The Nature of Volcanic Eruptions

Volcanic activity is perceived as a process that produces a picturesque, cone-shaped structure that periodically erupts in a violent manner. Although some eruptions may be very explosive, many are not. What determines whether a volcano extrudes magma violently or “gently”? The primary factors include the magma’s composition, its temperature, and the amount of dissolved gases it contains. These factors affect the magma’s mobility, or viscosity (viscos = stickly). The more viscous the material, the greater its resistance to flow.

The effect of temperature on viscosity is easily seen. Just as heating syrup makes it more fluid (less viscous), the mobility of lava is strongly influenced by temperature. As lava cools and begins to congeal, its mobility decreases and eventually the flow halts. A more significant factor influencing volcanic behavior is the chemical composition of the magma. A major difference among various igneous rocks is their silica (SiO₂) content. Magmas that produce mafic rocks such as basalt contain about 50 percent silica, whereas magmas that produce felsic rocks (granite and its extrusive equivalent, rhyolite) contain more than 70 % silica. Intermediate rock types—andesite and diorite—contain about 60 % silica. A magma’s viscosity is directly related to its silica content—the more silica in magma, the greater its viscosity. Silica impedes the flow of magma because silicate structures start to link together into long chains early in the crystallization process. Consequently, rhyolitic (felsic) lavas are very viscous and tend to form comparatively short, thick flows. By contrast, basaltic lavas, which contain less silica, are relatively fluid and have been known to travel 150 km or more before congealing. The amount of volatiles (the gaseous components of magma, mainly water) contained in magma also affects its mobility. Water dissolved in the magma tends to increase fluidity because it reduces polymerization (formation of long silicate chains) by breaking silicon–oxygen bonds. It follows, therefore, that the loss of gases renders magma (lava) more viscous.

Task 3. Answer the following questions, using the vocabulary from Task 1.

1. In what way is volcanic activity perceived?

2. Are all eruptions explosive?

3. What are the primary factors that determine whether a volcano extrudes magma violently or “gently”?

4. How does viscosity affect the resistance of the material to flow?

5. What is the effect of temperature on viscosity?

6. How does the chemical composition of the magma influence volcanic behaviour?

7. Is a magma’s viscosity directly related to its silica content? If yes, in what way?

8. How does the amount of volatiles contained in magma affect its mobility?

Task 4. Fill in the gaps with the correct numbers:

50, 60, 70, 80, 100

Magmas that produce mafic rocks such as basalt contain about … % silica, whereas magmas that produce felsic rocks (granite and its extrusive equivalent, rhyolite) contain more than … % silica. Intermediate rock types—andesite and diorite—contain about % silica.

Task 5. Look at Figure 6.1. The table shows different compositions of magmas, which cause their properties to vary. Answer the questions:

1. How much gas content does intermediate (Andesitic) magma have?

2. Which type of magma – felsic or mafic has the least viscosity? The greatest viscosity?

3. Which type of magma has the most gas content? The least gas content?

Task 6. Find English equivalents for the following (see the text). Try to build up your own sentences with them.

Бурхливо вивергатись; основні фактори; рухливість магми; хімічний склад магми; різні вулканічні породи; вміст двоокису кремнію; зменшуватись; збільшуватись; бути прямо пов’язаним з; поєднуватись у довгі ланцюги; вода, розчинена у магмі; рвати хімічні зв’язки між кремнієм та киснем.

Task 7. Give a short summary of the text from Task 2.