Individual work

Task 1. Read the text and translate it into Ukrainian (in written form). Build up a list of key terms to the text.

Other Compositional Groups

Rocks with a composition between granitic and basaltic rocks are said to have an intermediate rocks contain at least 25% dark silicate minerals, mainly amphibole, pyroxene, and biotite mica with the other dominant mineral being plagioclase feldspar. This important category of igneous rocks is associated with volcanic activity that is typically confined to the margins of the continents. Another important igneous rock, peridotite, contains mostly olivine and pyroxene and thus falls on the opposite side of the compositional spectrum from granitic rocks. Because peridotite is composed almost entirely of ferromagnesian minerals, its chemical composition is referred to as ultramafic. Although ultramafic rocks are rare at Earth’s surface, peridotite is the main constituent of the upper mantle. An important aspect of the chemical composition of igneous rocks is silica (SiO₂) content. Typically, the silica content of crustal rocks ranges from a low of about 40% in ultramafic rocks to a high of more than 70% in granitic rocks. The amount of silica present in magma strongly influences its behavior. Granitic magma, which has a high silica content, is quite viscous (“thick”) and may erupt at temperatures as low as 700 °C. On the other hand, basaltic magmas are low in silica and are generally more fluid.

Task 2. Make up a list of all the names of rocks mentioned in the text. Provide the transcription of the words.

Task 3. Answer the questions:

1. Which category of rocks is associated with volcanic activity?

2. Characterize peridotite. Why is its chemical composition referred to as ultramafic?

3. In what way does the amount of silica present in magma strongly influence its behaviour?

Test yourself

Task 1. Tell whether the sentences are true or false. Correct the false ones.

1. Lava is molten rock which reaches Earth’s surface.

2. Magma is completely or partly molten rock, which on cooling solidifies to form an igneous rock composed of silicate minerals.

3. Most magmas consist of two distinct parts—a solid component and a gaseous phase.

4. The solid components in magma are nonsilicate minerals.

5. When magma loses its mobility before reaching the surface it crystallizes to form plutonic rocks.

6. Igneous rocks that form when molten rock solidifies at the surface are called volcanic rocks.

7. Igneous rocks are rarely composed of silicate minerals.

8. Olivine, pyroxene, amphibole, and biotite mica are the common light silicate minerals of Earth’s crust.

9. The light silicates include quartz, muscovite mica, and the feldspars.

10. An important aspect of the chemical composition of igneous rocks is silica content.

Task 2. Using the information from the previous text and analysing Figure 8.1 try to complete the summary of the topic “The Nature of Magma. Igneous Processes. Igneous Compositions. Other Compositional Groups”.

In summary, igneous rocks can be divided into broad groups according to the proportions of …and …minerals they contain. …(felsic) rocks, which are composed almost entirely of the lightcolored minerals quartz and feldspar, are at one end of the compositional spectrum. …(mafic) rocks,which contain abundant dark silicate minerals in addition to plagioclase feldspar, make up the other major igneous rock group of Earth’s crust. Between these groups are rocks with an …(andesitic) composition. …rocks, which lack light-colored minerals, lie at the far end of the compositional spectrum from granitic rocks.