Our Planet

Contents

Internal Structure of the EarthTheory of Continental DriftHypothesis of Sea-floor spreadingTheory of Convection CurrentsTheory of Plate Tectonics

Internal Structure of the Earth

The CrustThe MantleThe Core

Internal structure of the earth

The CrustIt is the outermost and thinnest layer of the lithosphere.Thickness: 13km (ocean) to 50km (mountains)Gravity: 2.5 to 3.4Crustal rocks vary not only in thickness and density, but also in composition.

Ocean Basins: heavier SIMA (2.9)Continents:

• Upper layer: lighter SIAL (2.7)• Lower layer: heavier SIMA (2.9)

The CrustThe boundary of SIAL and SIMA is called “Conard Discontinuity”.The base of the crust is called “Mohorovicic Discontinuity” MOHO.

The MantleBeneath the “MOHO”, there is a 2880km thick intermediate zone called the Mantle.The specific gravity: 3.3 (upper mantle) to 8.0 (bottom)Upper mantle is a “Low-Velocity Zone – (LVZ)”. Magma is found in this layer.LVZ is also called “Asthenosphere”Lithosphere is composed by the crust and the Asthenosphere (upper mantle).

The CoreGenerally, the core is only 15% of the total volume of the earth, but it is 32% of the total mass of the earth.The diameter is 3475 km with very hot (3700oC), dense and under tremendous pressure.It is composed largely by nickel and iron (NIFE)It is divided into “Outer Core” (liquid) and “Inner Core” (solid).The specific gravity of outer core: 12.0 or more.The specific gravity of inner core: 17.0 or more.

Theory of Continental DriftDevelopmentEvidences

The fitness of the edges of continentsEvidences from GlaciationEvidences from GeologyThe matching of rock layersEvidences from PaleontologyEvidences from ClimatologyPalaeomagnetism

DevelopmentIn the 17th and 18th centuries, many scientists commented on the similarity of the shape of the coastlines on either side of the Atlantic.19th century, an Austrian geologist Eduard Suess suggest that Africa, South America, Australia and India were once part of a super-continent (Gondwanaland).In 1910, a German Meteorologist, Alfred Wegener, proposed that at the beginning of the Mesozoic era (200 million years ago), there was a single super-continent which was called “Pangaea”The vast Pangaea began to break up by lateral crustal movement, forming a northern continent (Laurasia) and southern continent (Gondwanaland) spearated by a long narrow ocean known as Tethys.

Continental Drift

EvidencesThe fitness of the edges of continents

The similarity in shape of a contour (500 fathoms) drawn on the continental slope of eastern south America and the same contour drawn on the West African slope is quite striking.

EvidencesEvidence from Glaciation

All four southern continents reveal sign of a period of large-scale glaciation in Carboniferous-Permian (250 million years ago)

EvidencesEvidences from Geology

Similar fauna and flora fossils and geological structures were found in the corresponding continents.Coal fields of Appalachian Mts. Extends to Newfoundland, stop at Atlantic Ocean, but the extension can be found at the East Atlantic edge in Ireland.

Evidences

The matching of rock layerSimilarity of age, time and sequence of rock layers can be found.The Cretaceous and Jurassic sedimentary basins in West Africa and Brazil.

EvidencesPaleontology

Fossils of Lystrosaurus was found in South Africa, South America and Antarctica at Permian.Lystrosaurus could not swim.

Evidences

ClimatologyCoalfields can be found in AntarcticaAncient climate of Antarctica must be very warm and humid for vegetation growth.A huge ice-cap has been covering Antarctica now.The location of Antarctica is not fixed but moved.

Palaeomagnetism

Hypothesis of Sea-Floor SpreadingIntroduction:In 1960, improved techniques had enabled scientists to examine the detailed topography of large areas of ocean floors.They were revealed ‘mountain ranges’.These ranges were not isolated but formed part of a world-wide continuous network.They were seismically active and youngest regions of the earth.

Ocean mountain ranges

HypothesisThis hypothesis was formulated by Harry Hess in 1960.Ocean floors are young and ephemeral features.They regenerated at ridge crests and destroyed in the trench systems.Hess state that no materials greater than 100 million years in age from the deep ocean floor or oceanic island.Continents have been passively drifted apart and together on the backs of mantle-wide convection cells.The orientations of magnetic anomalies are roughly symmetrical about the ridge axis.

Mantle-wide convection and orientations of the magnetic anomalies

The Theory of convection currentsIt was developed by A. Holmes which provides a support the theory of continental drift.Radioactive heat in the mantle sets up convection currents moving in a series of “cell” through the upper mantle and the simatic layer.These currents rise under the oceans, then move out horizontally in either direction, taking with them the “continental drift” of the lighter sialic material in opposite directions.

Convection cells in upper mantle

Table for crustal moving rate

Theory of Plate TectonicsIt was developed in the concepts of sea-floors spreading, continental drift, crustal structures, world patterns of seismic and volcanic activities.Surface of the Earth comprises a series of internally rigid but relatively thin plates.They are continuously in motion.All seismicity, vulcanicity and tectonic activities are associated with different motion between adjacent platesThe plates (lithosphere) consist of crust and upper mantle.Beneath the lithosphere lies the soft, weak layer (Asthenosphere).

Three types of plate boundary / margin

Constructive marginDestructive marginConservative margin

Constructive marginDuring the spreading process, new crust is created and moves away from the ridge along with the underlying, uppermost mantle.The ridge represents a zone of two plates moving away from each other (eg. Mid-ocean Ridges)

Destructive MarginIt occurs at the deep ocean trenches or fold mountain belts where two plates approach each other and one slips down under the margin of the other at an angle of about 45o

The zone is called subduction zone.(eg. Himalayas, Java Trench)

Conservative MarginThese are margins at which plates past each other.Due to the friction arising from the lateral movement of the plates, earthquakes are usually caused. (eg. San Andreas Fault)

San Andreas Fault

Mechanism of plates movementThe mechanism for the plate movement is very complex and not yet very clear.It is thought to be related to circulation of convection current which is driven by the heat derived from the radioactive elements in the asthenosphere.Ascending current may locate at the constructive plate margin and drags the plates apart.Descending current may located at the destructive plate margin and pulls the plates to be plunged into mantle.