khoahoc.vietjack.com

Danh sách câu hỏi ( Có 2,554,227 câu hỏi trên 51,085 trang )

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.For a century and a half the piano has been one of the most popular solo instruments for Western music. Unlike string and wind instruments, the piano is completely self- sufficient, as it is able to play both the melody and its accompanying harmony at the same time. For this reason, it became the favorite household instrument of the nineteenth century.The ancestry of the piano can be traced to the early keyboard instruments of the fifteenth and sixteenth centuries-the spinet, the dulcimer, and the virginal. In the seventeenth century the organ, the clavichord, and the harpsichord became the chief instruments of the keyboard group, a supremacy they maintained until the piano supplanted them at the end of the eighteenth century. The clavichord's tone was never powerful, nevertheless, because of the variety of tone possible to it, many composers found the clavichord a sympathetic instrument for intimate chamber music. The harpsichord with its bright, vigorous tone was the favorite instrument for supporting the bass of the small orchestra of the period and for concert use but the character of the tone could not be varied save by mechanical or structural devices .The piano was perfected in the early eighteenth century by a harpsichord maker in Italy (though musicologists point out several previous instances of the instrument). This instrument was called a piano e forte (soft Mid loud), to indicate its dynamic versatility; its strings were struck by a recoiling hammer with a felt-padded head. The wires were much heavier in. the earlier instruments. A series of mechanical improvements continuing well into the nineteenth century, including the introduction of pedals to sustain tone or to soften it, the perfection of a metal frame, and steel wire of the finest quality, finally produced an instrument capable of myriad tonal effects from the most delicate harmonies to an almost orchestral fullness of sound, from a liquid, singing tone to sharp, percussive brilliance.What does the passage mainly discuss?

Xem chi tiết 1.7 K lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet te indicate the correct answer to each of the questions from 31 to 35.The walnut tree produces wood that is used for countless purposes, and is considered the finest wood in the world. The wood is easy to work with, yet it is very hard and durable and when it is polished, it produces a rich, dark luster. It also shrinks and swells less than any other wood, which makes it especially desirable for fine furniture, flooring, and even gun stocks.In fact, just about every part of the walnut is unusually hard and strong. The nut of the tree is encased inside a very hard shell, which itself is enclosed in a leathery outer covering called a husk. It requires real effort to break through those layers to get at the tasty meat inside.Yet every part of the walnut is useful to people. The outer husk produces a dark reddish stain that is hard to remove from the hands of the person who opens the nut, and this pigment is widely used in dyes and wood stains. The inner shell is used as an abrasive substance to clean jet engines. And the meat of the nut is extensively used in cooking, ice cream, flavorings-and just eaten raw.Walnut trees exude a chemical into the soil near their roots which can be poisonous to some trees and shrubs. Fruit trees, for example, will not survive if planted too close to a walnut. Many other plants, such as maple trees or ivy, are not affected by the walnut's presence, and are well-suited to grow in its vicinity.As used in the passage, the underlined word "exude" most nearly means_____________

Xem chi tiết 540 lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet te indicate the correct answer to each of the questions from 31 to 35.The walnut tree produces wood that is used for countless purposes, and is considered the finest wood in the world. The wood is easy to work with, yet it is very hard and durable and when it is polished, it produces a rich, dark luster. It also shrinks and swells less than any other wood, which makes it especially desirable for fine furniture, flooring, and even gun stocks.In fact, just about every part of the walnut is unusually hard and strong. The nut of the tree is encased inside a very hard shell, which itself is enclosed in a leathery outer covering called a husk. It requires real effort to break through those layers to get at the tasty meat inside.Yet every part of the walnut is useful to people. The outer husk produces a dark reddish stain that is hard to remove from the hands of the person who opens the nut, and this pigment is widely used in dyes and wood stains. The inner shell is used as an abrasive substance to clean jet engines. And the meat of the nut is extensively used in cooking, ice cream, flavorings-and just eaten raw.Walnut trees exude a chemical into the soil near their roots which can be poisonous to some trees and shrubs. Fruit trees, for example, will not survive if planted too close to a walnut. Many other plants, such as maple trees or ivy, are not affected by the walnut's presence, and are well-suited to grow in its vicinity.The author of the passage probably believes that_____________.

