Some intrusive rocks, known as subvolcanic, were not formed at great depth but were instead injected near the surface where lower temperatures result in a more rapid cooling process; these tend to be aphanitic and are referred to as Geologic materials cycle through various forms.
Sediments composed of weathered rock lithify to form sedimentary rock, which then becomes metamorphic rock under the pressure of Earth's crust.
On the other hand, magma erupted at the surface is chilled so quickly that the individual minerals have little or no chance to grow.
As a result, the rock is either composed of minerals that can be seen only with the aid of a microscope (called intrusive igneous rocks that solidified deep within the crust and (2) volcanic, or extrusive, igneous rocks formed at Earth’s surface.
Their emanations have provided the water for the oceans, the gases for the primordial oxygen-free atmosphere, and many valuable mineral deposits.
Clarke estimated that 95 percent of crustal rocks are of igneous origin (formed from molten silicate masses, or magmas).
These are planar or linear features that result from flowage of magma with or without contained crystals.
(Denudation is the wearing away of the terrestrial surface by processes including weathering and erosion.) Generally, the intrusive rocks have cross-cutting contacts with the country rocks that they have invaded, and in many cases the country rocks show evidence of having been baked and thermally metamorphosed at these contacts.It may settle within the crust or erupt at the surface from a volcano as a lava flow.Rocks formed from the cooling and solidification of magma deep within the crust are distinct from those erupted at the surface mainly owing to the differences in physical and chemical conditions prevalent in the two environments.As the magma cools and begins to crystallize, silica is taken from the magma to be combined with the other cationic oxides to form the silicate minerals.For example, one mole of Si O), along with the pyroxene, since the olivine requires only one-half as much silica for every mole of magnesium oxide.Within Earth’s deep crust the temperatures and pressures are much higher than at its surface; consequently, the hot magma cools slowly and crystallizes completely, leaving no trace of the liquid magma.The slow cooling promotes the growth of minerals large enough to be identified visually without the aid of a microscope (called , meaning “visible”).The major ) are depleted in the rocks that have a silica content of less than about 45 weight percent, but, above 45 percent, calcium oxide can be as high as 10 percent; this amount decreases gradually as the silica increases.Alumina in rocks that contain more than 45 percent silica is generally above approximately 14 weight percent, with the greatest abundance occurring at an intermediate silica content of about 56 weight percent.situation, state of affairs - the general state of things; the combination of circumstances at a given time; "the present international situation is dangerous"; "wondered how such a state of affairs had come about"; "eternal truths will be neither true nor eternal unless they have fresh meaning for every new social situation"- Franklin D.Igneous rock, any of various crystalline or glassy rocks formed by the cooling and solidification of molten earth material.