Types of Rocks Three Types of Rocks Type of rock and source material Rock forming process IGNEOUS Melting of rocks in hot, deep Crust and upper mantle Crystallization (solidification of magma) SEDIMENTARY Weathering and erosion of rocks exposed at the surface Deposition, burial, and lithification Laminated sandstone Recrystallization in solid state Of new minerals Gneiss. Metamorphosed granite METAMORPHIC Rocks under high temperatures And pressures in deep crust and Upper mantle Example Coarsely crystallised granite Proportions of the Rock Types Think about CPT Sedimentary Camps Bay IGNEOUS Melting of rocks in hot, deep Crust and upper mantle Crystallization (solidification of magma) Coarsely crystallised granite Crystallise directly from magma, either at depth or at the surface Igneous rocks can be intrusive or intrusive Mantle - magma Lava Extrusive igneous rock Intrusive igneous rock Active subaerial volcanoes of the world (80% of them are at convergent plate boundaries) Genetic Classification of Igneous rocks Intrusive: crystallised from slowly cooling magma intruded with the Earth’s crust e.g., granite, gabbro Extrusive: crystallised from rapidly cooling magma extruded on the surface of the earth or erupted as pyroclastic material. Genetic Classification of Igneous rocks First tier - Intruded / extruded Second tier - Chemistry (e.g., SiO2) controlled by mineralogy Felsic: (acid) SiO2 rich Intermediate Mafic: (basic) Mg and Fe rich Ultramafic Extrusive Intrusive Observation – Mafic rocks are dark, Where felsic rocks are light Classification diagram for hand specimens Types of intrusive and extrusive igneous structures Intrusive Extrusive Ship rock, New Mexico. Stock or plug with associated radial dyke swarm Let’s look at Igneous rocks in SA Check out https://www.youtube.com/watch?v=cuFGMk_sDa4 Bushveld Igneous Complex > 2 Ga Largest known igneous intrusion on Earth 70% of the world’s platinum and significant quantities of palladium, rhodium, chromium, and vanadium Paarl Rock (SA) One of the many granites belonging to the Cape Granite Suite Granites, Boulders Beach Cape Granite Suite (~525 Ma) Kimberlite pipes Check out: https://www.youtube.com/watch?v=EAmab-UlQo4 Kimberly “The big hole” (mined Kimberlite) The pit has yielded some of the largest diamonds in the world. Nearly 15 million diamonds were extracted from the Kimberley Diamond Mine, discovered in 1871. Excavation ended in August 1914. Dolerite dykes (~135 Ma) – Intruded during extension. Break up of Gondwana (Africa, South America, Antarctica, India) Cross section of top right figure. Dyke Drakensberg Flood Basalt ~183 Ma Drakensberg Part of the Karoo Igneous province – one of the largest volcanic episodes in the Earth History. Heralded the break up of the supercontinent Gondwana Clarens Sandstone Mt Alex Du Toit Check this out: https://www.youtube.com/watch?v=T1-cES1Ekto Sandstone, looks Like the Clarens 180 Ma basalt lvas overlying sandstone in Kirwanveggen Escarpment, Antarctica. Once joined to northern KZN Alex Du Toit “Our wondering continents” Alex Du Toit, 1937 Continental drift Theory of plate tectonics SEDIMENTARY Weathering and erosion of rocks exposed at the surface Deposition, burial, and lithification Laminated sandstone Can be CLASTIC (TERRIGENOUS) or NON-CLASTIC Clastic: A rock resulting from the consolidation of loose sediment that has been derived from previously existing rocks and accumulated in layers Non-clastic: a rock formed by the precipitation of minerals from solution by either organic or inorganic processes (chemical) Sand and gravel Conglomerate (clastic) Non-clastic shells (calcite/aragonite precipitated) Sedimentary rocks can be made up of pieces of igneous rocks Let’s take a closer look at clastic rocks Understanding the key processes of the rock cycle is essential in sedimentology which is the science of sediments and sedimentary rocks. Most of the process take place on The Earth’s surface or at very shallow depths. Therefore, the observation, analysis and evaluation of these can be – in general- easily carried out. Clastic sedimentary stages of the rock cycle • • • • Weathering