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Lecture

Fall Term: 3rd Lecture - Rocks and Volcanoes

8 Pages
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Department
Natural Science
Course Code
NATS 1750
Professor
James Williams

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Earth and the Atmosphere Lecture 3 – September 26 Sedimentary Rock 1. Comprises large part of exposed surface rock (75%), but only a small smart of the upper crust 2. Sedimentary rocks are those rocks laid down from weathering or chemical processes 3. Surface action moves material from the location to another where small particles can accumulate and “lithify” into new rock Material weathers at the surface and is transported mainly by water (liquid or ice (like glaciers… it will form all kind of sedimentary deposits as the advance or retreat) – though wind can also move material. Laid down elsewhere (ex. River – picks up sediment, gets down to land that flattens out, then the sediment will drop out) Over time accumulates to great depths. This results in compaction into denser material Water can also transport material into spaces and cement materials together Sedimentary rock VERY much an indication of surface processes. This includes like as many sediments have fossils or some evidence of life trapped in them. Almost all fossils are buried in sedimentary rock. Sedimentary rock is all about surface process. Features - Sedimentary rock forms in beds or strato - Divided by bedding planes – often split easily along these lines - These planes represent changes in conditions that laid down the rock Types - Main division detrital (small particles) or chemical (includes biochemical) Rock that would form coral skeletons would be: Chemical Something that is living or in a reef, it will include some chemical process. Detrital - Particles broken off by weathering and carried in suspension (breaking down into smaller and smaller bits, breaking down and carrying it with it. Strictly mechanical process) - Usually classified by particle size - Particle size and indication of mechanisms involves - Bigger particle – more energetic - Most common elements clay (from chemical weathering of silicates) and quartz As material buried deeper and deeper is compressed more and more…particularly for fine particles Cementation also binds particles together Water precipitates matter onto larger particles eventually joining particles Calcit, silica, iron oxide most common cements Chemical/Biochemical - Material dissolves in water and is carried to another location and deposited - Precipitates out as concentrations change to physical changes or biochemical action Limestone most abundant type (part of the carbon cycle) Formed mostly from biochem processes – small skeletal remains from microorganisms Some formed more directly by precipitation of calcium carbonate from water (like limestone in caves) Coal is also sedimentary rock Forms when plant material accumulates and it’s buried and compressed Peat (uncompacted coal) to lignite to bituminous coals are all sedimentary Also a part of the carbon cycle Carbon cycle and sedimentary rock Important aspect of carbon cycle Carbon transferred back and forth between solid and gas states Link between atmosphere and lithosphere Formation of limestone would: Decrease carbon dioxide in air - Silica and other materials also precipitate out of water – can be inorganic or organic but can be very hard to tell which - Evaporates like halite (salt beds) form when concentration become too high and material comes out of solution Metamorphic Rock - Existing rock can be transformed by the physical and chemical environment it is exposed to - Changes can range from slight to substantial - Rock remains essentially solid, but is transformed - Doesn’t melt - Driven by heat, pressure and chemically active fluids though not in equal proportions and these proportions vary by circumstance - Low grade – slight changes like shale to slate - High grade – substantial changes - may show no clue of original rock Heat - Most important, triggers crystallization - Contact metamorphism – heat from intruding magma “bakes” adjacent rock - Temperature also increases with depth, so as rock buried will experience higher temps – regional metamorphism – large scale metamorphism Pressure - Can be either confining or differential - When rock is buried, pressure roughly equal all directions – confining pressure - Deeper you go, like with heat, pressure increases - Rock tends to be more compact as is squeezed Chemically Active Fluids - Fluids – usually water with other materials dissolves may act as a catalyst - Hotter the fluid, the more reactive the dissolved ions become - Materials dissolve more easily in areas of high stress and are precipitated out in areas of lower stress - Minerals will grow/accumulate is direction at right angle to stress - texture – foliated and non-foliated Resources from rock and mineral - although most elements awe think of as commercially valuable are distributed in throughout most rock – the process in the rock cycle can concentrate these elements like gold, silver, nickel, etc - one way heavier materials are concentrated (like platinum) as they settle to the bottom of a magma chamber - pegamatite are large crystals that occur as hot mineral rich fluids Hydrothermal - hot metal rich fluids will accumulate at the top of magma deposit - fluids can also flow large distances along bedding planes or fractures in surrounding rock creating vein deposits - disseminated deposits also occur where the fluids are dispersed through the rock near the parent igneous body Volcanoes are associated with which type of rock?: basalt Types All volcanoes are the result of magma becoming liquid under the surface, rising and erupting as lava (often very forcefully) at the surface The mechanism that produces the magma and the local geology where the magma and the local geology where the magma exits the crust can produce a number of different structures. There are three main types of volcano - strato (or cone) (Mt. St Helens) - shield (or hot spot) (Hawaiian Volcanoes) - cinder cone There are also other structures that result from large scale eruption of lava producing much larger feature
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