EARTH'S CRUSTAL STRUCTURE

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(1)EARTH'S CRUSTAL STRUCTURE:

Earth's surface is divided into two categories:

  • Continents
  • Ocean

∙Earth's crust beneath the continents comprises three layers-

(1) Uppermost Layer : Sedimentary rocks
  • Seismic Velocity : 1.8 to 5 Km/s
  • Density : 2.2gm/cc (Sedimentary rocks)
  • Thickness of layer : 10Km

(2) Middle Layer : Made up of Granite and Metamorphic rocks.
  • Seismic velocity : 5 to 6 Km/s
  • Density : 2.4 to 2.6 gm/cc
  • Thickness of layer : 15 to 20 Km

(3) Lowermost Layer : Basaltic Layer; made up of basic and ultrabasic rocks, such as basalt and gabbro(Igneous Rocks).

  • Seismic velocity : 6 to 7.6 Km/s
  • Density : 2.8 to 3.3 gm/cc
  • Thickness of layer : 35Km and 50 to 70Km for young mountains like Himalaya.

∙Structure of the earth's crust is very different beneath the oceanic floor.
  • Thickness : 5 Km
  • Predominantly made up of basaltic layer.

(2) TECTONIC ELEMENTS OF CONTINENTS:

  • A major part of the continents is made up of plains that are only a few tens of meters above the mean sea level.
  • The peneplanation (Featureless plain, place where no more erosion could occur) on the continents is brought about by a long period of erosion that is governed mainly by exogenic process(processes that take place at or near earth's surface).

∙These plains are interrupted by a series of : Mountain Chains and Plateau Mountains

(1) Mountain Chains:
  • These are narrow belts of land that were raised above the average heights of the continental plains during tectonic events related to orogeny.
  • These are known as folded mountains since they are made up of folded rocks.
  • Example : Younger mountains- Himalaya and older mountains - Aravalli, have undergone denudations(erosion leading to reduction in elevation) and have been reduced to lesser heights.

(2) Plateau Mountains:
  • These are uplifted blocks of the continent that are made up of horizontally bedded or gently dipping rocks.






On the basis of geomorphology, structure and tectonics, the continental area can be broadly divided into two types:

(1) CRATONS : 
  • Relatively stable area of the earth's surface, which have not experienced orogenic type tectonic activity since the close of Precambrian era.



Based on two structural levels:

(1) Shield area :
  • Older unit occupying the lower structural level is made up of strongly deformed rocks.
  • Term shield signifies hardened and stable rock complexes.
  • Generally occupy the central part of craton.
  • Made up of Archean and lower Proterozoic sequence.
  • Shield areas constitute the Precambrian Basement for the younger sequence of the platform cover. 
(2) Platform cover :
  • Overlying younger sequence of shield area that show very little deformation.
  • Made up of Upper Proterozoic and Phanerozoic sequences.
  • Occupying both the central and peripheral part of the craton.

∙The shield areas of various cratons seem to have evolved in four stages :

(1) 'older' greenstone belts (remnants of primordial crust) - Earliest
(2) Emplacement of granitic gneiss and migmatites.
(3) Elongate and relatively narrow troughs over granitic gneiss and migmatites; such sedimentary basins are known as 'Avlacogens'.
Avlacogens then were filled by Volcano-sedimentary sequences which are also referred to as 'newer greenstone belts.
(4) Younger sequences were intensely deformed and metamorphosed and intruded by granitic plutons.




∙Syneclise : 
  • Sedimentary basins that covered large areas of similar widths and lengths and that remained tectonically active for long period of geologic time (slow subsidence)
  • Syneclise subsequently uplifted to form plateau mountains, such as Vindhyachal and Sahyadri mountains of indian peninsula.

∙Graben : Graben constituents the second group of sedimentary basins of platform cover, which were narrow elongate sedimentary basins bounded by a pair of normal faults.


(2) Folded Mountain Belts : 
  • Products of orogenic events of Phanerozoic Eon.
  • FMB are considered to be the end product of the evolution of geosyncline.
-Geosyncline : Sedimentary basin that is characterized by a faster pace of sedimentation as compared to other basins.

Evolution of geosyncline consists of three phases :

(1) Geosyncline Phase : Prolonged phase of sedimentation(phase of subsidence).

(2) Orogenic Phase : Intense deformation, metamorphism and emplacement of granitic plutons in the deeper levels of sedimentary column(Phase of uplift).
∙Orogenic phase that is also known as inversion phase give rise to a foredeep(a kind of sedimentary basin in front of rising mountains).

(3) Post orogenic phase : Post orogenic phase of folding, faulting and uplift.


∙The continent of Europe is characterized by the presence of three groups of Phanerozoic mountain belts.
  • Caledonian = Early Paleozoic Time
  • Hercynian = close of Paleozoic Era
  • Alpine = Tertiary Period

∙The Tertiary mountain chains constitute the loftiest(Highest) mountains of the world.
  • Distributed in two groups:
  • (1) Circum-Pacific Belt : Bounded on one side by Oceanic crust of pacific ocean.
  • (2) Mediterranean Belt : Alpine - Himalayan chains (central part of Europe and Asia)




TECTONIC ELEMENTS OF OCEAN:

  • The circum-pacific belts of oceanic trenches and volcanic island arcs constitute the "Ring of Fire" of the pacific zone.
  • The belts are also marked by active earthquake zone(Benioff Zone) that is related to the subduction of the lithospheric plates.

Abyssal plains : Large, flat area of deep ocean floor.
∙Vast areas of the abyssal plains of the oceanic floor are largely made up of volcanic and sub-volcanic rocks.

Four major mid-oceanic ridges are - 
  • Indian Ocean Ridge
  • Carlsberg Ridge
  • Mid-Atlantic Ridge
  • East-Pacific Ridge


THANKS FOR READING

-Ichbinaasif
 


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