Page 62 - Laboratory Manual in Physical Geology

(rigid rocks) and change their behavior to that of viscous, all of the data, from all directions, it is possible to
plastic-ductile solids. The zone where this happens generate seismic tomography images of Earth. These
even has a name—the brittle-ductile transition zone . So images can be viewed individually or combined to form
mantle rocks are rheids—they flow and permanently three- dimensional perspectives. The computer can
deform (change shape) like viscous fluids. Rocks of the also assist in false coloring seismic tomography images
lithosphere and rock samples in your hand are not. They to show bodies of mantle rock that are significantly
are viscoelastic solids that behave much more like elastic warmer or cooler than the rest of the mantle
(to brittle) solids than viscous fluids. ( FIGURE 2.6 ).

Mantle Convection ACTIVITY
While much is known about plate tectonics, and the plates
have been identified and named ( FIGURE 2.5 ), there has 2.4 Paleomagnetic Stripes and
been uncertainty about how mantle rocks beneath the Seafloor Spreading
asthenosphere may influence this process. In the 1930s, an
English geologist named Arthur Holmes speculated that THINK
the mantle may contain convection cells in which there is | How are plate boundaries identified?
a circular flow of material like in boiling pot of soup. He About It How and at what rates does plate
proposed that such flow could carry continents about the tectonics affect Earth’s surface?
Earth like a giant conveyor belt. This idea was also adapted
in the 1960s by Harry Hess, who hypothesized that OBJECTIVE Analyze paleomagnetic stripes and

mantle flow is the driving mechanism of plate tectonics. infer how seafloor spreading is related to Cascade
New technologies provide an opportunity to evaluate this Range volcanoes.
hypothesis. For example, seismic tomography now provides PROCEDURES
sound evidence that processes at least 660 kilometers deep 1. Before you begin , read about Earth’s magnetism
inside the mantle may have dramatic effects on plate tec- and paleomagnetism below. Also, this is what
tonics at the surface.
you will need to do the activity:
____ calculator, ruler
Seismic Tomography ____ Activity 2.4 Worksheets (pp. 61 – 62 ) and pencil
Earth’s mantle is nearly 3000 km thick and occurs 2. Answer every question on the worksheet in a
between the crust and the molten outer core. Although way that makes sense to you and be prepared to
mantle rocks behave like a brittle solid on short time- compare your work and inferences with others.
scales associated with earthquakes, they seem to become
rheids and flow like a viscous fluid on longer times-
cales of hundreds to thousands of years. Geologists
use a technique called seismic tomography to detect this Earth’s Magnetism and
mantle flow.
The word tomography (Greek: tomos = slice, graphe Paleomagnetism
= drawing) refers to the process of making drawings of If you drop a pen, it falls to the floor. The pen is under
slices through an object or person. Geologists use seis- the influence of Earth’s gravity—an invisible force field
mic tomography to view slices of Earth’s interior similar that pulls everything and everyone towards the center
to the way that medical technologists view slices of the of our planet. But Earth has another force field that
human body. The human body slices are known as CAT is not so obvious, its magnetic field. It is as though a
(computer axial tomography) scans and are constructed giant bar magnet resides inside Earth, giving our planet
using X-rays to penetrate and image the human body. both a magnetic north pole and a magnetic south pole.
The tomography scans of Earth’s interior are con- Invisible lines of the magnetic force field arc out through
structed using seismic waves to penetrate and image the space from the south magnetic pole, travel around the
body of Earth. outside of Earth at its equator, then arc back into the
In seismic tomography, geologists collect data north magnetic pole. The strength of a magnetic field
on the velocity (rate and direction) of many thou- is measured in units called teslas, and a microtesla is a
sands of seismic waves as they pass through Earth. millionth of a tesla. Small magnets used to hold notes
The waves travel fastest through rocks that are the on refrigerator doors have a strength of about 50,000
densest and presumed to be coolest. The waves travel microteslas, and Earth’s magnetic field strength ranges
slower through rocks that are less dense and presumed from just 30 microteslas at the equator to about 50
to be warmer. When a computer is used to analyze microteslas at the poles. Therefore, a refrigerator magnet

Plate Tectonics and the Origin of Magma ■ 47