that happen to be there when the mountains were uplifted. The rocks have to form first - then they are exposed within a growing mountain range. Rocks comprising the Sierra Nevada can be divided into two broad groups: the Sierran Batholith and the Western Metamorphic Belt
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2 Sierran Batholith When you visit Yosemite and see massive cliffs of apparently homogeneous rock like El Capitan or Half Dome you are looking at rock that solidified deep within the earth in a magma chamber. Rocks that form by crystallization from a molten magma or lava are called igneous rocks. Solidification of igneous rocks may occur either inside the earth in a magma chamber or on the surface of the earth along the flanks of a volcano. The main mass of the Sierra is composed of rocks that solidified from molten magma deep within the ground. These types of igneous rocks are called intrusive (or plutonic ). (As opposed to extrusive volcanic rocks that solidify as lava or pyroclastic debris on the surface).
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_06_SierraNevadaNotes_w12 - GEOLOGY 20 - LECTURE 6 - Sierra...

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