Xem chi tiết 514 lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet te indicate the correct answer to each of the questions from 31 to 35.The walnut tree produces wood that is used for countless purposes, and is considered the finest wood in the world. The wood is easy to work with, yet it is very hard and durable and when it is polished, it produces a rich, dark luster. It also shrinks and swells less than any other wood, which makes it especially desirable for fine furniture, flooring, and even gun stocks.In fact, just about every part of the walnut is unusually hard and strong. The nut of the tree is encased inside a very hard shell, which itself is enclosed in a leathery outer covering called a husk. It requires real effort to break through those layers to get at the tasty meat inside.Yet every part of the walnut is useful to people. The outer husk produces a dark reddish stain that is hard to remove from the hands of the person who opens the nut, and this pigment is widely used in dyes and wood stains. The inner shell is used as an abrasive substance to clean jet engines. And the meat of the nut is extensively used in cooking, ice cream, flavorings-and just eaten raw.Walnut trees exude a chemical into the soil near their roots which can be poisonous to some trees and shrubs. Fruit trees, for example, will not survive if planted too close to a walnut. Many other plants, such as maple trees or ivy, are not affected by the walnut's presence, and are well-suited to grow in its vicinityAs used in the passage, the underlined word “abrasive” most nearly means_____________.

Xem chi tiết 1.4 K lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet te indicate the correct answer to each of the questions from 31 to 35.The walnut tree produces wood that is used for countless purposes, and is considered the finest wood in the world. The wood is easy to work with, yet it is very hard and durable and when it is polished, it produces a rich, dark luster. It also shrinks and swells less than any other wood, which makes it especially desirable for fine furniture, flooring, and even gun stocks.In fact, just about every part of the walnut is unusually hard and strong. The nut of the tree is encased inside a very hard shell, which itself is enclosed in a leathery outer covering called a husk. It requires real effort to break through those layers to get at the tasty meat inside.Yet every part of the walnut is useful to people. The outer husk produces a dark reddish stain that is hard to remove from the hands of the person who opens the nut, and this pigment is widely used in dyes and wood stains. The inner shell is used as an abrasive substance to clean jet engines. And the meat of the nut is extensively used in cooking, ice cream, flavorings-and just eaten raw.Walnut trees exude a chemical into the soil near their roots which can be poisonous to some trees and shrubs. Fruit trees, for example, will not survive if planted too close to a walnut. Many other plants, such as maple trees or ivy, are not affected by the walnut's presence, and are well-suited to grow in its vicinityWhat is the main idea of the passage?

Xem chi tiết 0.9 K lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet te indicate the correct answer to each of the questions from 31 to 35.The walnut tree produces wood that is used for countless purposes, and is considered the finest wood in the world. The wood is easy to work with, yet it is very hard and durable and when it is polished, it produces a rich, dark luster. It also shrinks and swells less than any other wood, which makes it especially desirable for fine furniture, flooring, and even gun stocks.In fact, just about every part of the walnut is unusually hard and strong. The nut of the tree is encased inside a very hard shell, which itself is enclosed in a leathery outer covering called a husk. It requires real effort to break through those layers to get at the tasty meat inside.Yet every part of the walnut is useful to people. The outer husk produces a dark reddish stain that is hard to remove from the hands of the person who opens the nut, and this pigment is widely used in dyes and wood stains. The inner shell is used as an abrasive substance to clean jet engines. And the meat of the nut is extensively used in cooking, ice cream, flavorings-and just eaten raw.Walnut trees exude a chemical into the soil near their roots which can be poisonous to some trees and shrubs. Fruit trees, for example, will not survive if planted too close to a walnut. Many other plants, such as maple trees or ivy, are not affected by the walnut's presence, and are well-suited to grow in its vicinityWhat is the topic of this passage?

