1. Summer School Geosciences
Geology
Lecture 4 Rocks and the Rock Cycle

2. 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

3. Granite
Aeolionite
Flint
Purple Lithic Sandstone
Gneiss
Sandstone

4. The Rock Cycle

5. 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

6. 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

7. 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

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

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

10. 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

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

12. 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

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

14. 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