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GEOS 251 — Physical Geology. 11 February 2014 Quiz today, 15 minutes, 25 pts Starts promptly at 11:02 Closed book and notes; pencil or pen only Covers topics through Igneous Rocks (4 Feb) Handout Lecture Summary. Last Time: Magmatic Processes. Magmatic processes - origin
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GEOS 251 — Physical Geology 11 February 2014 • Quiz today, 15 minutes, 25 pts • Starts promptly at 11:02 • Closed book and notes; pencil or pen only • Covers topics through Igneous Rocks (4 Feb) • Handout • Lecture Summary
Last Time: Magmatic Processes • Magmatic processes - origin • Melt of rock caused by ∆T, ∆P, ∆H2O (tectonics) • Basalt originates by melting of mantle peridotite • Granite originates by melting of mafic crust • Magmatic processes - evolution and emplacement • Magmas are modified by • Removal of crystals (“crystal fractionation”) • Mixing with other magmas or assimilation of other rocks • Emplacement of magmas: buoyancy (lower density) vs. impedance (increasing viscosity) • Result: A spectrum of igneous rocks that varies with tectonic setting
Melting produces magmas • Rocks melt with (1) increasing temperature (2) decreasing pressure (3) addition of water— Why do these factors change in the Earth? • Source rocks:— mantle (peridotite)— crust (andesite or shale — most abundant compositions)
Causes of Melting • Increase in temperature • Crustal thickening / subduction • Heat from other magmas • Decrease in pressure • Upwelling of mantle • Increase in water content • Subduction zones • Hydrous crust (what minerals?) • Next examine experimental data– a test in the lab
heating melting paths (how?) heating melting paths (how?) addition of water melting paths (why?) decompression melting paths (where?) Wet Dry
Melting in the Mantle:The dry (decompression) process • Process: rising mantle intersects the melting curve as it decompresses • Where: (1) under mid-ocean ridges, (2) in hot spots (“mantle plumes”) (e.g., Hawaii) • Produces: basalt (pyroxene + plagioclase feldspar) from mantle (olivine > pyroxene > an Al mineral such as Ca-rich plagioclase)
Q: Why do we get basalt (a mafic composition) from mantle peridotite (an ultramafic composition)? In a chemically complex material, crystals and melt typically have different compositions. Thus when peridotite melts >>>
Analogous to Ice + Salt melting to make Brine (salt water) Removal of low-T melting fraction (“partial melting”) generates mafic melt (basalt) from ultramafic mantle (peridotite)
Water from slab(mainly metamorphic reactions, not from sediment) Origin of magmatic arcs • Cycling of water back into the mantle via subduction of hydrous minerals; their breakdown triggers melting • Where does the liquid water go? • Where does the heat beneath arcs originate?
Melt generation processesby tectonic setting Melting by heating of lower crust Decompression melting Water-induced melting
Changes in magmas byfractionalcrystallization • Can you think of analogies?
How might fractional crystallization lead to the concentration of rare elements? • Tourmaline (gemstone) • Contains the rare elements • lithium (Li, Z =3) • boron (B, Z=5) • Can anyone suggest why these elements might be rare? • Why might these very light elements not readily substitute in common mineral structures?
Sedimentation and volcanism • Both take place at the Earth’s surface • Both involve fluids • Water and air for sedimentary rocks; but cool • Melt (± air or water) for volcanic rocks; but hot
Lecture 8: Volcanoes and Volcanism • Volcanic rocks and processes • Ejecta (lavas, pyroclastics, gases), volcaniclastics (lahars, sands, muds) • Other features: earthquakes, geothermal systems • Types of volcanoes • Basic types: central vent, fissure, caldera . . . • Controls on types: magma composition, T, flux • Tectonic distribution and controls • Next time finish with: Volcanoes and people: hazards, resources (geothermal, minerals, water, soil), climate • Then on to: Weathering and erosion (Chap. 16, 5)
What happens in volcanoes?Why does this happen?What do volcanoes produce?How are volcanoes constructed?How do they differ and why?(what, where, how come)
Volcanic MaterialsTwo groups – what are they? • Lavas — move (mainly) as massive magma • Massive to brecciated flows (pahoehoe, aa, pillows) Fragmental materials • Pyroclastics — fragmented magma / rock • Ash (and lapilli), cinders, bombs (increasing size) • Pyroclastic flows (“ash flows” or “ignimbrites” can be welded by heat, depending on size) • Volcaniclastics — reworked, mainly by water • Lahars / mudflows — start on or near volcano • Epiclastic rocks (reworked volcanic sands / muds)
LavasCoherent bodies of magma emplaced by flow at surface • Wide differences in form and textureWhy might this be?
