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magma, a molten mixture of rock-forming minerals and usually volatiles such as gases and steam.

Since their constituent minerals are crystallized from molten material, igneous rocks are formed at high temperatures.

The volumetric portion of bulk rock that is not occupied by grains, crystals, or natural cementing material is termed porosity.

That is to say, porosity is the ratio of void volume to the bulk volume (grains plus void space).

This void space consists of pore space between grains or crystals, in addition to crack space.

In sedimentary rocks, the amount of pore space depends on the degree of compaction of the sediment (with compaction generally increasing with depth of burial), on the packing arrangement and shape of grains, on the amount of cementation, and on the degree of ).

Most are deposited from the land surface to the bottoms of lakes, rivers, and oceans.

Sedimentary rocks are generally stratified—Metamorphic rocks are those formed by changes in preexisting rocks under the influence of high temperature, pressure, and chemically active solutions.

Depending on the type of close-packing of the grains, porosity can be substantial.

It should be noted that in engineering usage— Total porosity encompasses all the void space, including those pores that are interconnected to the surface of the sample as well as those that are sealed off by natural cement or other obstructions.

These three classes, in turn, are subdivided into numerous groups and types on the basis of various factors, the most important of which are chemical, mineralogical, and textural attributes.

It is important to distinguish between the high strength of a block of solid or intact rock and the much lower strength of the rock mass consisting of strong rock blocks separated by much weaker joints and other…

The recrystallization that takes place does so essentially in the solid state, rather than by complete remelting, and can be aided by ductile deformation and the presence of interstitial fluids such as water.

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