Lecture4

Download
Share  Add to  Flag Embed
Uploaded from authorPOINTLite
Views:
 
Category: Entertainment
     
 
License:   All Rights Reserved

Presentation Description

No description available.

Comments

By: ProudRVian (8 month(s) ago)

Send to my email: lemonadecat@gmail.com Thanks

By: ProudRVian (8 month(s) ago)

I like these slides, they're more detailed than the others! :D May i download them please

Presentation Transcript

ROCKS: 

ROCKS

Slide2: 

The study of the Earth’s rocks is called PETROLOGY Rocks are aggregates of minerals

Slide3: 

Three main types of rocks: Igneous Rocks: formed from the direct cooling and solidification of molten material Sedimentary rocks: formed near the Earth’s surface by the deposition and lithification of sediments Metamorphic rocks: a rock of any origin is physically transformed by being subjected to high pressures and temperatures.

Slide4: 

Igneous and metamorphic rocks make up 95% by volume of all of the Earth’s crust. However, sedimentary rocks cover most of the Earth’s surface. Much of the crust consists of a large thickness of igneous rock, covered by a relatively thin layer of sedimentary rock.

IGNEOUS ROCKS: 

IGNEOUS ROCKS Molten silicate material underneath Earth’s surface is called magma. Magma reaching the surface in liquid form is called lava. Igneous rocks may form either by eruption of lava at the surface (volcanoes) to form EXTRUSIVE igneous rocks, or by solidification of magma below the surface to form INTRUSIVE igneous rocks

Types of Igneous Rocks: 

Types of Igneous Rocks Igneous rocks may be classified in terms of composition (the minerals they contain) or texture. Texture refers to the size of the mineral grains (crystals) in the rock. Slow cooling  large crystals (coarse-grained) Rapid cooling  small crystals (fine-grained) Very rapid cooling  amorphous, or glassy

Slide7: 

Extrusive igneous rock Intrusive igneous rock Magma chamber

Texture types: 

Texture types Phaneritic : all the crystals are large enough to be seen with the naked eye Aphanitic: no crystals are visible to the naked eye Porphyritic: The bulk of the rock is fine-grained, but contains some large crystals, called phenocrysts, visible by eye. This rock results from 2-stage cooling – magma begins to cool slowly within the crust, forming large crystals. The rock then erupts to the surface and cools rapidly.

Slide9: 

Granite- a phaneritic rock, containing crystals of feldspar, quartz, biotite

Slide10: 

Basalt – an aphanitic rock Obsidian – a glassy rock Porphyritic Rock

Slide12: 

Note that crustal rocks e.g. granite, rhyolite are rich in silica (felsic), while further down into the earth, the silica content decreases (mafic, ultramafic) e.g. olivine

Differentiation of igneous rocks: 

Differentiation of igneous rocks The range of compositions of igneous rocks is very vast. The type of minerals that crytallise from a magma depends on the chemical composition of the magma. The crystal size depends on the cooling rate. As a magma cools (say from 1500oC), the minerals with the highest melting point crystallize out first and form a solid at the bottom of the chamber. Progressively , more minerals crystallise out, at the same time changing the composition of the remaining magma. The first mineral to crystallise out is olivine. This process is called fractional crystallisation

Igneous Intrusions: 

Igneous Intrusions

Sedimentary Rocks: 

Sedimentary Rocks Most sedimentary rocks are formed from fragmented rocks or minerals of any type. Sediments always form at, or near, the earth’s surface The rocks from which sediments originate are called source rock types. This influences the mineralogy of the final sedimentary rock. Other rocks originate by accumulation of fragments of organic material (eg shells of marine animals) or by chemical precipitation from sea water or other solutions.

Sedimentary systems: 

Sedimentary systems Source Rock Type Weathering, Erosion Transport Mechanism Site of deposition Organic material Chemical Precipitation

Types of Sediment: 

Types of Sediment Detrital (or Clastic) Sediments: formed from loose grains produced by weathering of pre-existing rock Biochemical Sediments: composed of material of organic origin, such as shell and coral fragments Chemical Sediments: made up of inorganic materials such as salt, precipitated from solution.

