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containing some large crystals move to a different environment—for example, by
erupting at the surface—the remaining liquid portion of the lava would cool more
quickly. The resulting rock, which has large crystals embedded in a matrix of smaller
crystals, is said to have porphyritic texture.
Glassy texture. During some volcanic eruptions, molten rock is ejected into the
atmosphere, where it is quenched quickly. Rapid cooling of this type may generate
rocks having a glassy texture.
Pyroclastic (fragmental) texture. Another group of igneous rocks is formed from
the consolidation of individual rock fragments that are ejected during a violent volcanic
eruption. The ejected particles might be very fine ash, molten blobs, or large angular
blocks torn from the walls of the vent during the eruption. Igneous rocks composed of
these rock fragments are said to have a pyroclastic texture or a fragmental texture.
Pegmatitic texture. Under special conditions, exceptionally coarse-grained igneous
rocks, called pegmatites, may form. These rocks, which are composed of interlocking
crystals all larger than a centimeter in diameter, are said to have a pegmatitic texture.

Task 4. Answer the following questions, using the vocabulary from Task 1.
1. What does the term texture mean?
2. Is texture an important property?
3. What does texture reveal?
4. What are three factors which influence the textures of igneous rocks?
5. Why is aphanitic texture called so?
6. Are the crystals that make up aphanitic rocks small or large?
7. How do geologists commonly characterize fine-grained rocks?
8. What does phaneritic texture mean?
9. Are the crystals that make up phaneritic rocks small or large?
10. What type of texture is said to be porphyritic?
11. When do the rocks have a glassy texture?
12. In what way is pyroclastic texture of igneous rocks formed?
13. What rocks are said to have a pegmatitic texture?

Task 5. Read the descriptions of the rocks and try to name their texture.
1. These rocks consist of a mass of intergrown crystals that are roughly equal in size
and large enough so that the individual minerals can be identified without the aid
of a microscope.
2. Igneous rocks composed of this texture have the ejected particles which might be
very fine ash, molten blobs, or large angular blocks torn from the walls of the
vent during the eruption.
3. The rocks of this type are light, intermediate, or dark in color and contain
primarily light-colored nonferromagnesian silicate minerals.
4. These rocks are composed of interlocking crystals all larger than a centimeter in
diameter.

Task 6. Look at Figure 15.1. It shows igneous rock textures. Answer the
questions:
1. What type of texture should have been at A?
2. Show extrusive igneous activity. (Which textures belong to extrusive igneous
activity?) Show intrusive igneous activity. (Which textures belong to intrusive
igneous activity?)

Task 7. Find English equivalents for the following (see the text). Try to
build up your own sentences with them.

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