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Isostatic Equilibrium Lab
Why does the Earth
have high places
(the continents) and
low places (the
ocean basins)?
What controls the
elevation of
mountains,
lowlands, and
various regions of
the ocean floor?
Adopted from a presentation
by Ray Rector
Density Calculation
1) The density of a substance is a measure of how much
mass is present in a given unit of volume.
DENSITY = MASS / VOLUME
Example: density of liquid water =
8.33 lb/gal = 64 lb/ft3 = 1.0 gm/cm3
2) We will measure all masses in grams and volumes
in cubic centimeters (cm3), so all of our densities
will be in grams per cubic centimeter (gm/cm3).
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Density Calculation
Measuring mass is easy; we use a gram scale.
We’ll use two methods to measure volume:
1) Linear dimensions (for objects that are rectilinear):
Volume (cm3) = length (cm) x width (cm) x height (cm)
2) Water displacement (for objects that have an
irregular shape)
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Density Calculation
An object’s volume will displace an equal
volume of water in a graduated cylinder.
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Density Calculation
An object’s density is a function of:
• Open spaces inside
• Chemical composition (atomic weight of the
elements that make up the substance)
Atomic weight = the number of protons plus neutrons
in an element
Example:
wood is mostly carbon (C): atomic weight = 12
a fishing weight is mostly lead (Pb): atomic weight = 207
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Isostatic Equilibrium Lab
Why does the Earth
have high places
(the continents) and
low places (the
ocean basins)?
What controls the
elevation of
mountains,
lowlands, and
various regions of
the ocean floor?
Answer: parts of the
crust vary in
thickness & density.
Adopted from a presentation
by Ray Rector
Earth’s Layered Structure
Core
Mantle
Oceanic crust
Continental crust
Lithosphere: the crust (either oceanic or
continental) plus the uppermost part of the
mantle; forms a layer of cool, rigid rock that
is broken up into moving plates.
Crust
Earth’s Layered Structure
The lithosphere is broken up into plates that move
horizontally across the Earth.
Earth’s Layered Structure
Today’s lab is not about
horizontal movements
of the Earth’s plates,
but rather about vertical
movements and vertical
elevations of the
Earth’s crust.
Elevation of Earth’s Crust
Two Main Types of Crust
Continental Crust
Oceanic Crust
The Concept of Isostasy
Isostasy is the principle
of buoyancy applied to
the Earth’s crust.
Isostasy: a state of gravitational equilibrium in which an
area of crust “floats” in a balanced way on the denser rock
of the mantle below.
The elevation of any part of the Earth’s crust is a
function of the THICKNESS and DENSITY of the crust.
The Concept of Isostasy
These figures show how either thickness differences
or density differences determine how high the wood
blocks will float.
Isostasy and Isostatic Equilibrium
The top of the wood will ride higher if:
-The wood block is thicker
-The wood block is less dense
Isostatic Adjustment – Ice Caps
Isostatic Response to
Changing Ice Weight
Glacial Adjustment
Isostatic Adjustment - Orogeny