2.5 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) _________.
3. Reading comprehension
3.1 Scanning - Igneous rocks (r.P. -5.5)
Read the following text and fulfill the after-reading exercise
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.
Classification and chemical differentiation of igneous rocks
Igneous rocks are distinguished on contrasting chemical composition. Because the bulk-rock chemistry is reflected in the nature and proportions of the mineral present, we will utilize a simple classification based on the proportions of quarts, feldspars and dark ferromagnesian, minerals and on the composition of the plagioclase.
The most refractory minerals (i.e. those stable at the highest magmatic temperatures) form first and as the melt cools, crystallize, in some cases replacing earlier ones. This crystallization takes place discontinuously; formation of another group of minerals proceeds continuously, as in the feldspars.
Bulk compositions reflect differences in mineral proportions among the various rock types. The solid and the liquid can be separated at this stage as well. The process is known as igneous differentiation and explains the contrasts in compositions of melts and solids produced during either partial fusion and / or partial freezing.
Fig. 26. Classification of igneous rocks
3.1.1. Correct the following statements where necessary (Igneous rocks)
1. Igneous rocks comprise the smallest proportion of the continental crust and the largest of the oceanic crust.
2. Most igneous rocks are formed from the crust.
3. Most igneous rocks are remelted oceanic and continental materials.
4. Igneous rocks have rather limited compositional variations.
5. The main constituent is oxide.
6. Silica has an average weight in the common rock types.
7. Magnesium and iron oxides are also major components.
8. There are two categories of igneous rocks-extrusive and intrusive.
9. Intrusive is those that have come to the surface of the Earth in a molten condition.
10. The second category is composed of those that are crystallized from magma that solidified before reaching the surface of the Earth.
11. The processes of the formation of igneous rocks can be studied in uplifted and deeply eroded parts of the crust.