- Created by: Andrew Murray
- Created on: 24-05-11 20:51
2.1.1 The General Rock Cycle
- Extrusion- Emission of Magma on the Earth's surface where it forms a lava flow.
- Weathering- The breakdown of rocks in situ
- Erosion- The removal of weatheredmaterial, usually due to the physical action of transported fragment
- Transport- The meeans by which weathered material is removed, by water, wind, ice or gravity.
- Deposition- The laying down of sediment that occurs when a transporting agent loses energy.
- Burial- occurs when sefiment is covered by younger layers of sediment accumulating on top of it.
- Diagenesis- All processes that take place in sediments at low temperature and pressure
- Recrystallisation- The solid state process that changes minerals into new crystalline metamorphic minerals
- Metamorphism- The changing of rocks in the Earth's crust by heat and/or pressure and/ or volatile content. It is isochemical and occurs in the solid state.
- Partial Melting- The incomplete melting of the rock in the lower crust or upper mantle
- Magma accumulation ia magma collecting within a magma chamber
- Crystallisation occurs during the cooling of magma or lava so that solid mineral crystals form.
- Uplift- The return of buried rock to the surface by tectonic forces
2.1.2 Rock Forming Minerals
Minerals have many different characteristics to be identified-
- Habit- The shape of some mineral crystals i.e. Halite, cube shaped
- Colour- Not very reliable as some minerals can occur in different colours
- Hardness- Moh's Scale of Hardness can be used
- Cleavage and fracture- Cleavage planes are planes of weakness due to their atomic structure can be termed perfect, good or porr depending on how easily it splits.
- Lustre- Ability to reflect light, most have a vitreous lustre, like glass.
- Specific Gravity- The ratio of the mass of a mineral compard with the mass of water.
- Reaction with acid- Use dilute HCl to distinguish Calcite
- A rock- is an aggregate or mixture of one or more minerals
- Mineral- is a naturally occurring inorganic crystalline compound witha definite chemical composition.
2.1.4 The Geological Column
- The geological column is split into era ( a major unit of time that consists of several periods/systems). These are then divided further into systems (refers to rocks laid down in a named time period, shorter than a era)
There are a few methods of relative dating:
- Law of superposition- All sedimentary layers are deposited one on top of the other
- Law of original horizontality- All layers must be originally nearly horizontal.
- Law of cross cutting relationships- State that any sedimentary rock that has included fragments of another rock must be younger than the fragments
- Distinctive fossils of a particular period can also be used
Absolute dating involves looking at radiometric dating using radioactive decay.
2.2.1 Igneous Rocks
Igneous Rocks cool from magma (Molten rock), lava on the other hand is molten rock cooled at the surface. You can classify rocks in a number of different ways
2.2.1 Igneous Rock Classification
- Crystal Grain size- Fine grained (<1mm), Medium (1-5mm), Coarse (>5mm)
- Silica percentage- can be used also, but the SiO2 percentage can be linked with colour the Silicic rocks are rich in quartz and feldspars giving a lighter colour, but Mafic minerals like olivine and augite can give a darker appearance
- Silicic rocks (More than 66%)
- Intermediate rocks (66-52%)
- Mafic (52-42%)
- Ultramafic (<45%)
Felsic minerals are rich in Feldspar and Silica, therefore the minerals are feldspar and quartz
Mafic minerals are rich in Magnesium and Ferric (Iron)
2.2.1 Igneous minerals (FELSIC MINERALS)
- Hardness 7
- White/Grey/transparent colour
- No cleavage
- Glassy/vitreous lustre
- Composition SiO2
Potassium Feldspar (Orthoclase):
- Hardness 6
- Pink colour
- Good cleavage in two directions
- Vitreous lustre/glassy
- Grey colour
2.2.1 Igneous Mineral (MAFIC MINERALS)
Micas, Biotite & Muscovite
- Hardness 2.5 and colour isBlack for biotite; Silvery for Muscovite
- One perfect cleavage (makes flakes)
- Hardness 5.5 and Black colour
- Two Cleavages at 120 degrees
- Hardness 5.5 and Colour is black to dark green
- Two cleavages at 90 degrees form 8 sided crystals
- Hardness 6.5 and colour is light green
- No cleavage
2.2.2 Rates of Cooling
The deeper the intrusion, the slower it cools, the slower something cools the longer it has to grow crystals.
Small crystals- Short Cooling time
Crystal grain size can then be linked to depth of intrusion
- Glassy- No crystals therefore very quick cooling rate (hours) at the surface always volcanic extrusive products
- Fine- Crystals <1mm forms due to rapid cooling (weeks/months) at the surface in extrusive volcanic rocks and in chilled margins in minor intrusions.
