Page 35 - Laboratory Manual in Physical Geology

axis (independent variable), you get two lines. So this floating objects around 225 B.C. When he placed a block
kind of graph is called a two-line graph . of wood in a bucket of water, he noticed that the block
floated and the water level rose ( FIGURE 1.10A ). When he

■ Bar graphs or histograms use the length/height pushed down on the wood block, the water level rose even
of a bar or column to show how frequently a more. And when he removed his fingers from the wood
measurement occurs. The bars are labeled according block, the water pushed it back up to its original level of
to the class interval of measurements that you choose floating. Archimedes eventually realized that every floating
(independent variable). The length/height of the bars object is pulled down (toward Earth’s center) by gravity,
is their frequency , how many times (how frequently) so the object displaces fluid and causes the fluid level to
data values occur in each class interval. In Excel™, rise. However, Archimedes also realized that every floating
histograms are called bar graphs if the bars are object is also pushed upward by a buoyant force that is
horizontal and column graphs if the bars are vertical.
equal to the weight of the displaced fluid. This is now
called Archimedes’ Principle.
Graphing—The Math You Need Buoyant force (buoyancy) is caused as gravity pulls on
the mass of a fluid, causing it to exert a fluid pressure on
You can learn more about graphing and how submerged objects that increases steadily with increasing
to use graphs in the geosciences at this site depth in the fluid. The deeper (greater amount of) a
featuring The Math You Need, When You fluid, the more it weighs, so deep water exerts greater fluid
Need It math tutorials for students in pressure than shallow fluid. Therefore, the lowest surfaces of
introductory geoscience courses: http://serc. a submerged object are squeezed more (by the fluid pressure)

carleton.edu/mathyouneed/graphing/ than the upper surfaces. This creates the wedge of buoyant
index.html
force that pushes the object upward and opposes the
downward pull of gravity (white arrows in FIGURE 1.10B ).

ACTIVITY An object will sink if it is heavier than the fluid it displaces
(is denser than the fluid it displaces). An object will rise if
1.5 Density, Gravity, and Isostasy it is lighter than the fluid it displaces (is less dense than the
fluid it displaces). But a floating object is balanced between
THINK | How is the distribution of Earth sinking and rising. The object sinks until it displaces a
About It materials related to their density? volume of fluid that has the same mass as the entire floating
object. When the object achieves a motionless floating
condition, it is balanced between the downward pull of
OBJECTIVE Develop and test models of isostasy, gravity and the upward push of the buoyant force.
measure rock densities, and calculate the isostasy of
oceanic and continental crust.
Isostasy
PROCEDURES In the 1880s, geologists began to realize the abundant
1. Before you begin , read the following background evidence that levels of shoreline along lakes and oceans
information on density, gravity, and isostasy, and had changed often throughout geologic time in all parts
equations.This is what you will need : of the world. Geologists like Edward Suess hypothesized
____ Activity 1.5 Worksheet (p. 35 ) and pencil that changes in sea level can occur if the volume of ocean
with eraser water changes in response to climate. Global atmospheric
warming leads to sea level rise caused by melting of glaciers
____ calculator, ruler
(cryosphere), and global atmospheric cooling leads to a
____ other materials provided in the lab: wood
drop in sea level as more of Earth’s hydrosphere gets stored
blocks (oak, pine), basin of water, gram
in thicker glaciers. However, an American geologist named
balance or scale
Clarence Dutton suggested that shorelines can also change
2. Then follow your instructor’s directions for if the level of the land changes (and the volume of water
completing the worksheets. remains the same).
Dutton reasoned that if blocks of Earth’s crust
are supported by the mantle beneath them (solid rock
capable of a slow flow) then they must float in the mantle
according to Archimedes’ Principle (like wood blocks,
icebergs, and boats floating in water). Therefore, he
proposed that Earth’s crust consists of buoyant blocks of
Density, Gravity, and Isostasy
rock that float in gravitational balance in the top of the
Scientists have wondered for centuries about how the mantle. He called this floating condition isostasy (Greek
distribution of Earth materials is related to their density for “equal standing”). Loading a crustal block (by adding
and gravity. Curious about buoyancy, the Greek scientist lava flows, sediments, glaciers, water, etc.) will decrease its
and mathematician, Archimedes, experimented with buoyancy, and the block will sink (like pushing down on a

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