Xem chi tiết 1.2 K lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 26 to 30.Books are essential to learning as they (26) _____________ unlimited information to the readers. If you want to know about dogs, you can easily learn about dog behavior, breeds, caring, and various topics about our canine friends by reading books. You don't have to go to school just to gather quick information about a subject. (27) _____________you need to do is grab a book, read, and absorb the contents.People who are in need of data can acquire them from books. It is not necessary to attend school in order to get information. A quick scan on books will give you the details, facts, and data that you need about a topic.Books are actually one of the best sources of information. One of the main reasons why they are (28) _____________as top sources of knowledge is because they provide unlimited facts to the readers. If you do a research, you can definitely find (29) _____________you are looking for from books. This is why we often turn to books or any reading materials whenever we have a research about a project. Whether you are a student or a professional, you will definitely gain something new whenever you open a page of a book.Before the invention of laptops and iPads, books were known as the ultimate source of information. Unlike the internet, you can bring any book you want with you anywhere you go. (30) _____________, they don't take up much space in your bag. You can easily carry them when you go out on a trip or while traveling to school.

Xem chi tiết 1 K lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 26 to 30.Books are essential to learning as they (26) _____________ unlimited information to the readers. If you want to know about dogs, you can easily learn about dog behavior, breeds, caring, and various topics about our canine friends by reading books. You don't have to go to school just to gather quick information about a subject. (27) _____________you need to do is grab a book, read, and absorb the contents.People who are in need of data can acquire them from books. It is not necessary to attend school in order to get information. A quick scan on books will give you the details, facts, and data that you need about a topic.Books are actually one of the best sources of information. One of the main reasons why they are (28) _____________as top sources of knowledge is because they provide unlimited facts to the readers. If you do a research, you can definitely find (29) _____________you are looking for from books. This is why we often turn to books or any reading materials whenever we have a research about a project. Whether you are a student or a professional, you will definitely gain something new whenever you open a page of a book.Before the invention of laptops and iPads, books were known as the ultimate source of information. Unlike the internet, you can bring any book you want with you anywhere you go. (30) _____________, they don't take up much space in your bag. You can easily carry them when you go out on a trip or while traveling to school.

Xem chi tiết 630 lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 26 to 30.Books are essential to learning as they (26) _____________ unlimited information to the readers. If you want to know about dogs, you can easily learn about dog behavior, breeds, caring, and various topics about our canine friends by reading books. You don't have to go to school just to gather quick information about a subject. (27) _____________you need to do is grab a book, read, and absorb the contents.People who are in need of data can acquire them from books. It is not necessary to attend school in order to get information. A quick scan on books will give you the details, facts, and data that you need about a topic.Books are actually one of the best sources of information. One of the main reasons why they are (28) _____________as top sources of knowledge is because they provide unlimited facts to the readers. If you do a research, you can definitely find (29) _____________you are looking for from books. This is why we often turn to books or any reading materials whenever we have a research about a project. Whether you are a student or a professional, you will definitely gain something new whenever you open a page of a book.Before the invention of laptops and iPads, books were known as the ultimate source of information. Unlike the internet, you can bring any book you want with you anywhere you go. (30) _____________, they don't take up much space in your bag. You can easily carry them when you go out on a trip or while traveling to school.

Xem chi tiết 598 lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 26 to 30.Books are essential to learning as they (26) _____________ unlimited information to the readers. If you want to know about dogs, you can easily learn about dog behavior, breeds, caring, and various topics about our canine friends by reading books. You don't have to go to school just to gather quick information about a subject. (27) _____________you need to do is grab a book, read, and absorb the contents.People who are in need of data can acquire them from books. It is not necessary to attend school in order to get information. A quick scan on books will give you the details, facts, and data that you need about a topic.Books are actually one of the best sources of information. One of the main reasons why they are (28) _____________as top sources of knowledge is because they provide unlimited facts to the readers. If you do a research, you can definitely find (29) _____________you are looking for from books. This is why we often turn to books or any reading materials whenever we have a research about a project. Whether you are a student or a professional, you will definitely gain something new whenever you open a page of a book.Before the invention of laptops and iPads, books were known as the ultimate source of information. Unlike the internet, you can bring any book you want with you anywhere you go. (30) _____________, they don't take up much space in your bag. You can easily carry them when you go out on a trip or while traveling to school.

