Geoscience Reference
In-Depth Information
white dots representing quartz are missing in basalt. Basalt prevails in the Earth's
crust. Having their origin deep within the Earth, igneous rocks are classifi ed and
grouped together. If they are subsequently subjected to increased pressure and heat,
they lose their original form and appear within the group called metamorphic rocks.
Rocks are usually stable unless their external pressure and temperature condi-
tions change as a result of being shifted vertically. These changes have the conse-
quence of decreasing mechanical and chemical stability if the rock is lifted up. On
the other hand, the submersion of the rocks into great depths is accompanied by
heating and by an increase of pressure. The rock starts to be melted and gains the
appearance of a very dense pulp or paste. Extremely great changes of pressure and
temperature occur when lithospheric plates hit one another or when such a plate is
broken. A lithospheric plate is the scientifi c designation for a piece of the Earth's
fractured crust. Lithospheric comes from the
Greek lithos
meaning stone or rock
and
sphaira
meaning sphere. These plates are also known as tectonic plates. Tectonic
is derived from the Greek
tekton
which means builder. Today, we recognize that
continents are huge lithospheric plates slowly and continually fl oating on liquid
magma kilometers below them. Several times in the past, these plates were con-
nected forming a single supercontinent that later broke apart. Subsequently, some of
the remaining plates collided to form and build waves with their tops becoming
mountains. The hypothesis about this continental drift appeared at the end of the
sixteenth century and was theoretically explained in the twentieth century by Alfred
Wegener. A meteorologist by university education, he actively participated and con-
ducted meteorological expeditions in high levels of the atmosphere using weather
balloons and in the extreme polar climate. Incidentally, in 1906, he together his
brother on the occasion of taking meteorological data set world records in the height
and in the continuous length of a balloon fl ight. He was also the fi rst scientist made
famous by overwintering on the inland Greenland ice sheet where he organized the
fi rst boring of ice cores in addition to taking meteorological data. He was excep-
tional in combining adventurous expeditions with utilization of his own measured
data in theoretic developments in many domains of geophysics - among them is the
formulation of the fi rst continental drift theory that was not fully accepted until after
40 years when it was experimentally confi rmed and broadly accepted in the 1950s.
Many new measuring procedures provided support for continental drift theory and
for the model of plate tectonics. The model contributes to the explanation of many
earlier not well-understood discoveries, e.g., why recent deserts like the Sahara did
not exist several million years ago.
Locally some parts of the Earth's broken crust were pushed downward and
heated. This was the situation when the initially igneous rocks gained their meta-
morphic features. Gneiss, a typical example of these metamorphized rocks, is found
in abundant quantities as a major part of the Earth's lower crust.
When the rocks are formed from external materials like sands or seawater pre-
cipitates, we denote them as sedimentary rocks. An example of the fi rst group is
sandstone. The second group is represented by limestone that may contain plant