Erosion Transportation Deposition (sedimentation) • Burial • Diagenesis Sedimentary processes Drakensberg Flood Basalt Clarens Formation sandstone Can you identify parts of Sedimentary rock cycle in this photograph? The river will erode sediments if it has high energy. At higher gradients, the river will have higher energy Wind will blow and transport loose sediment Sediment is deposited The river will transport sediment Lithification or Diagenesis lithification =Transformation of unconsolidated sediments into sedimentary rocks at low temperatures and pressures after deposition of the loose sediments Involves 2 main steps: 1, Compaction 2, Cementation lithification Burial = more sediment is added onto a previous layer by the weight of sediment building up above them Sediment particles are forced close together Compaction = pore space is reduced by the weight of the overburden Diagenesis =partial or total cementation (=lithification), binding together by cement, the glue of sedimentary rocks (e.g., carbonates/calcite, silica/quartz, iron oxides) Mud, sand, gravel, angular debris breccia compaction + cementation mudstone, sandstone, conglomerate, Rounded vs angular gravel Properties to describe terrigenous clastic rocks We’ll focus on the highlighted ones • Grain size • Grain shape • Roundness • Sphericity • Sorting • Maturity • Porosity • Permeability When you are asked to describe a rock sample, you should be thinking about all of these properties. You may not be able to observe all of them for every rock you encounter Classification of clastic rocks is primarily based on the size of the constituent particles i.e., grain size Clastic sedimentary rock Non-clastic sedimentary rock Grain size Can see grains - Sand Cannot see Grains - mud Grain size Sorting The range of grain sizes present in the sediment. The narrower the range, the better the sorting of the sediment Beach sand is very well-sorted. Constantly being reworked by waves Sorting Sorting is affected by: • The type of transport. Wind vs glaciers – wind is a selective transporting agent, sands and muds can be picked up but not gravel. Sediments transported by wind are well-sorted. – Glaciers are high energy modes of transport. They can entrain large boulders and smaller material. Once sediments are lodged in the glacier, they do not have any further sorting, when the ice melts it dumps all of the sediment it entrained. Therefore, the sediments are poorly sorted The duration of transport. Longer transporting times typically result in better sorted sediments. Think about a river. Closer to the source, it has higher energy and can entrain a variety of sediments. Further along, it looses energy and begins to drop the larger clasts, by the end only fine sediments are entrained – wellsorted. Dust storm Downslope in a river, decreasing energy increasing travel time Sorting Roundness The sharpness (angularity) of the corners and edges of the particles Sphericity The degree to which a particle approaches a spherical shape Roundness and sphericity will increase with increasing transporting duration. More time for particles to bump around and smooth Check out: https://www.youtube.com/watch?v=lFh8u7xmzr0 Roundness Let’s take a closer look at non- clastic rocks Carbonates Principal minerals – calcite, aragonite (unstable), dolomite (diagenetic). Can be biogenic or non-biogenic Biogenic – carbonate forming organism include both plants and animals. Skeletal elements can be complete or broken hard body parts of organisms that use calcium carbonate minerals in their structure. Non-biogenic - calcium carbonate precipitates when cold water gets warmed up. This is because bicarbonate is less soluble in warm water than cold water e.g., the Bahamas. • Ooids spherical bodies of calcium carbonate. White cliffs of Dover – chalk (CaCO3) Transvaal Supergroup, SA. 2.65 Billion years old Contains 1 km thick carbonate – Great oxidation Event. First free oxygen on Earth Banded Iron Formation Precipitate comprising of alternating bands of iron oxides (hematite, magnetite) and chert/iron poor shale Formation related to the Great