Introduction features such as rivers, lakes and

IntroductionThe crust is one of the three main concentriclayers which make up the Earth’s interior. It is a very thin layer of solidrock which forms the outermost shell of the planet that supports livingorganisms as well as natural surface features such as rivers, lakes andmountains. The Crust is significantly thinner than both the core and the mantle(the other two main layers making up the Earth’s interior). As a matter offact, the crust accounts for less than 1 % of the earth’s total volume. Figure 1-Image showing the concentric layers which make up the Earth’sinteriorProperties and compositionThe crust not only varies in thickness withthe other concentric layers but also differs due to its properties and composition.

The solid crust is not only significantly thinner when compared to the otherlayers but is also less dense and less hot. Due to its solid nature, relativethinness and low density, the crust is brittle and prone to cracking. Moreover,the crust is also not uniform in its thickness with some of its regions beingless thick 1 km thick and other regions being more than 80 km thick. The crust consists of mixture of chemical elements,minerals and rock types. The most abundant elements present in the crust areoxygen (47%), silicon (8.

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1%), aluminium (8.1%) and iron (5.0%).

Other elementssuch as calcium, sodium, potassium and magnesium are also present, howeverthese are present in more minute quantities. The elements in the crust areoften found combined with one another to form various compounds. Such compoundsgive rise to minerals.

       Minerals are the building blocks of rock.By definition, minerals are naturally occurring inorganic solids with definitechemical compositions and well-ordered internal structures. Minerals aregenerally composed of 2 or more elements.

The crust is made up of over 2000different minerals. However, many of these are present in very smallquantities. As a matter of fact, the crust is mainly composed of 6 minerals whichare feldspar (50%) , quartz (12%) , pyroxene (11%), amphibole (5%)  mica (5%)  and olivine (3%)Feldspar is the most abundant mineralpresent in the crust. It consists of silicon, oxygen and other metallicelements such as sodium, potassium, calcium and aluminium. Feldspar may existin different varieties according to which metal element is present. Two maintypes of feldspar exist which are known as plagioclase (39%) and alkalifeldspar (12%).

This mineral has a light cream to salmon pink colour. Thesecond most abundant mineral is quartz. Quartz is one of the primary componentsof granite and sand. It is a hard, water insoluble mineral which mainlyconsists of silica (SiO2). Quartz is usually colourless or white.

Otherminerals such as pyroxene, amphibole ,mica and olivine are also present howeverin smaller abundances. Figure 3- Pie chart showing the composition of the Earths’ crust withrespect to mineralsThe minerals present in the crust (mainly feldspar and quartz) mixtogether to form different rock types. Rocks can be classified into threegroups which are igneous rocks, sedimentary rocks and metamorphic rocks.Igneous rock is the most abundant rock type found in the Earth’s crust.

Rocks of this type form when molten rock such as magma or lava cools andsolidifies. Igneous rocks are sometimes called the ‘primary rocks’ or the’parents of all rocks’ since they formed the Earth’s first crust and gave riseto all other types of rock. Igneous rocks can be classified as being eitherintrusive or extrusive based on the mode of formation and occurrence. Intrusiverocks are those rocks which form when magma solidifies beneath the earth’ssurface. Some examples of this type of rock include granite, diorite andgabbro.

Extrusive rocks on the other hand refer to rocks which form when lavacools on the earth’s surface. Such rocks include basalt, andesite and rhyolite.Sedimentary rock forms from the accumulation of sediment and organicmatter. Sediments come from weathered or eroded previously existing igneous ormetamorphic rock.

When sediments accumulate, increased pressure causes thesediments to compress and form sedimentary rocks. This process is known asLithification. Sedimentary rocks are found mainly in the upper parts of thecrust since such types of rock are not very stable under high temperatures andhigh pressure.  Some examples ofsedimentary rock are shale, sandstone and limestone.Metamorphic rock refers to rock which forms when igneous or sedimentaryrocks undergo changes in their structure due to high pressure and hightemperatures. The influence of heat and pressure causes recrystallization andreorganisation of molecules within original rocks which results in overallchanges in the rocks’ hardness and colour. This process is known asmetamorphism.

This explains why sedimentary rock is not very stable in lowerparts of the crust. Examples of metamorphic rock include marble (originatingfrom limestone), quartzite (originating from sandstone) and blueschist (originatingfrom basalt).Differenttypes of crustThe crust can be divided into two types – continental crust and oceaniccrust. In general continental crust is the part of the crust which gives riseto continents whereas oceanic crust is the part of the crust which underliesthe earth’s oceans.

These types of crust differ from one another in terms ofthickness and density and composition.Continental crust covers approximately 40%of the planet. This type of crust is mostly exposed to the air. It is older andthicker than oceanic crust. In fact, continental crust is around 2 billionyears old and is on average 35-40 km thick. The rocks of continental crust are sometimesreferred to as ‘Sial’ by some geologists.

This is due to the fact thatcontinental crust is mainly composed of granite which has silica (SiO2) andalumina (AlO3) as its most abundant chemicals. Contitnental crust has a higheramount of oxygen when compared to oceanic crust. This is due to continentalcrust being more exposed to the atmosphere. Due to the chemical composition ofgranitic rocks, continental crust has a relatively low density when compared tooceanic crust. In fact continental crust has an average density of 2.7-3.0 g/cm3.In terms of its minerals continental crust can be described as being felsic sincethe most abundant minerals in granite are feldspar and quartz whereas mineralssuch as amphibole , pyroxene and olivine are only present in trace amounts.