Slide18: 

Weathering may be mechanical (breaking down action of temperature changes, wind, plant roots, “sand blasting” etc) Or chemical eg solution of limestone in rain water. Transport of fragments may take place by wind, water (rivers) or gravity. Deposition of sediments, usually on a seabed, initially takes place in horizontal layers, called strata

Slide19: 

An example of sedimentary strata (layers). The Maltese islands are composed entirely of a sedimentary sequence of clastic and biochemical rocks.

1. Clastic Rocks : 

1. Clastic Rocks Clastic sedimentary rocks are classified according to texture (size of grains they contain). Weathered mountain material is carried down by rivers. Large boulders stop rolling earlier on, while smaller stones and particles get transported progressively further. As these fragments get carried along, they rub together and become more rounded. A long way from the mountains, stream sediment is mostly sand-sized.

Clastic Sedimentary Rocks: 

Clastic Sedimentary Rocks

Slide22: 

conglomerate breccia sandstone mudstone

Mineral Composition of clastic sediments: 

Mineral Composition of clastic sediments The minerals present in a sedimentary rock are ultimately related to the source rock type. E.g. We cannot have quartz sand on Maltese beaches because our sand is derived from limestone! As sediment progresses downstream, more soluble minerals get washed away, e.g. olivine. Plagioclase feldspar cleaves easily and breaks into ever smaller fragments. Quartz is very hard and insoluble, therefore survives longest.

2. Chemical Sedimentary Rocks: 

2. Chemical Sedimentary Rocks Sea water is a solution of many salts, esp. sodium chloride NaCl. If the concentration of a dissolved salt becomes too high (e.g. by excessive evaporation) the salts precipitate out of solution and form a layer on the sea or lake bed. Within the sediments on the floor of the Mediterranean basin, we find a layer of salt, about 2km thick. This occurs because the Mediterranean dried up almost completely about 6 million years ago.

3. Biochemical Sediments: 

3. Biochemical Sediments Many marine organisms, such as molluscs, and coral, grow shells or skeletons made of calcium carbonate. Some are made of silica SiO2. When the animals die, the shell and skeleton fragments settle on the sea bed. Limestone is almost pure calcium carbonate. In the deep ocean, microscopic organisms, foraminifera, fall to the sea bed and form a fine “ooze”, like mud. Limestone may contain well-preserved fossils.

Slide26: 

Fossils in limestone, Malta

Slide27: 

GRADED BEDDING: Each layer represents a depositional episode, in which a volume of mixed sediment is carried. Of this volume, large fragments settle first to the bottom, whereas finer particles are laid down more slowly.

Slide28: 

Loosely packed grains. Spaces filled with water Compaction. Water squeezed out from between grains Cementation. Increased pressure at contact points causes some minerals to melt, acting as a “cement”. Most cements are silica and calcite. Increasing Pressure LITHIFICATION

Metamorphic Rocks: 

Metamorphic Rocks Metamorphism is the process by which rocks are subjected to heat, pressure and chemical reaction and thereby transformed. The transformation may involve texture, mineralogy or even chemical composition. The high temperatures and pressures required for metamorphism are usually generated in processes of plate tectonics (later lectures), e.g in regions of mountain building or subduction. Metamorphism may also occur by burial of sediments to great depths and pressures, e.g. mudmudstoneslate(flaky)schist(micas)gneiss

Types of Metamorphism: 

Types of Metamorphism Regional metamorphism: Generally occurs in regions of mountain building (collision of two plates). The crust is compressed, and parts of it are buried by overthrusting crust and are subjected to high pressures and temperatures. Marble, quartzite, gneiss High Pressure Metamorphism: usually at subduction zones. Subducted plate remains cool, but at high pressure. Mostly basalt  eclogite Contact metamorphism: igneous intrusion heats up the rock surrounding it. High temperature, low pressure.

Slide32: 

Contact Metamorphism