Unit 5 igneous rocks
Words to be remembered:
to solidify – затвердевать, твердеть; уплотняться
solidification – затвердевание
either … or – или…или, либо … либо
to occur – залегать
occurrence – залегание
mode of occurrence – условия залегания
intrusive = plutonic – интрузивный, внедрившийся = плутонический
extrusive = volcanic – эффузивный, излившийся = вулканический
bodies = masses – массы
grain size – размер зерна
be referred to – называться
fine – мелкий
to expose – выходить на поверхность, обнажаться
to cool – остывать, охлаждаться
to occupy – занимать
crack – трещина
volcanic ash – вулканическая зола
etc. = et cetera – и так далее
Igneous rocks have crystallized from solidified magma. Igneous rocks can be classified based on the mode of occurrence. They occur either as intrusive (below the surface) bodies or as extrusive masses solidified at the Earth’s surface. The terms ‘intrusive’ and ‘extrusive’ refer to the place where rocks solidified.
The grain size of igneous rocks depends on their occurrence. The intrusive rocks generally cool more slowly and crystallize to a larger grain size. They are referred to as intrusive igneous rocks because they intruded into older pre-existing rocks. They are also called plutonic. Extrusive or volcanic rocks have finer grains.
Exposed igneous rocks are most numerous in mountain zone. The largest bodies of igneous rocks are called batholiths. Batholiths cooled very slowly. This slow cooling permitted large mineral grains to form. It is not surprising that batholiths are composed mainly of granitic rocks with large crystals called plutons. Granites and diorites also belong to the group of intrusive or plutonic rocks formed by solidification of igneous mass under the Earth's crust.
Laccoliths and sills are intruded between sedimentary rocks. Sills are thin and they may be horizontal, inclined or vertical. Laccoliths are thicker bodies and in some cases they form mountains.
Dykes are also intrusive bodies. They range in thickness from a few inches to several thousand feet. Dykes are generally much longer than they are wide. Most dykes occupy cracks and have straight parallel walls. These bodies cool much more rapidly and are commonly fine-grained.
Extrusive igneous rocks have been formed from lava flows which come from fissures to the surface and form fields of volcanic rocks such as basalt, volcanic ashes, dust, tuff, etc. As a rule these volcanic rocks cool very rapidly and are fine-grained. It is, interesting to note that basalt is the principal rock type of the ocean floor.
Igneous rocks are rich in minerals that are important economically or have great scientific value. Igneous rocks and their veins are rich in iron, gold, zinc, nickel and other ferrous metals.
Questions on the text:
1) Have igneous rocks crystallized from magma or have they been formed by sedimentation? 2) Which types of igneous rocks do you know? 3) What does the grain size of igneous rocks depend on? 4) Can you give an example of intrusive or plutonic rocks? 5) Are diorites intrusive or extrusive formations? 6) What do you know about batholiths? 7) What can you say about laccoliths and sills? 8) What are dykes? What can you say about volcanic rocks? 9) What minerals are igneous rocks rich in?