 Oceanic crust covers approximately 60% ofthe planet. This type of crust is thin and relatively young. It is no more thanabout 20 km thick and is on average 7-10km thick.  Moreover, it no older than180 million years (approximately). Old oceanic crust is destroyed atsubduction zones.

Oceanic crust is formed at mid-ocean ridges as a result ofthe process of seafloor spreading whereby plates are pulled apart. This causespressure in the underlying mantle to be released. Such pressure causes part ofthe peridotite(igneous rock present in the mantle) to melt. The melted peridotite gives riseto basaltic lava, which rises, cools, solidifies and forms new oceanic crust.Oceanic crust is denser than continental crust since it is mainly composed ofbasalt.

This type of crust has an average density of 3.0-3.3 g/cm3. Basalticrocks are sometimes called ‘Sima’ by geologists due to the presence of silicaand magnesium. In mineral terms, basalt is considered as being a mafic rock dueto feldspar, amphibole and pyroxene being the most abundant minerals present. The Moho-discontinuityThere exists a boundary between the crustand the upper part of the mantle. This boundary is called theMoho-discontinuity and it was named after Andrija Mohorovicic , theseismologist who discovered it. Mohorivicic discovered a sharp discontinuitywhereby the velocities of P-waves and S-waves increased in an abrupt fashion.

He understood that there was a relationship between the velocity of seismicwaves and the density of the material the waves are moving through and thus heinterpreted this discontinuity as a change in composition within our planet. Heconcluded that this sharp increase in seismic wave velocity is due to a low densitycrust being present over a high density mantle.Figure 4- Diagram showing the position of the Moho discontinuity The LithosphereThe crust and the rigid top part of themantle together form what is known as the lithosphere. The lithosphere issubdivided into a number of plates. The lithosphere is made up of 7 majorplates and several minor plates. These plates lie on top of the asthenosphere(the mantle’s softer and less rigid layer).

The plates move as a result ofconvection currents generated when magma rises and sinks in this part of the mantle.The movement of plates is responsible for both the creation and destruction ofcrust in addition to numerous volcanic and seismic activity.Figure 5- Map showing the major and some of the minor plates which make upthe lithosphere. Figure 6- Diagram showing the convection currents present in theasthenosphere.

Plate BoundariesAdjacent plates can interact with eachother in different ways giving rise to different plate boundaries. Plateboundaries are classified into three types; divergent boundaries, convergentboundaries or transform faults. The type of boundary depends on the directionin which the adjacent plates are moving.With divergent boundaries two plates movein opposite directions away from each other. When two adjacent oceanic platesmove apart through the process of sea floor spreading, the magma underneath theplates upwells, cools and solidifies to give rise to new crust.

This type ofboundary is found at mid ocean ridges such as the Mid-Atlantic Ridge (boundarybetween the North American plate and the Eurasian plate) The formation ofshield volcanoes or volcanic islands such as Iceland is common at this type ofboundary. Figure 7- Diagram showing a divergent plate boundary with oceanic crust.When continental plates move apart, thetensional forces generated when the plates get pulled apart causes cracks andfaults to appear within the crust. As the plates continue to pull apart fromeach other, rock present between faults sinks downwards to give what is knownas a rift valley. An example of a rift valley is the East African rift valleyin Kenya, Africa.Figure 8- Diagram showing the formation of a rift valley from divergent plateboundaries.Convergent boundaries involve the movementof two plates against each other.

When oceanic crust converges with continentalcrust, the thicker , denser oceanic crust sinks downwards beneath the thinner ,lessdense continental crust into the mantle via subduction to form a subductionzone. As the crust subducts, heat, pressure and friction cause the crust tomelt into magma. This magma rises up through cracks in the crust and can giverise to composite volcanoes. Volcanic and earthquake activity is common at thistype of boundary. An example of this type of boundary is the subduction of thePacific plate as it converges with the Eurasian plate.Figure 9- Diagram showing subduction with convergent plate boundaries.When two continental plates converge intoone another, neither plate sinks downwards due to both plates having similardensities. As a result, when the plates push into each other, they pushoverlying sediment upwards to form fold mountains.

Magma can also rise throughcracks present between the plates to give rise to volcanoes. Examples of fold mountainsinclude the Andes mountain range and the Himalayas. Figure 10- Diagram showing the formation of fold mountains with convergentplate boundaries.Transform faults are a type of plateboundary whereby crust is neither created nor destroyed.

With this type ofboundary two plates slide past each other in opposite directions. When twoplates slide past each other, friction causes the two plates to get stuck. Whenthis occurs, pressure builds up between the plates . Eventually the pressure isreleased in the form of an earthquake. The San Andreas Fault is an example ofthis type of boundary.Figure 11- Diagram showing transform fault plate boundaries ConclusionIn spite of the crust occupying less than1% of the earth’s total volume, it serves an important role in supporting allliving organisms. The crust exists in two different types, both of which havingdifferent chemical compositions and properties.

The crust also forms part ofthe lithosphere and gives rise to plates and tectonic activity such asearthquakes and volcanoes.


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