Pyroclasticvolcanic products • Fragmental materials produced during eruption (right) • Compare and contrast lavas (above) and clastic sediments
Fragmental Volcanic Products • Pyroclastic types • depend on nature of eruption (and thus nature of volcano) • proximal (near the vent) ejecta and more far traveled pyroclastic fall (“air fall” tuff) and pyroclastic flow (ash-flow tuff) deposits • fragment size diminishes with distance from vent • Pyroclastic processes and products • ballistic • mix with air and be carried away (-> pyroclastic fall / ash-fall tuff) • eruptive column collapse (-> pyroclastic flow / ash-flow tuff) • volcano collapse (-> debris avalanches / pyroclastic flows / lahars (cooler)) • Contrast reworked cool materials around volcanoes • mudflows, lahars, and volcaniclastic rocks Many parallels with sedimentary processes (later lectures)
Types of Eruptionswhat are they and what governs their styles? • Types — as seen above Passive – mainly lava flows Explosive — pyroclastic fall and flow deposits • What are the main factors? • Energy for eruption (from what?) Buoyancy, especially dissolved gases that exsolve and expand • Viscosity – how easy it is to deform magma • So what will be the effects of magma composition (and thus geologic setting)?
Types (geometries) of volcanoes • Central volcanoes (varied compositions) • Cinder cones, domes, stratovolcanoes • Calderas (most commonly felsic) • Fissure eruptions from ring-fractures over collapsing magma chambers • Fissure eruptions (typically mafic) • Flood basalts, mid-ocean ridges • Type (geometry) of volcano largely reflects • Type and volume of magma erupted, thus • Passive vs. explosive eruptions, and • Tectonic setting
Stratovolcanoes and Calderas • Stratovolcanoes built of multiple lava flows and pyroclastic rocks (“strata”) • Steep cones, typically of andesitic arc volcanism • Shield volcanoes built of many lava flows • Gentle cones, typically of basaltic volcanism, especially in intraplate (hot spot) settings • Calderas are collapse features formed over rapidly evacuated magma chambers • Most common over rhyolitic magma chamber, commonly without central volcano precursor, but • May also develop late in the history of a cluster of andesitic stratovolcanoes
Andesitic stratovolcanoes— Composite (many eruptions), steep sided— Commonly violent (e.g., Mount St. Helens, WA)
Mt. Pinatubo • Crater collapses during eruption to form caldera • Soon fills with water to form a lake
Shield volcanoes— Gentle slopes— Basaltic, passive eruptions (e.g., Hawaii)
Calderas instratovolcanoes- Andesitic: Crater Lake, Oregon- Basaltic: Kilauea, Hawaii • Crater Lake caldera formed by collapse during & following of massive eruption of ancient Mt. Mazama
Rhyolitic calderas• Enormous, violent eruptions; single eruptions (over a few days) can be 100s to 1000s of km3 (e.g., San Juan Mtns, CO)
Flood basalts and large igneous provinces • High-volume fissure eruptions related to plumes from deep mantle (hot spots) • Can cover very large areas and can be erupted quickly • Some eruptions contemporaneous with mass extinctions • Most easily recognized on continents • Columbia Plateau / Snake River Plain, Siberian traps (Russia), Deccan traps (India) • But also form large oceanic plateaus • Ontong Java Plateau (SW Pacific), Kerguelen Plateau (Southern Ocean)