Biology Reference
In-Depth Information
and since the last reversal it has moved from northernmost Canada toward
Siberian Russia. The cause of the reversals is unknown, but they are pre-
ceded by a period of several hundred to a few thousand years when the
magnetic fi eld weakens, reverses, and then returns to normal strength. The
last switch from normal (that is, present-day) to mostly reversed polarity
was about 780,000 years ago, and it is called the Brunhes-Matuyama re-
versal. The pattern was discovered when magnometers were being towed
by the U.S. Navy to map the topography of the ocean fl oor, and later dur-
ing studies of plate tectonics. Long parallel stripes of lava pour out along
either side of the Mid-Atlantic Ridge, and these were found to have ferric
(iron) inclusions alternately oriented according to the present poles and in
reverse alignment. The different intervals of normal and reversed polarity,
called chrons, have been spliced together from sedimentary and igneous
rocks from different parts of the world into a sequence that extends into the
very remote geologic past. Among the many values of magnetostratigraphy,
in addition to revealing pole reversals and for correlating lava and basal-
tic rocks, is their use in determining the past position of continents. As
hematite or magnetite settles out during deposition, it orients toward the
magnetic poles (inclination), and the tilt (declimation) reveals the position
of the site on the curved surface of the Earth. The two vectors measured
through a section track changes in poles and continents through relative
time. The patterns of geomagnetic reversals for an early interval is shown
in fi gure 3.7.
Other geohistoric and biohistoric information that can be read in chron-
ological order comes from the sediments that have accumulated on the sea
fl oor. A time limit of about 180 million years is imposed by the disappear-
ance or subduction of the ocean fl oor into the great trenches of the world.
For example, the western edge of the Pacifi c basin is subducting beneath
Asia along the Mariana Trench, and the northern edge is entering the Aleu-
tian Trench bordering the Bering region between North America and Rus-
sia. The remaining unsubducted sediments reveal whether the water was
deep or shallow, the proximity of land masses, pole and continent posi-
tions from the magnetite inclusions, volcanism along midocean ridges, and
changes in these features over time.
The value of magnetic and sedimentary sequences is that they afford
ways of placing strata, structures, processes, and events in Earth history
in proper chronological order. This constitutes the science of stratigra-
phy, which began with the early work of British geologist William Smith
(1769-1839) as recounted by Simon Winchester in
The Map that Changed
the World
(2001). Smith widely advocated that strata, and the fossils and