- Medium- 1-5mm due to fairly slow cooling over thousands of years, found below the surface in minor intrustions about 1km down (hyperbyssal depths)
- Coarse- Crystals >5mm cooled over millions of years, found in major intrusions at great depths >10km, as batholiths or (plutonic rocks)
- Pegamites can produce large crystals in a short geological period of time and form in te last stages of crystallisations.
2.2.3- Igneous Textures
- Equigranular texture- All of the crystals are of equal size, the size of which does not matter, REMEMBER the longer the cooling time the larger the crystal.
- Vesicular texture- Gas bubbles become trapped in the lava as it rapidly cools leaving holes where the gas was present. Vesicles are usually found to the top of the lava flow, usually found in basalt and pumice
- Amygdaloidal texture is where vesicles are later infilled with mineral deposits from groundwater percolating into the vesicles.
- Flowbanding- Very common with rhyolite as layers of dark and light minerals form due to speration in a silicic lava flow, it was viscous and so flowed very slowly.
- Porphyritic- Has two main stages of cooling resulting in large phenocrysts surrounded by a fine crystal matrix/groundmass.
2.2.4 Identifying Silicic, intermediate and Mafic
Silicic igneous rocks
- Obsidian-Glassy with a glassy texture
- Pumice- Glassy shards and fine crystals, vesicular texture
- Rhyolite- Fine Crystals, and usually flow banded
- Granite and Granodiorite- Coarse and can be porphyritic or equigranular.
Intermediate igneous rocks
- Andesite - Fine and can have vesicular, amygdaloidal, porphyritic or equigranular texture
- Diorite- Coarse and can have a equigranular or porphyritic texture
- Basalt- Fine and can have vesicular, amygdaloidal, porphyritic or equigranular texture
- Dolerite- Medium and can be porphyritic or equigranular
- Gabbro- Coarse and Equigranular
2.2.7 Magma differentiation
Bowen's reaction series, there are two series in which mineral crystalise out of magma. Below is a diagram and an explanation follows:
2.2.7 Differentiation of Magmas Part 2
- In the discontinuous series all of the minerals are mafic with olivine to form first. As the temperature lowers other minerals form as the olivine reacts with the magma to form pyroxene, if there is enough silica present, if it erupted quite early on the olivine will be preserved.
- In the continuous reaction series, Ca rich plagioclase will form first at high temperatures and sodium rich at low temperatures but with intermediate compositions of the two minerals in between the two as the plagioclase is being continually reacted. Sometimes individual crystals show zoning with Ca rich in the centre and Na rich towards the edge.
- The two reaction series converge at a low temperature, minerals remain that will not react with the remaining liquid and these final group of minerals are felsic and are rich in silica.
2.2.7 Magmatic differentiation
- As Olivine and Pyroxene form at high temperatures, they use iron and magnesium from the magma. The high temperature plagioclase crystals are rich in calcium. The remaining liquid becomes depleted in iron, magnesium and calcium and so the melt changes in overall composition.
- Early formed minerals with a higher density that the surrounding minerals like olivine sink to form a layer at the base of the intrusion
- Gravity settling removes the crystals from the remaining liquid so they do not react with the remaining magma
- Due to the weight of the overlying crystals during the crystallisation of magam as the liquid gets squeezed out forming a seperate layer, depleted in the elements incorporated in the early crystals
2.2.9 Intrusive igneous rock
- Sills are usually concordant, sheet-like intrusions and parallel to the beds, but some occasionally cut across the beds transgressive sills
- Dykes are sheet-like intrusions that are discordant and often vertical , they form straight, long, thin features and are often intruded along zones of weakness in country rock.
- Batholiths is an example of a major intrusion (occurs at plutonic depths), the outcrop is usually very large and are normally composed of granite.
- Granite magmas form from the partial melting of the continental crust as xenoliths of the country rock surrounding the magma is assimilated into the magma as they gradually melt to become more magma.
2.2.10 Differences between intrusive and extrusive
- Chilled margin- is where the igneous rock has cooled rapidly and so has fine crystals
- Baked margin- in the country rock where it has been heated by the intrusion and altered, the rock is recrystallised (Contact metamorphism)
Differences between sills and lava flows
- Sills have two chilled margins, whereas the beds are deposited above after the lava flow has cooled and so has one margin.
- Sills have two baked margin, whereas lava flows have one.
- Xenoliths from above and below the sill, may have some from below in a lava flow
- May show differentiation of magma in a sill, whereas lava flows will have flowbanding or crystals aligned with the direction of flow.
- Dolerite is the most common rock in sills, with basalt in lava flows
- Vesicles are common in lava flows but rare in sills.
- Lava flow may have clasts in the beds above due to erosion and deposition
- Lava flows have a irregular upper surface, which is regular with sills.
