Page 37 - Laboratory Manual in Physical Geology

floating wood block). Unloading materials from a crustal Isostasy and Earth’s Global
block will increase its buoyancy, and the block will rise. Topography
Therefore, you can also think of isostasy as the equilibrium
(balancing) condition between any floating object (such Clarence Dutton applied his isostasy hypothesis in 1889

as the iceberg in FIGURE 1.10 ) and the more dense fluid to explain how the shorelines of lakes or oceans could be

in which it is floating (such as the water in FIGURE 1.10 ). elevated by vertical motions of Earth’s crust. At that time,

Gravity pulls the iceberg down toward Earth’s center (this little was known about Earth’s mantle or topography of
is called gravitational force ), so the submerged root of the the seafloor. Modern data show that Dutton’s isostasy
iceberg displaces water. At the same time, gravity also tries hypothesis has broader application for understanding
to pull the displaced water back into its original place global topography.
(now occupied by the iceberg’s root). This creates fluid
pressure that increases with depth along the iceberg’s root, Global Topography: The Hypsometric
so the iceberg is squeezed and wedged (pushed) upward. Curve
This squeezing and upward-pushing force is called buoyant Radar and laser imaging technologies carried aboard
force . Isostatic equilibrium (balanced floating) occurs satellites now measure Earth’s topography very exactly,
when the buoyant force equals (is in equilibrium with) and the data can be used to form very precise relief
the gravitational force that opposes it. An equilibrium images of the height of landforms and depths of
line (like the waterline on a boat) separates the iceberg’s ocean basins. For example, satellite data was used to
submerged root from its exposed top.
construct the image in FIGURE 1.11A of Earth with

ocean water removed. The seafloor is shaded blue and
Equations—The Math You Need
includes features such as shallow continental shelves,
Activity 1.6 involves writing and rearranging submarine mountains (mid-ocean ridges), deep
equations. You can learn more about equations abyssal plains, and even deeper trenches. Land areas
(including practice isostasy problems) at this site (continents) are shaded green (lowlands) and brown
featuring The Math You Need, When You Need (mountains).
It math tutorials for students in introductory The histogram (bar diagram) of Earth’s topography in
geoscience courses: http://serc.carleton.edu/ FIGURE 1.11B shows the percentage of Earth’s surface for

mathyouneed/equations/ManEqSP.html each depth or height class (bar) in kilometers. Notice that
ACTIVITY the histogram is bimodal (shows two levels of elevation
that are most common on Earth). One of the elevation
1.6 Isostasy and Earth’s Global modes occurs above sea level and corresponds to the
continents. The other elevation mode occurs below sea
Topography
level and corresponds to the ocean floor.

FIGURE 1.11C is called a hypsometric curve

THINK | How is the distribution of Earth (or hypsographic curve ) and shows the cumulative
About It materials related to their density? percentage of Earth’s spherical surface that occurs at
specific elevations or depths in relation to sea level.
OBJECTIVE Analyze Earth’s global topography and This curve is not the profile of a continent, because it

infer how the presence of continents and oceans it represents Earth’s entire spherical surface. Notice that
may be related to isostasy. the cumulative percentage of land is only 29.2% of
Earth’s surface, and most of the land is lowlands. The
PROCEDURES remaining 70.8 cumulative percent of Earth’s surface is
1. Before you begin , read the following background covered by ocean, and most of the seafloor is more than
information on isostasy and Earth’s global 3 km deep.
topography. This is what you will need :
____ Activity 1.6 Worksheets (pp. 36 – 38 ) and Hypsometric Curve—The Math You Need
pencil with eraser
You can learn more about the hypsometric
____ calculator curve and how to read and use it at this
____ other materials provided in the lab: 500 mL site featuring The Math You Need, When
or 1000 mL graduated cylinder, small You Need It math tutorials for students in

samples (about 30–50 g) of basalt and introductory geoscience courses: http://serc.

granite that fit into the graduated cylinder, carleton.edu/mathyouneed/hypsometric/

a gram balance or scale, and water index.html
2. Then follow your instructor’s directions for
completing the worksheets.

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