Summer School Geosciences Geology Lecture 4 Rocks and the Rock Cycle

Rock A 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

Granite Aeolionite Flint Purple Lithic Sandstone Gneiss Sandstone

The Rock Cycle

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

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

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

Sedimentary Rock Classification Exogentic – detrital rocks & sediments Endogenetic – chemical & biochemical rocks and sediments

Sedimentary Rock Important Features Firstly Grain size Adjective Sediment Name Rudaceous >/= 2mm Gravel Arenaceous 1/16 - 2mm 1/256mm –1/16mm Sand Silt Argillaceoous </=1/256mm Clay, mud

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

Sedimentary Rock Physiochemical – precipitated limestones, dolomites, sedimentary iron ores, flints, evaporites Biochemical Constructed – reef rocks (coral & algal limestones) Detrital – shell sands, carbonaceous rocks

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

Stromatolites algal fossil mats Copyright Cambridge Carbonates Coral Reef Calcite Concretions Copyright Cambridge Carbonates

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