2.2.11 Volcanic products
- Volcanic Gases- The main volcanic gas is water vapour 70% but the other gases can combine with hydrogen and oxygen to form toxic compounds such as HCl and HF, therefore acid rain can form as a result.
- Mafic lava flows have a low viscosity and so flows of basalt are widespread and thin, they can cool to form aa lava flows with a rough blocky or jagged surface; or they can form pahoehoe flows with a smooth or ropy surface. The pahoehoe flows are less viscous but as it cools it can become an aa flow. Pillow lavas form underwater.
- Intermediate and silicic lavas, tend to be very sticky and so can form lava domes within the crater as they tend to be thick and confide to a small area
Pyroclastic material can consist of:
- Ash, less than 2mm that can form tuff when consolidated
- Lapilli are particles between 2mm and 64mm and form lapilli tuff.
- Bombs and blocks are the coarsest and form pyroclastic agglomerate.
- Nuee ardentes are the gaseous magma that forms the fast moving pyroclastic flow that cool to form ignimbrite
2.2.13 Volcanic eruptions of Mafic magma
- A shield volcano is characterised by gentle slopes of less than 10 degress, composed of thin basalt lava and so dow not build a high thickness, very littel pyroclastic material is found near a shield volcano.
- Fissure eruptions, can erupt large quantities of fluid lava that spread over a large area, these can build up to form plateaus.
- Submarine eruptions are when lava is erupted underwater and cools very rapid and pressure of more lava behind it causes it to break forming pillow shaped lava.
- Basaltic lava has been described as being effusive ( non-explosive, fluid)
Volcanic Explosivity Index (VEI)
- It is an index of increasing explosivity, each number is an increase by a factor 10, and ranges from 0 to 8.
2.2.14 Explosive Volcanoes
- Hawaiian eruptions- have large amounts of fluid basaltic magma from which gases escape, few pyroclasts
- Strombolian eruption- More explosive less fluid basalt and andesitr, regular explosions of gas and pyroclasts
- Vulcanian eruptions- violet with viscous andesitic lava and large quantities of pyroclasts
- Plinian eruptions- extremely explosive with viscous gas filled andesite and rhyolite and huge amounts of pyroclatic material blasted out.
Strato-volcanoes are the most common volcanoes on the earth surface and have a conical shape due to a silicic and viscous lava that does not flow and so builds up to form steep sided volcanoes.
Calderas form when nthe magma chamber starts to become empty underneath a volcanoe and becomes weakened as a result and so may collapse compressing what is left forming some final more violent eruptions, finally the entire cone will collapse and can in fact cause tsunamis as a result.
2.2.15 Prediction of earthquakes
- Historic pattern of eruption can be used to predict the time frame in which the next eruption could occur
- Ground deformation as swelling of a volcano suggests the accumulation and movement of magma upwards under the volcano
- Gas emissions can occur as the pressure decreases due to magma being close to the surface, gases escape as a result
- Changes in groundwater as gas pressure increasing will cause water levels to rise and suddenly drop right before an eruption.
- Seismicity as earthquakes are common due to increased gas pressure in a magma chamber and fracturing of brittle rock as the magma forces its way upwards.
- Carbonation, the reaction between carbonic acid and minerals, carbonic acid is formed as carbon dioxide gas reacts with rain water, this method is important with the weathering of limestone
- Hydrolysis, the reaction between water and silicate minerals which causes the minerals to decompose.
- Exfoliation occurs when sheets of rock split off this is beacuse different minerals expand and contract by differnet amounts and causes the rock to disintergrate
- Frost shattering, water enters cracks and joints and if the temperature fluctuates around 0 degrees the water will expand and contract causes a shattering action
- Pressure release, when pressure is released by rocks due to erosion they can expand forming fractures.
Weathered material can be carried by gravity, wind, rivers, the sea, ice. But erosion can form by either abrasion by the wind ice or water as it hurled over or at the ground. Attrition is the wearing down of sediment by collisions with other sediment types.
- Solution, the transport of ions dissolved in the water
- Suspension, the transport of material without touching the ground
- Saltation, the transport of material by bouncing
- Traction- Sliding or rolling of material along a surface
Grains can be descrubed by there roundness and there shape, a grain can either be rounded or angular or sub rounded or sub angular. Shape can have a high or low sphericity.
Grain size is measure by the phi scale with 0 phi representing 1mm and increasing the phi number to 1 goes down by a half i.e. 2 phi = 0.25 mm
2.3.3 Grain size analysis
Coefficient of sorting = (84-16)/2 <0.5 well sorted 0.5-1.0 moderately sorted >1 poorly sorted
Sorting- The degree to which particles are the same size
- Dune Sane, Fine-medium sand, all quartz, well rounded, very well sorted
- Glacial till, Varying grain size, Varied composition, angular/sub-angular, very poorly sorted
- River, Coarse to fine sand, quartz and rock fragments, angular near to the source and poorly sorted
- Beach or offshore bar, medium sand sometime coase, neary all quartz, sub-rounded to rounded, moderately sorted.
