Read the following text and fulfill the after-reading exercise

Date: 2015-10-07; view: 598.

SCANNING - Igneous rocks (R.P. -5.5)

READING COMPREHENSION

Use the terms and fill in the gaps

A cross section (1) ________the distinctive features. The magma (2) ________ lies many miles below the Earth's surface. It is a reservoir of gas-rich (3) _______rock under (4) _____. This magma may (5) _______ outward, which results in the escaping of pressure through a (6) _______ in the crust. As the (7) _______ rises, the pressure is reduced. Dissolved gases are freed as (8) ______ ______.This force of (9) ______blasts opens and forms a circular (10) _______. This is an (11) _________ for ash, cinder and (12) _________. Explosions shape the volcano's top as an (13) _____ cone or (14) _________.

These types of rock constitute approximately 75% of the continental crust and well over 90% of the oceanic crust. Some have been derived directly from the mantle, whereas others represent partly remelted oceanic or continental crustal materials.

Chemically speaking, igneous rocks exhibit limited compositional variations. Their principal constituent oxide is silica (SiO₂), which ranges from about 40 to 75% by weight in the common rock types. Where the silica content is low, magnesium and iron oxides commonly are major components.

Occurrences of igneous rocks belong to two different categories: extrusive and intrusive. To the first group belong those that have reached the surface of the Earth in a largely molten condition. The second group of igneous rocks consists of those that crystallized from magmas that solidified before reaching the surface of the Earth. Processes that formed them are thus hidden from view and must be deduced from the study of uplifted and deeply eroded parts of the crust.