Page 66 - Laboratory Manual in Physical Geology

long-lived narrow plumes of hot rock rising rapidly indicate that nearly all of Earth’s mantle and crust are
from Earth’s mantle (like a stream of heated lava rising solid rock—not magma. Therefore, except for some
in a lava lamp), or b) the slow melting of a large mass specific locations where active volcanoes occur, there is
of hot mantle rock in the upper mantle that persists for no reservoir or layer of magma beneath Earth’s surface
a long interval of geologic time. just waiting to erupt. On a global scale, the volume
of magma that feeds active volcanoes is actually very
The Hawaiian Hot Spot and Pacific Plate small. What, then, are the special conditions that cause
Motion these rare bodies of upper mantle and lower crust
magma to form?
As a lithospheric plate migrates across a stationary hot Magma generally forms in three plate tectonic settings
spot, a volcano develops directly above the hot spot. (divergent plate boundaries, convergent plate boundaries,
When the plate slides on, the volcano that was over the and hot spots). Its origin (rock melting) is also influenced
hot spot becomes dormant, and over time, it migrates by underground temperature, underground pressure
many kilometers from the hot spot. Meanwhile, a new (lithostatic pressure), and the kind of minerals that
volcano arises as new lithosphere passes over the hot spot. comprise underground rocks.
The result is a string of volcanoes, with one end of the
line located over the hot spot and quite active, and the Temperature (T)
other end distant and inactive. In between is a succession
of volcanoes that are progressively older with distance Rocks are mostly masses of solid mineral crystals. There-
from the hot spot. The Hawaiian Islands and Emperor fore, some or all of the mineral crystals must melt to form

Seamount chain ( FIGURE 2.7 ) are thought to represent magma. According to the Kinetic Theory, a solid mineral

such a line of volcanoes that formed over the Hawaiian crystal will melt if its kinetic energy (motion of its atoms
hot spot. and molecules) exceeds the attractive forces that hold
together its orderly crystalline structure. Heating a crystal
is the most obvious way to melt it. If enough heat energy
is applied to the crystal, then its kinetic energy level may
ACTIVITY rise enough to cause melting. The specific temperature
at which crystals of a given mineral begin to melt is the
2.9 The Origin of Magma m ineral’s melting point .

THINK | How and where does magma form? Partial Melting. All minerals have different melting
About It points. So when heating a rock comprised of several
different kinds of mineral crystals, one part of the rock
OBJECTIVE Apply physical and graphical models of (one kind of mineral crystal) will melt before another
rock melting to infer how magma forms in relavtion to part (another kind of mineral crystal). Geologists

pressure, temperature, water, and plate tectonics. call this partial melting of rock. But where would
the heat come from to begin melting rocks below
PROCEDURES
the ground?
1. Before you begin , read about the origin of
magma below. Also, this is what you will need to
Geothermal Gradient. Unless you live near a volcano
do the activity:
or hot spring, you probably are not aware of Earth’s body
____ Materials provided in lab: hot plate, heat. But South African gold miners know all about it.
aluminum foil, sugar cubes, water, dropper The deeper they mine, the hotter it gets. In the deepest
____ calculator, ruler mine ( FIGURE 1.6 ), 3.8 kilometers below ground, rock
____ Activity 2.9 Worksheets (pp. 70 – 72 ) and pencil temperatures are 60 °C (140 °F) and the mine must be air
2. Answer every question on the worksheet in a conditioned. This gradient of increasing temperature with
way that makes sense to you and be prepared to depth is called the geothermal gradient . This gradient
compare your work and inferences with others. also varies between ocean crust and continental crust,
but the global average for all of Earth’s crust is about
25 °C (77 °F) per kilometer. In other words, rocks located
1 kilometer below your house are about 25 °C warmer than
the foundation of your house. If the geothermal gradient
The Origin of Magma
continued at this rate through the mantle, then the mantle
If you have watched videos of the fountains and riv- would eventually melt at depths of 100–150 kilometers.
ers of lava produced by Kilauea volcano in Hawaii, Seismology shows that this does not occur, so temperature
you may have wondered how much of Earth’s interior is not the only factor that determines whether a rock melts
is made up of melted rock, or magma. Seismic studies or remains solid. Pressure is also a factor.

Plate Tectonics and the Origin of Magma ■ 51