- Gravity, Varying grain size, Varied composition, angular to very angular, very poorly sorted.
2.3.5 Clastic Sedimentary Rocks
Breccia- Coarse grained (>2mm) clastic rock, angular clasts meaning short transport, coarse clasts are mixed within a fine matrix. Poorly sorted meaning the transport medium lost energy quickly
Conglomerate- Coarse grained clastic rock, that is rounded meaning longer transport time, clasts are held by a finer grained matrix and be poorly sorted as a result.
Orthoquartzite- Sandstone (0.0625mm-2mm), consists of only quartz grains in a quartz cement and are well sorted an rounded indicating a long transport period.
Desert Sandstone- Red in colour due to iron oxide surrounding the quartz grains as they are very well sorted and very well rounded as it is made up of quartz with an iron oxide cement.
Arkose- Medium/coarse graines with 25% K feldspar and quartz, arkose is pink in colour and moderately sorted and will be sub-angular/rounded.
Greywacke- Fine to coarse, dark coloured and poorly sorted with angular to sub-angular clasts that mainly consist of rock fragments and K feldspar held in a clay matrix.
2.3.6 Fine grained Clastic and non-clastic rocks
Mudstone- Dark grey and very fine grained, they contain clay mineral, mica and quartz, minerals do not alighn and so mudstones are not layered
Clay- Very fine grained and contain 40% of water, clay can have varying colours depending on the carbon and iron content, it forms layers with distinct bedding
Shale- Dark coloured, fine grained clastic rock that has distinctive layers due to mineral alignment, the clay minerals are flat and platy and so they align parallel to the beds.
Oolitic limestone- Chemically formed containing sub-spherical sand grains that act as a nucleus and concentric layers of calcium carbonate form around it.
Fossiliferous limestones- Biologically formed from fossils and are bioclastic thy have micrite matrix, and can be formed from bivalves.
Chalk- is biologically formed from coccolitihs, the chalk is white due to pure calcium
2.3.7 Sedimentary Structures
Cross Bedding- Sand grains flow in one direction and so can avalanche down slopes and so can form cross bedding that can be interupted due to erosion
Graded Bedding- The largest and heaviest particles are on the bottom of the bed with the finer particles at the top, so can be used as a way up structures.
Imbricate structures form as pebbles lean in the same direction as the current as they pile up against each other
Salt pseudomorphs form as halite crystals grow on the surface of a bed due to supersaline conditions, but then become dissolved again when more water arrives and so sediment then infills the mould left by the crystal.
Ripple marks- Form as sand frains are transported by saltation and can form symmetrical and asymetrical ripple marks in the surface as they have pointed troughs.
Dessication cracks- form in clay rich soil as evaporation causes the mud to crack to for V-shaped cracks and are then later infilled by sediment.
Flute casts- Is when turbidity currents erode into the soft muf and are then later infilled by sediment
Graded bedding, cross bedding, ripple marks and desiccation cracks can be used as way-up indicators.
2.3.10 Glacial environments
- When ice melts it deposits poorly sorted material in the form of boulder clay (till) which may become lithified to form tillite. It is poorly sorted, angular fragments, large clasts which are randomnly orientated, clays may be striated, may contain erratics.
- Fluvio-glacial depostist are sediments that have been transported by glacial melt water streams before being deposited in the outwash plains, and may show cross bedding and sometimes graded bedding in the braided streams
- Varves, when glacier ice melt lakes are formed and these transport fine sand, silt and clay, these settel to the floor of the lake and when it freeses fine grained laminated sediments called varves form in which the layers deposit themselves, these can be counted each one representing a year.
- Varves are therefore fine grained banded deposits from glacial lakes containing coarser pale material in the summer and finer dark material in the winter.
2.3.11 Fluvial environments
- alluvial fan breccias, conglomerates and arkose forms wher mountain streams flow onto a flat valley floor, reduction in velocity and energy, large volumes of sediment are deposited to form alluvial fans. Breccias form from angular and coarse scree fragments, conglomerates are compsed of rounded pebbles, arkose is formed from sand.
- Meandering streams flow in channels with curves the bank is eroded and deposited on the inside so you get upward fining
- Food plain clays and silts form at times of high flow where rivers flood the surrounding area forming a flood pain where clays are deposited and may contain fossil plants. Siltstone is also common on the flood plain with small scale cross bedding and the mudstones may form desiccation cracks and therefore the development of soils.