First, an aside:In 1902 Mt. Pelee on Martinique erupted with 4 deafening blasts, the pyroclastic flow killed all 30k people, only two people survived, one a murderer awaiting execution, protected by his dungeon.
=-=-=-=--reading notes on-=-=-=-=-=-=-=-"Geology of the Sierra Nevada" by Mary Hill-=-=-=-=-=-=-=-=--=-=-=-=-=-=-=-=-=-=-
"Batholith" is deep rock, its individual masses are called "Plutons." separated from one another by metamorphic or igneous rock. The Sierra plutonic rocks formed as crystals solidifying within liquid (in a thick mush, overcrowded crystals form til there's no more liquid (due to cooling or total crystalzation)). Darker rock was first to crystallize, lighter last, and Quartz (pure silica)is thought to be the residue. Actual coarse-grained dark granite is absent in Sierra, spots of darker rock are more often igneous rock that escaped metamorphic melting.
The first 4 billion years of what became the Sierra is unknown, the oldest rocks today were laid down 500 million years ago (Ordovician). They show that the sea washed over the land for 400million years. All older rocks (called "bedrock" or "subjacent series") were marine in origin. Sea receeded over the next 100million years.
Two great groups of rock formed the gold belt, Paleozoic (250M) and Mesozoic (75M) in age. All the rocks are now metamorphosed, but are often referred to (where poss) by what their orig names would have been (limestone, shale, sandstone), confusing amateur geologists. The Paleozoic beds were once mainly limestone, siltstone, and mud (collectively called "hornfels", they are even, fine-grained). Paleo+Mesozoic outcrops are now seen in the w.sierran foothills and along the crest. Uplift of granite and erosion have exposed the underlying granite. Where viz together, Paleozoic beds tilt at a different angle than Mesozoic, called an "unconformity." In higher country, color also distinguishes them. Paleozoic are reddish brown on their exposed surfaces while Mesozoic turn gray. Mixed in may be shiney green Serpentine, brought up from earth's mantle (source of chrome, asbestos, and nephrite jade.)
"Batholith" is deep rock, its individual masses are called "Plutons." separated from one another by metamorphic or igneous rock. The Sierra plutonic rocks formed as crystals solidifying within liquid (in a thick mush, overcrowded crystals form til there's no more liquid (due to cooling or total crystalzation)). Darker rock was first to crystallize, lighter last, and Quartz (pure silica)is thought to be the residue. Actual coarse-grained dark granite is absent in Sierra, spots of darker rock are more often igneous rock that escaped metamorphic melting.
The first 4 billion years of what became the Sierra is unknown, the oldest rocks today were laid down 500 million years ago (Ordovician). They show that the sea washed over the land for 400million years. All older rocks (called "bedrock" or "subjacent series") were marine in origin. Sea receeded over the next 100million years.
Two great groups of rock formed the gold belt, Paleozoic (250M) and Mesozoic (75M) in age. All the rocks are now metamorphosed, but are often referred to (where poss) by what their orig names would have been (limestone, shale, sandstone), confusing amateur geologists. The Paleozoic beds were once mainly limestone, siltstone, and mud (collectively called "hornfels", they are even, fine-grained). Paleo+Mesozoic outcrops are now seen in the w.sierran foothills and along the crest. Uplift of granite and erosion have exposed the underlying granite. Where viz together, Paleozoic beds tilt at a different angle than Mesozoic, called an "unconformity." In higher country, color also distinguishes them. Paleozoic are reddish brown on their exposed surfaces while Mesozoic turn gray. Mixed in may be shiney green Serpentine, brought up from earth's mantle (source of chrome, asbestos, and nephrite jade.)
Fossils - Cenezoic to Teritary Period (50-65M yrs ago) plants, winged seeds, laurels, figs, oaks, magnolia, witch hazel, small palms, cycads, hiclkorys and persimmon but no conifers. Not many fossils, no birds or bugs but near Knight's Ferry close to Tuolumne Table Mtn, two little horses (Nannipus and Hipparion), two camels, a pronghorn antelope and one tooth of a mastadon were found. (Yes there were n.american horses, but they vanished along with n.american zebras, tapirs, and camels in the ice age.)
Volcanoes are of three types: Basalt, Andesite, Rhyolite. Unlike Hawaiian basaltic lava flows that are calm, the initial Sierran Tertiary eruptions were rhyolitic, an explosion of steam from liquid lava that hurls dust, sand, blocks, and "bombs". Just as Krakatoa spewed its rhyolitic ash 7x around the planet (the sound was heard for 3k miles,) the Sierran Tertiary blasts 40M yrs ago left beds of ash 450' thick in gold country and 1200' thick near mountain the crest, north and northeast of Yosemite Natl Park. Other andesitic volcanoes erupted 20M yrs ago. These were hot mud flows (called "lahars" from the Indonesian term) cascading down streams and mtns inundating all but high country crests and some greenstone in the foothills. They carried masses of rock, boulders of granite and metmorphic rocks, torn and shattered trees.
Sierran passes are andesitic terrain, sparesely populated with bushes and digger pine for miles. Carson Spur on Hwy88 is an example, a mudlow perched above hundreds of feet of granite thru which the American River has carved its path. Weather has carved the volcanic cliff face into battlements and towers resembling a derelict fortress. Near Knight's Ferry on Tuolumne Table Mtn, the streaming lava flow through what was Stanislaus Rivery (Tertiary Period) hardened more than the surrounding hills which eroded away leaving a high, sinuous ridge (formerly a river valley!)
