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a word used to describe how the principle of buoyancy applies to blocks of the earth's crust as they rest on the mantle. asthenosphere: underlying denser, heat-softened, partially melted (plastic) – weak upper mantle. lithosphere: rigid, solid outer layer (brittle) –strong crust and uppermost mantle.
Isostasy and continental_drift
Isostasy and Continental Drift
• Isostasy (Greek ísos "equal", stasis "standstill")
• Term used in geology to refer to the state of gravitational
equilibrium between the earth’s lithosphere and
Isostasy: a state of gravitational
equilibrium in which an area of
crust “floats” in a balanced way on
the denser rock of the mantle
The elevation of any part of the
Earth’s crust is a function of the
THICKNESS and DENSITY of the
• Isostasy is the vertical movement
of the crust to attain “buoyancy”
in the mantle.
• The height a block of wood floats
in water depends on it’s density
• The “height” of the earth’s crust
also depends on it’s density and
The deflection of plumb bob
near mountain chains is less
than expected. Calculations
show that the actual
deflection may be explained
if the excess mass is canceled
by an equal mass deficiency
at greater depth.
The Concept of Isostasy
These figures show how either thickness differences or
density differences determine how high the wood blocks
• Isostasy used to denote the ideal state of
balance b/w different parts of the earth crust
due to difference of densities of two crustal
material lithosphere and asthenosphere.
• This state of balance tends to maintain the
certain level known as level of compensation.
Hypothesis of Isostasy
Mountain range supported by
density variations within the
Mountain range supported by
changes in crustal thickness;
crustal density relatively
Either way, the plate is relatively light and 'floats' on a fluid
The principle of isostasy suggests that the earth's crust should
adjust to any changes in mass that occur at the earth's surface
(we call these "isostatic adjustments"). There are basically two
types of responses:
a. Definition: the slow, sinking of the earth's crust
b. Cause: the addition of mass to the crust
c. Example: the advance of glacial ice sheets
a. Definition: the slow, vertical rise in earth's crust
b. Cause: the removal of mass from the earth's crust
c. Example: post-glacial rebound (Scandinavia) (U.S.)
It should also be noted that isostasy explains why continental
plates cannot be subducted: the forces that drive subduction
cannot overcome the "buoyancy" of these low density plates.
Subduction is a geological process that takes place at convergent boundaries of tectonic
plates where one plate moves under another and is forced or sinks due to gravity into the
mantle. Regions where this process occurs are known as subduction zones.
“see” isostatic adjustment today from load of glaciers on
crust during last glaciation and unloading from melting
(possible because response of asthenosphere is slow) process is called post-
• In 1915, the German geologist Alfred Wegener first
proposed the theory of continental drift, which
states that parts of the Earth's crust slowly drift atop
a liquid core.
• Wegner revived the early idea of continental drift,
contending that all of the present-day continents
were connected side by side as long as far
• He called the Super Continent mass “Pangaea”,
Greek for all Lands.
• The gradual movement of the continents across the
earth's surface through geological time.
• the lateral movement of continents resulting from
the motion of crustal plates.
• Earth's continents have been joined together and have
moved away from each other at different times in the
A term, no longer used by geologists, that refers to the
fact that continents are not stationary, but move across
the Earth 's surface. Continental drift is one feature of
the modern theory of plate tectonics.
• the hypothetical landmass that existed when all
continents were joined, from about 300 to 200 million
years ago. (Pangea
• a hypothetical protocontinent of the remote geologic
past that rifted apart to form the continents of today.
• Wegner summary was based on
the number of careful observation:
– Matching Rock , Fossils ,
Glacier , and structural
relations among different parts
of different continents.
• Pangaea started to break up into
two smaller supercontinents,
called Laurasia and Gondwanaland,
during the Jurassic period.
• By the end of the Cretaceous
period, the continents were
separating into land masses that
look like our modern-day
The presence of fossils only over small areas of now separate continents (how did they get
from continent to continent?).
Cause of Continental drift
• Continental drift occurs under the earth crust by
convection current in the mantle which drive the
plates, happening since development of earth.
• Deep material, hotter than their surroundings
would tend to flow upward. In approaching the cool
surface of earth, the mantle would loose its thermal
energy, cool and sink.
• Motion of mantle put in to action by convection
thus becomes mechanism for moving rigid pieces of
crust over some more actively flowing mantle