Page 58 - Petrophysics

32 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

the mantle. Theoretically, the mantle responds to the continuous stress
created by heat rising from the interior of the globe by developing current
cells of very slowly ascending and descending material. Continental
masses accumulate over the descending zones, and the ocean basins
lie over the ascending zones. Thus, the slow movement of the mantle,
as a plastic material, could be the mechanism causing the drift of the
continental masses and the spreading of the Ocean floor at mid-ocean
rifts around the globe. Continuous drifting motion of the crustal plates
also may be influenced by body forces generated by gravitational earth
tides and by the rotation of the earth.
Rocks and magma at volcanic eruptions that have apparently come
from the upper mantle are basic in composition, and are rich in
magnesium and iron. The density of the mantle is greater than the
lithosphere, approximately 3.3 g/cm3.
The boundary at the base of the mantle, where the S waves disappear
and the P-wave velocity decreases, marks the beginning of the outer
liquid core. The fact that the P waves increase in velocity once more at
a depth of 5,000 km suggests that the inner core is a solid. It is believed
to be composed principally of nickel and iron with a density of about
10.7 g/cm3, which is more than twice as dense as the mantle. The Earth’s
magnetic field is assumed to be created by an electric field resulting from
circulation of currents within the liquid core [ 1-53.

PLATE TECTONICS

Theories of plate tectonics are based on spreading of the sea floor
at mid-ocean rifts and the motion, or drift, of the continents. The
Earth’s lithosphere is composed of six major plates whose boundaries
are outlined by zones of high seismic activity [4]. The continents appear
to be moved by the convection currents within the mantle at rates of two
to three inches (5.1-7.6 cm) per year. The convection cells apparently
occur in pairs and thus provide the kinetic energy for movement of the
continental masses.
Mid-ocean ridges form a network of about 65,000 km of steep
mountains with branches circling the globe. Some of the mountains are
as high as 5,500 meters above the ocean floor, and some emerge above
the ocean as islands.
The crustal plates are manufactured from magma rising to the surface
through rifts at the sites of the mid-ocean ridges. Material from the
mantle apparently liquifies as it nears the surface and is relieved of
a great part of its pressure. The liquid, or magma, rises to the crust
and adds to the mass of the plate. As the plate moves across the ocean