During the ice age, a third sierran volcanic era began (just 1M yrs ago) in the Mono Lake area vents near 11k foot Glass Mtn. An avalanche of fire poured 2100' down and flowed 10miles, moving at 100MPH out burying 350M sq mi under 500' of 1200-2400F degree rhyolitic ash, erasing whole hills and canyons. Later, a basaltic flow issued from Mammoth Lakes basin west into Middle Fork Vly. Devils Postpile's 60' columns resulted from this 600k yr old basaltic flow that was 900' long by 200' high. Recent basaltic cones have formed 3mi south of Devils Postpile, steam can be seen in winter on Mammoth volcano.
Mammoth volcano is part of a string of obisian glass and pumice domes called Mono Craters which have pushed 9000' above sea level, but remain dwarfed by the Sierra west of them. Still active as hot springs attest. An 1890 eruption releasing sulfur fumes and boiling the normally cold waters of Mono Lake is the most recent known activity. Oddly, Mono Lake has fresh spgs in the depths of its caustic, alkaline waters.
Basic mineral families=
Quartz (won't scratch, white, often clear, sometimes milky)
Feldspar (marked by lines, less glassy than quartz)
Mica (soft, flaky, thin sheets, glimmers (foolsgold))
Amphibole (dk green/black, lacquered, breaks obliquely)
Pyrozene (close to Amphibole but breaks at right angles, short stubby square)
Calcite (Pearly, Glassy, breaks obliq, knife scratches)
Iron - only small deposits, but the high slopes of the Minarets may falsely deflect your compass needle.
Mammoth volcano is part of a string of obisian glass and pumice domes called Mono Craters which have pushed 9000' above sea level, but remain dwarfed by the Sierra west of them. Still active as hot springs attest. An 1890 eruption releasing sulfur fumes and boiling the normally cold waters of Mono Lake is the most recent known activity. Oddly, Mono Lake has fresh spgs in the depths of its caustic, alkaline waters.
Basic mineral families=
Quartz (won't scratch, white, often clear, sometimes milky)
Feldspar (marked by lines, less glassy than quartz)
Mica (soft, flaky, thin sheets, glimmers (foolsgold))
Amphibole (dk green/black, lacquered, breaks obliquely)
Pyrozene (close to Amphibole but breaks at right angles, short stubby square)
Calcite (Pearly, Glassy, breaks obliq, knife scratches)
Iron - only small deposits, but the high slopes of the Minarets may falsely deflect your compass needle.
Gold (shiney, soft) In 1854 a 160 pound chunk came from Carson Hill mine near Melones. Great rips in earth during Jurassic (150M yrs) allowed mineralizing solutions to rise from the depths. Those faults "healed" with quartz, etc including gold and silver in sheets ("veins") and holes ("vugs") 10k feet+ deep filled with ore-bearing minerals and metal shot thru the rock on either side. Gold bearing quartz was usually last to form, esp where veins turned or split. Miners believe milkier quartz was more likely "live" with gold. 25M yrs of erosion during Cretaceous period removed 9 vrical miles of rock (.5-1.5 feet per 1k years) to expose gold bearing quartz veins. 1860s gold miners using huge hydraulic hoses eroded 1555M cubic yds (8x the panama canal) out of the sierra and into farmlands, the fastest erosion in history til an 1884 court injunction halted it.
Other Sierran minerals of note:
Tungsten - (used to harden steel) world's largest mine up Pine Creek (near Bishop).
Garnet (not gem quality, used as an abrasive) pale reddish brown.
Diamond - hard to distinguish from Quartz, miners took several hundred, probly ignored many more.
Plutonic Igneous Rock families=
Granite typically shows white quartz, black biotite mica, and feldspar. Salt n pepper, or sometimes pink.
Diorite is mostly feldspar, darker than granite with little or no quartz
Gabbro is dk gray to black, equal parts feldspar (plagioclase variety) and dark minerals. No quartz.
Peridotite = grainy dark green/black minerals, no quartz or feldspar. Most in Sierra has wholly or partly metamorphosed to serpentine.
Porphyry= granite with lumpy chunks of feldspar crystals as large as several inches diameter. See rhyolite dike west of convict creek and Mr. Morrison quadrangle.
Other Sierran minerals of note:
Tungsten - (used to harden steel) world's largest mine up Pine Creek (near Bishop).
Garnet (not gem quality, used as an abrasive) pale reddish brown.
Diamond - hard to distinguish from Quartz, miners took several hundred, probly ignored many more.
Plutonic Igneous Rock families=
Granite typically shows white quartz, black biotite mica, and feldspar. Salt n pepper, or sometimes pink.
Diorite is mostly feldspar, darker than granite with little or no quartz
Gabbro is dk gray to black, equal parts feldspar (plagioclase variety) and dark minerals. No quartz.
Peridotite = grainy dark green/black minerals, no quartz or feldspar. Most in Sierra has wholly or partly metamorphosed to serpentine.
Porphyry= granite with lumpy chunks of feldspar crystals as large as several inches diameter. See rhyolite dike west of convict creek and Mr. Morrison quadrangle.
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