summer school geosciences geology lecture 4 rocks and the rock cycle
TRANSCRIPT
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Summer SchoolGeosciences
Geology
Lecture 4 Rocks and the Rock Cycle
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RockA rock is any naturally formed
aggregate or mass of mineral matter that constitutes an integral part of the Earth’s crust, not a single fossil or a single individual mineral.
e.g. granite, Sandstone, marble,gneiss
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Granite
Aeolionite Flint
GneissSandstone
Purple Lithic Sandstone
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The Rock Cycle
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Sedimentary Rock
• Sedimentary rocks are formed by the disintegration of existing rocks, with chemical and biochemical deposits and are a result of deposition at the Earth’s surface
• Disintegrated rock fragments – solid material in the form of mineral grains and rock fragments, together with material in solution
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Sedimentary Rock
• Chemical deposits – new minerals deposited from solutions within the sediment or by direct precipitation
• Biochemical deposits – many animals and plants build skeletons which disintegrate and act as sedimentary particles e.g. shells
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Sedimentary Rock• Important features
• Sorting – the range in grain size in the rock e.g. well sorted means all same size
• Grain shape – round or spherical
• Composition – use %s, quartz, rock fragments, fossils & type
• Colour – often controlled by cement
• Sedimentary structures – bedding, lamination, ripples
• Physical features – fissility, porosity
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Sedimentary Rock
Classification
• Exogentic – detrital rocks & sediments
• Endogenetic – chemical & biochemical rocks and sediments
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Sedimentary Rock
Important Features
• Firstly Grain sizeAdjective Grain Size Sediment
Name
Rudaceous >/= 2mm Gravel
Arenaceous 1/16 - 2mm
1/256mm –1/16mm
Sand
Silt
Argillaceoous </=1/256mm Clay, mud
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Sedimentary Rock
Secondly Rock Composition particularly sandstones
• Orthoquartzite – 95% quartz• Feldspathic sandstone – high % feldspar• Arkose – 25% Feldspar• Lithic sandstone – with appreciable rock
fragments• Greywacke – with feldspar &/or rock
fragments and detrital clay matrix
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Sedimentary Rock
Physiochemical – precipitated limestones, dolomites, sedimentary iron ores, flints, evaporites
Biochemical
Constructed – reef rocks (coral & algal limestones)
Detrital – shell sands, carbonaceous rocks
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Sedimentary Rock
• Bioconstructed Rocks
• These rocks have a rigid framework, constructed by the activities of living organisms
• The modern example are coral reefs, where coral colonies & algae act as a frame builder
• In the geological past, several groups of organisms have acted as frame builders, corals and algae are the most important
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Coral Reef
Stromatolites algal fossil matsCopyright Cambridge Carbonates
Calcite ConcretionsCopyright Cambridge Carbonates
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Sedimentary Rock
• Carbonaceous Rocks
• These are formed from plant material in various stages of decay
• They consist of complex hydrocarbons, sulphur and nitrogen compounds and varying amounts of detrital material
• Although rare, these rocks are of great economic significance e.g. coal, oil shales