Oxidation Event Economic value – iron ore. No BIF forming in modern times Kolomela, mining iron in SA Evaporites Dissolved salts precipitate due to increased concentration during evaporation. The least soluble compounds precipitate. Form in hot, dry environments. Etosha Salt Pans, Namibia Coal Large quantities of organic matter First stage of peat formation is aerobic – biological breakdown of plant tissue on the land surface To preserve plant material, need an environment with limited oxygen Burial; increase in temperature and pressure Volatiles are removed; the proportion of carbon in the pear increases METAMORPHIC Rocks under high temperatures And pressures in deep crust and Upper mantle Recrystallization in solid state Of new minerals Gneiss. Metamorphosed granite How do rocks metamorphose? • From the partial or complete recrystallization of minerals in the rocks over long periods of time • Rocks remain essentially solid during metamorphism • Changes in pressure and temp. • Metamorphic reactions often Involve dehydration (loss of H2O) &/ Decarbonation (loss of CO2) reactions That produce a fluid phase Commonly used unit of pressure in geology is the kilobar (kb). 1 kb = 1000 times atmospheric pressure, ~0.1 Gpa. Each kb pressure increase corresponds to an increase in depth of ~3km Metamorphic grade Environments of metamorphism Contact metamorphism Sedimentary Igneous intrusion Igneous rock intrudes into rock sedimentary rock – heats it, melts it – it metamorphoses At Sea Point Promenade – contact metamorphism • In deep successions of young sedimentary or volcanic strata – burial metamorphism • At active plate margins – regional metamorphism • In close proximity to igneous intrusions – contact metamorphism • In fault and shear zones – dynamic metamorphism • Where chemically active fluids circulate through rocks – hydrothermal metamophsim / metasomatism • At the sites of meteorite impact – impact metamorphism Excluding impact metamorphism, the general types grade into one another Check out: https://www.youtube.com/watch?v=oFOIWOoaK0s • The protolith – “original” “rock” (prior to metamorphism) The protolith can be igneous, sedimentary or metamorphic. Marble Protolith is limestone • The mineral assemblage of metamorphic rocks is a function of 1. Bulk composition 2. Conditions of formation (P, T and fluid composition) • There are 3 major compositional types of metamorphic rocks: 1. Metapelite [protolith – argillaceous sediments (contains clay)] 2. Metabasites [protolith – basalts and gabbros (igneous)] 3. Calc-silicates [protolith = impure limestones] Gneiss Protolith is granite Schist Protolith mica rich mudstone 1. Metapelites: Clays decompose to give Al-rich minerals (muscovite, staurolite, garnet, and the Al2SiO5 polymorphs – kyanite, sillimanite, andalusite). Muscovite Garnet Kyanite 2. Metabasites: Characteristic minerals are chlorite, epidote, amphibole, plagioclase, garnet, serpentine. Al-rich silicates are not common. Chlorite Epidote Serpentine 3. Calc-silicates: Quartz, calcite, and dolomite react together with water to make minerals such as talc, wollastonite, diopside and forsterite. Talc Wollastonite Diopside Fabrics in metamorphic rocks • Metamorphism is very often accompanied by deformation • Deformation forms new fabrics • Different fabrics form depending on the metamorphic grade Development of cleavage This is different to the mineral characteristic cleavage we previously discussed • Sedimentary layers are folded • Clay minerals are re-oriented by applied stress Slate is used for roofing, gravestones .. Schistosity and Gneissosity • At higher metamorphic grades, the fabric develops into schistosity (medium grade) and gneissosity (high grade) Metamorphic rocks without fabrics hornfels Not all metamorphic rocks have fabrics • Contact metamorphism not accompanied by deformation – contact metamorphic rocks are called hornfels • Some rocks do not contain any micas to align – Sandstone becomes quartzite quartzite – Limestone becomes marble hornfels texture marble Starting to feel like a geologist? Check out: https://www.youtube.com/watch?v=1NU51lJIdrg
