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Our Planet
Contents
Internal Structure of the Earth
Theory of Continental Drift
Hypothesis of Sea-floor spreading
Theory of Convection Currents
Theory of Plate Tectonics
Internal Structure of the Earth
The Crust
The Mantle
The Core
Internal structure of the earth
The Crust
It is the outermost and thinnest layer of the
lithosphere.
Thickness: 13km (ocean) to 50km (mountains)
Gravity: 2.5 to 3.4
Crustal 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 Crust
The boundary of SIAL and SIMA is called
“Conard Discontinuity”.
The base of the crust is called “Mohorovicic
Discontinuity” MOHO.
The Mantle
Beneath 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 Core
Generally, 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 Drift
Development
Evidences
The fitness of the edges of continents
Evidences from Glaciation
Evidences from Geology
The matching of rock layers
Evidences from Paleontology
Evidences from Climatology
Palaeomagnetism
Development
In 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
Evidences
The 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.
Evidences
Evidence from Glaciation
All four southern
continents reveal sign
of a period of largescale glaciation in
Carboniferous-Permian
(250 million years ago)
Evidences
Evidences 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 layer
Similarity of age, time and sequence of
rock layers can be found.
The Cretaceous and Jurassic sedimentary
basins in West Africa and Brazil.
Evidences
Paleontology
Fossils of Lystrosaurus was found in South Africa, South
America and Antarctica at Permian.
Lystrosaurus could not swim.
Evidences
Climatology
Coalfields can be found in Antarctica
Ancient 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 Spreading
Introduction:
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
Hypothesis
This 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 currents
It 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 Tectonics
It 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 plates
The plates (lithosphere) consist of crust and upper mantle.
Beneath the lithosphere lies the soft, weak layer
(Asthenosphere).
Three types of plate boundary / margin
Constructive margin
Destructive margin
Conservative margin
Constructive margin
During 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 Margin
It 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 Margin
These 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 movement
The 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.