Summer SchoolGeosciences

Geology

Lecture 4 Rocks and the Rock Cycle

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

Granite

Aeolionite Flint

GneissSandstone

Purple Lithic 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 sizeAdjective Grain Size 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

Coral Reef

Stromatolites algal fossil matsCopyright Cambridge Carbonates

Calcite ConcretionsCopyright 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