(on the average) in oceanic regions is the most accessible to direct observations
and thus is best known by marine geologists. During the last 120 years of
investigations on land and in the deep sea, scientists have recognized several
oceanic features based on their different types of environment and setting, and also
based on their rock compositions. These major underwater structures include (1)
spreading ridge systems, (2) fracture zones, (3) islands, underwater volcanoes and
seamount chains, (4) deep trenches associated with subduction zones, (5) large
volcanic plateaus which form the outer shell of our planet (see Chap. 4).
Thanks to deep-sea drilling operations, plus direct field observations of deep
fracture zones and/or based on the upwelling of deep-seated molten material
giving rise to volcanism on the surface, geologists have managed to probe the
composition and structure of the lithosphere. Also, extrapolations have been made
based on indirect studies involving seismic profiles and gravity anomaly measurements. However, we need to be careful when trying to define the stratigraphy
of the crust. It is often found that the sequential variation of rock types is not
uniformly distributed and sometimes the rocks recovered may vary considerably
within the same structure. This is due to tectonic activities such as fracturing and
fissuring as well as to the rheology (deformation) of the material forming the
lithosphere and exposing different types of material. Diapiric upwelling of deepseated material locally intruding the crust (see Chap. 4) gives rise to a large
amount of tectonic instability and rock composition differences. In order to generalize the sequential compositional variation of a section of the lithosphere, it is
necessary to have a good knowledge of the geological setting and the compositional variability of the rocks found within a given area.
Earth’s Magnetism
Sailors are familiar with the effect of Earth’s magnetic field and the preferential
direction shown by the needle of a compass. The property of the magnetized
needle in relationship to the Earth’s magnetic field and the invention of the
compass was probably first used by Chinese navigators and in fact, the first person
recorded to have used the compass as a navigational tool was Zheng He
(1371–1435), from the Yunnan province in China, who made seven ocean voyages
between 1405 and 1433. (Mary Bellis, About.com Guide). William Gilbert wrote
the earliest scientific treatises on magnetism in Latin, in 1600. However, the
earliest evidence of Earth’s field magnetization is likely to date back to the
eleventh century (Jacobs 1967).
In modern science, the geomagnetic field has been used to explore the dynamics
of the Earth’s interior and for mapping mineral deposits. Furthermore, scientists
have learned that the magnetic poles have jumped from north to south and vice
versa several times during the Earth’s history. Thus, the study of paleo-magnetism
has given indications about the location of the poles during the emplacement of
iron rich lava.
48
2 Our Haven, Planet Earth
and thus is best known by marine geologists. During the last 120 years of
investigations on land and in the deep sea, scientists have recognized several
oceanic features based on their different types of environment and setting, and also
based on their rock compositions. These major underwater structures include (1)
spreading ridge systems, (2) fracture zones, (3) islands, underwater volcanoes and
seamount chains, (4) deep trenches associated with subduction zones, (5) large
volcanic plateaus which form the outer shell of our planet (see Chap. 4).
Thanks to deep-sea drilling operations, plus direct field observations of deep
fracture zones and/or based on the upwelling of deep-seated molten material
giving rise to volcanism on the surface, geologists have managed to probe the
composition and structure of the lithosphere. Also, extrapolations have been made
based on indirect studies involving seismic profiles and gravity anomaly measurements. However, we need to be careful when trying to define the stratigraphy
of the crust. It is often found that the sequential variation of rock types is not
uniformly distributed and sometimes the rocks recovered may vary considerably
within the same structure. This is due to tectonic activities such as fracturing and
fissuring as well as to the rheology (deformation) of the material forming the
lithosphere and exposing different types of material. Diapiric upwelling of deepseated material locally intruding the crust (see Chap. 4) gives rise to a large
amount of tectonic instability and rock composition differences. In order to generalize the sequential compositional variation of a section of the lithosphere, it is
necessary to have a good knowledge of the geological setting and the compositional variability of the rocks found within a given area.
Earth’s Magnetism
Sailors are familiar with the effect of Earth’s magnetic field and the preferential
direction shown by the needle of a compass. The property of the magnetized
needle in relationship to the Earth’s magnetic field and the invention of the
compass was probably first used by Chinese navigators and in fact, the first person
recorded to have used the compass as a navigational tool was Zheng He
(1371–1435), from the Yunnan province in China, who made seven ocean voyages
between 1405 and 1433. (Mary Bellis, About.com Guide). William Gilbert wrote
the earliest scientific treatises on magnetism in Latin, in 1600. However, the
earliest evidence of Earth’s field magnetization is likely to date back to the
eleventh century (Jacobs 1967).
In modern science, the geomagnetic field has been used to explore the dynamics
of the Earth’s interior and for mapping mineral deposits. Furthermore, scientists
have learned that the magnetic poles have jumped from north to south and vice
versa several times during the Earth’s history. Thus, the study of paleo-magnetism
has given indications about the location of the poles during the emplacement of
iron rich lava.
48
2 Our Haven, Planet Earth