Text 2

texture – структура, будова

intergrowth – проростання, зростання

randomly - випадково

plentiful – багатий, рясний

pebble - галька

outpouring – оголення, вихід; голова пласта

If we examine a few dozen stones we discover that there are many different kinds – in fact it is difficult to find two exactly alike. Looking even more closely, we see that all the differences of color and texture are caused by differences in the kinds and shapes of particles that make up each rock, and in the way these particles are arranged. The particles are mineral grains and the proportions of the different kinds constitute the mineral composition of the rock. Whether the grains are large or small or variable in size; rounded or irregular or geometric in shape; intergrown, cemented together, randomly distributed, or in a layered arrangement – these determine the texture of the rock. It is on the basis of these two fundamental attributes, mineral composition and texture, that almost all rocks are most reliably distinguished.

Among the varieties of rock, all of which (and more) are present in larger masses in the mountains, several are distinctive. One is plentiful in several parts of the mountains but rare as pebbles because, being soft and somewhat soluble, it does not survive much travel. Several others are so rare that pieces could not be found.

All the facts about the composition of the gravels are consistent with the hypothesis that they are deposits left by repeated outpourings from the mountains, and that the crude stratification reflects successive floods.

Text 3

congeal – замерзати, заморожувати

molten – розтоплений, рідкий

confining – обмежувальний

Violent – інтенсивний

toss – струс, падіння

conduit – трубопровід, акведук; канал

tuff – туф

Almost every rock has two things to tell. First, it has within it a record of the conditions under which it formed, whether under the sea or on land, in volcano or deep in the earth’s crust. Second, it commonly records some of the things that have happened to it since it were formed – bending, breaking, squeezing, partial recrystallization, or merely the fragmentation and surface rounding exhibited by the gravels just considered.

Among the important rock types, lavas have the most obvious origin. Hundreds of thousands of people have witnessed eruptions, and millions have seen pictures of such active volcanoes as Mauna Loa, Vesuvius, or Paricutin. In simplest terms, molten material wells up, usually accompanied by some gas, flows out on the surface and congeals. In a few hours, or at most a few days, it is hard rock, though it may take much longer to cool completely.

The two important ingredients in all volcanic eruptions are the molten rock (called magma as long as it is still liquid underground and molten lava when it reaches the surface) and the accompanying gases (mostly water vapor). The gases are dissolved in the magma and held there by the pressures existing within the earth’s crust. As the magma rises in the throat of the volcano, this confining pressure is reduced, until the gas – like that in a bottle of carbonated drink that has been uncapped – forms bubbles within the magma that rush to the surface, in a process that h somewhat resembles violent boiling.

If the amount of gas is relatively small, the lava pours out fairly quietly, perhaps forming a few fountains or tossing an occasional hot plastic blob of lava into the air. If the proportion of gas is relatively high or the magma viscous there may be numerous explosions. In extreme cases much or all of the molten lava may be blown high the air, sometimes accompanied by fragments from the walls of the volcanic conduit. By the time the mixture of lava and rock particles settles back to earth it is so dispersed as to consist almost entirely of small fragments, collectively known as volcanic ash when it first settles and as tuff if and when it later becomes consolidated into rock.

Since lava, ash, and tuff are “born of fire,” coming directly from the molten state, they belong to the great group of igneous rocks (from Latin: ignis, fire; compare ignite). Igneous rocks are almost surely more abundant than any other kind in the crust of the earth, and those that erupt on the earth’s surface, the volcanic rocks, are an important variety.

The two characteristics – the presence of vesicles and a texture of very fine grains embedded in glass – are earmarks of lava. They can be found far from any other evidence of volcanic activity.