Engineering Geology

1. ISOSTASY
Presentation
Engr. Faisal 13-CE-26
&
Group

2. Definition of Isostasy
• Huge plates of crustal and upper mantle
material (lithosphere) “float” on more dense,
plastically flowing rocks of the asthenosphere.
The “depth” to which a plate, or block of crust,
sinks is a function of its weight and varies as
the weight changes. This equilibrium, or
balance, between blocks of crust and the
underlying mantle is called isostasy.

3. Contd.
• The taller a block of crust is (such as a
mountainous region), the deeper it penetrates
into the mantle because of its greater mass and
weight. Isostasy occurs when each block settles
into an equilibrium with the underlying mantle.
Blocks of crust that are separated by faults will
“settle” at different elevations according to their
relative mass

4. Isostatic models
• Three principal models of isostasy are used:
• The Airy-Heiskanen Model - where different
topographic heights are accommodated by changes
in crustal thickness, in which the crust has a constant
density
• The Pratt-Hayford Model - where different topographic
heights are accommodated by lateral changes
in rock density.
• The Vening Meinesz, or flexural isostasy model -
where the lithosphere acts as an elastic plate and its
inherent rigidity distributes local topographic loads over
a broad region by bending.
• Airy and Pratt isostasy are statements of buoyancy,
while flexural isostasy is a statement of buoyancy while
deflecting a sheet of finite elastic strength.

5. Important to Note
1. ISOSTATIC EFFECTS OF DEPOSITION AND
EROSION
2. ISOSTATIC EFFECTS OF PLATE
TECTONICS
3. ISOSTATIC EFFECTS OF ICE SHEETS

6. ISOSTATIC EFFECTS OF DEPOSITION AND EROSION
• When large amounts of sediment are deposited on a particular
region, the immense weight of the new sediment may cause
the crust below to sink. Similarly, when large amounts of
material are eroded away from a region, the land may rise to
compensate. Therefore, as a mountain range is eroded down,
the (reduced) range rebounds upwards (to a certain extent) to
be eroded further. Some of the rock strata now visible at the
ground surface may have spent much of their history at great
depths below the surface buried under other strata, to be
eventually exposed as those other strata are eroded away and
the lower layers rebound upwards again.
• An analogy may be made with an iceberg - it always floats
with a certain proportion of its mass below the surface of the
water. If more ice is added to the top of the iceberg, the
iceberg will sink lower in the water. If a layer of ice is
somehow sliced off the top of the iceberg, the remaining
iceberg will rise. Similarly, the Earth's lithosphere "floats" in
the asthenosphere.

7. ISOSTATIC EFFECTS OF PLATE TECTONICS
• When continents collide, the continental crust may thicken at their edges in the
collision. If this happens, much of the thickened crust may move downwards rather
than up as with the iceberg analogy. The idea of continental collisions building
mountains "up" is therefore rather a simplification. Instead, the
crust thickens and the upper part of the thickened crust may become a mountain
range.[citation needed]
• However, some continental collisions are far more complex than this, and the
region may not be in isostatic equilibrium, so this subject has to be treated with
caution.[citation needed]
• The formation of ice sheets can cause the Earth's surface to sink. Conversely,
isostatic post-glacial rebound is observed in areas once covered by ice sheets that
have now melted, such as around the Baltic Sea and Hudson Bay. As the ice
retreats, the load on the lithosphere and asthenosphere is reduced and
they rebound back towards their equilibrium levels. In this way, it is possible to find
former sea cliffs and associated wave-cut platforms hundreds of metres above
present-day sea level. The rebound movements are so slow that the uplift caused
by the ending of the last glacial period is still continuing.
• In addition to the vertical movement of the land and sea, isostatic adjustment of
the Earth also involves horizontal movements. It can cause changes in
the gravitational field and rotation rate of the Earth, polar wander,
and earthquakes.

8. ISOSTATIC EFFECTS OF ICE SHEETS
• The formation of ice sheets can cause the Earth's surface
to sink. Conversely, isostatic post-glacial rebound is
observed in areas once covered by ice sheets that have
now melted, such as around the Baltic Sea and Hudson
Bay. As the ice retreats, the load on
the lithosphere and asthenosphere is reduced and
they rebound back towards their equilibrium levels. In this
way, it is possible to find former sea cliffs and
associated wave-cut platforms hundreds of metres above
present-day sea level. The rebound movements are so
slow that the uplift caused by the ending of the last glacial
period is still continuing.
• In addition to the vertical movement of the land and sea,
isostatic adjustment of the Earth also involves horizontal
movements. It can cause changes in the gravitational
field and rotation rate of the Earth, polar wander,
and earthquakes.