Earth's Crust

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Earth’s Crust


the uppermost of the earth’s solid shells. The lower boundary of the earth’s crust is considered to be the division surface where longitudinal seismic waves passing from above to below undergo a sudden increase in velocity from 6.7-7.6 km/sec to 7.9-8.2 km/sec; this surface is also known as the Mohorovicic discontinuity. This is an indicatioryhat less elastic material has given way to more elastic ami dense material. The layer of the upper mantle which underlies the earth’s crust is often called the substratum. Together with the earth’s crust it constitutes the lithosphere. The earth’s crust on continents differs from the crust under the ocean. Continental crust is usually 35-45 km thick, attaining a thickness of up to 70 km in mountain regions. The upper part of the continental crust is made up of a fragmented sedimentary layer consisting of unchanged or slightly changed sedimentary and igneous rocks of different ages. The layers are often folded, broken, and displaced along faults. In some places (on shields) the sedimentary shell is absent. The entire remaining thickness of the continental crust is divided into two parts according to the velocity of seismic waves; the upper part is conventionally called the “granite” layer (velocity of longitudinal waves up to 6.4 km/sec) and the lower part, the “basalt” layer (6.4-7.6 km/sec). It appears that the granite layer is made up of granites and gneisses; the basalt layer is composed of basalts, gabbro, and very strongly metamorphized sedimentary rocks in varying ratios. These two layers are often divided by the Conrad discontinuity, where seismic waves undergo a sudden increase in velocity as they pass through the discontinuity. It appears that the silica content of the earth’s crust de-creases with depth and the content of oxides of iron and magnesium increases; this occurs to an even greater degree with the transition from the crust to the substratum.

The oceanic crust is 5-10 km thick (together with the water depth it amounts to 9-12 km). It is divided into three layers: under a thin (less than 1 km) layer of marine sediments lies the “second” layer where longitudinal seismic waves have velocities of 4-6 km/sec. It is 1-2.5 km thick and is probably composed of serpentinite and basalt, possibly with sedimentary interlayers. The lower “oceanic” layer has an average thickness of about 5 km, and seismic waves pass through it at a velocity of 6.4-7.0 km/sec; it is probably composed of gabbro. The thickness of the layer of sediments on the ocean floor varies, and in some places there are none at all. In the transitional zone between the continent and the ocean an intermediate type of crust may be observed.

The earth’s crust is subject to constant movements and changes. In its irreversible development the mobile areas (geosynclines) are transformed by prolonged transformation into relatively stable areas (platforms). There are a number of tectonic hypotheses that explain the process of development of geosynclines, platforms, continents, and oceans, and the causes of the development of the earth’s crust as a whole. There is no doubt that the primary reasons for the development of the earth’s crust lie deeper within the earth’s interior; this fact renders the study of the interaction between the earth’s crust and the upper mantle of special interest.

The earth’s crust is close to a state of isostasy (equilibrium): the heavier (which is to say the thicker and the more dense) a particular sector of the crust is, the deeper it sinks into the substratum. Tectonic forces disrupt isostasy, but when they weaken, the earth’s crust returns to a state of equilibrium.


References in periodicals archive ?
On board, marine geologist Henry Dick sent dredge after dredge through the ice to the seafloor, searching for telltale rocks that would help shed light on how Earth's crust forms.
They also suggested that the magma had to be injected into the Earth's crust at a high rate to reach a large enough volume and pressure to cause an eruption.
When the Rio Negro rises and floods, Earth's crust in the region sinks, they note.
The lithosphere is a somewhat flexible layer comprising Earth's crust and the upper part of the mantle.
More than seven years after seismologists were jolted into action by the 1994 Northridge Earthquake, scientists will be able to record as little as a millimeter of movement in the Earth's crust using the Southern California Integrated Global Positioning System Network.
Participants in the session could see diagrams of the earth's crust, analyze data from an onscreen table, and discuss the results with scientists at external institutions.
University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science professor Keir Becker report on the first observatory experiment to study the dynamic microbial life of an ever-changing environment inside Earth's crust.
Silicon is the second most abundant element in Earth's crust, but nature's primary sources of silicon--sand and quartz--are tainted with metals.
The caldera was like a blister underneath Earth's crust [outermost layer]," explains park ranger Mark Williams.
Instead of sort of pushing pieces of the Earth's crust together, you're sort of pulling them apart,'' he said.
Radium (Ra) cannot be seen, tasted or smelled in your drinking water and is a naturally occurring radioactive element that is found in rocks and soil within the earth's crust.
Since subduction is needed to drag water into the crust, the finding also confirms that plate tectonics, the cycling of the Earth's crust, was happening at this time, albeit in a different way, according to the researchers.