Xem chi tiết 2.6 K lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 26 to 30.Books are essential to learning as they (26) _____________ unlimited information to the readers. If you want to know about dogs, you can easily learn about dog behavior, breeds, caring, and various topics about our canine friends by reading books. You don't have to go to school just to gather quick information about a subject. (27) _____________you need to do is grab a book, read, and absorb the contents.People who are in need of data can acquire them from books. It is not necessary to attend school in order to get information. A quick scan on books will give you the details, facts, and data that you need about a topic.Books are actually one of the best sources of information. One of the main reasons why they are (28) _____________as top sources of knowledge is because they provide unlimited facts to the readers. If you do a research, you can definitely find (29) _____________you are looking for from books. This is why we often turn to books or any reading materials whenever we have a research about a project. Whether you are a student or a professional, you will definitely gain something new whenever you open a page of a book.Before the invention of laptops and iPads, books were known as the ultimate source of information. Unlike the internet, you can bring any book you want with you anywhere you go. (30) _____________, they don't take up much space in your bag. You can easily carry them when you go out on a trip or while traveling to school.

Xem chi tiết 1.6 K lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.Any rock that has cooled and solidified from a molten state is an igneous rock. Therefore, if the Earth began as a superheated sphere in space, all the rocks making up its crust may well have been igneous and thus the ancestors of all other rocks. Even today, approximately 95 percent of the entire crust is igneous. Periodically, molten material wells out of the Earth's interior to invade the surface layers or to flow onto the surface itself. This material cools into a wide variety of igneous rocks. In the molten state, it is called magma as it pushes into the crust and lava when it runs out onto the surface.All magma consists basically of a variety of silicate minerals (high in silicon-oxygen compounds), but the chemical composition of any given flow may differ radically from that of any other. The resulting igneous rocks will reflect these differences. Igneous rocks also vary in texture as well as chemistry. Granite, for instance, is a coarse-grained igneous rock whose individual mineral crystals have formed to a size easily seen by the naked eye. A slow rate of cooling has allowed the crystals to reach this size. Normally, slow cooling occurs when the crust is invaded by magma that remains buried well below the surface. Granite may be found on the surface of the contemporary landscape, but from its coarse texture we know that it must have formed through slow cooling at a great depth and later been laid bare by erosion. Igneous rocks with this coarse-grained texture that formed at depth are called plutonic.On the other hand, if the same magma flows onto the surface and is quickly cooled by the atmosphere, the resulting rock will be fine-grained and appear quite different from granite, although the chemical composition will be identical. This kind of rock is called rhyolite. The most finely grained igneous rock is volcanic glass or obsidian, which has no crystals. Some researchers believe this is because of rapid cooling; others believe it is because of a lack of water vapor and other gases in the lava. The black obsidian cliffs of Yellowstone National Park are the result of a lava flow of basalt running head on into a glacier. Some of the glacier melted on contact, but suddenly there also appeared a huge black mass of glassy stone.Which of the following is another name for volcanic glass?

Xem chi tiết 1.1 K lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.Any rock that has cooled and solidified from a molten state is an igneous rock. Therefore, if the Earth began as a superheated sphere in space, all the rocks making up its crust may well have been igneous and thus the ancestors of all other rocks. Even today, approximately 95 percent of the entire crust is igneous. Periodically, molten material wells out of the Earth's interior to invade the surface layers or to flow onto the surface itself. This material cools into a wide variety of igneous rocks. In the molten state, it is called magma as it pushes into the crust and lava when it runs out onto the surface.All magma consists basically of a variety of silicate minerals (high in silicon-oxygen compounds), but the chemical composition of any given flow may differ radically from that of any other. The resulting igneous rocks will reflect these differences. Igneous rocks also vary in texture as well as chemistry. Granite, for instance, is a coarse-grained igneous rock whose individual mineral crystals have formed to a size easily seen by the naked eye. A slow rate of cooling has allowed the crystals to reach this size. Normally, slow cooling occurs when the crust is invaded by magma that remains buried well below the surface. Granite may be found on the surface of the contemporary landscape, but from its coarse texture we know that it must have formed through slow cooling at a great depth and later been laid bare by erosion. Igneous rocks with this coarse-grained texture that formed at depth are called plutonic.On the other hand, if the same magma flows onto the surface and is quickly cooled by the atmosphere, the resulting rock will be fine-grained and appear quite different from granite, although the chemical composition will be identical. This kind of rock is called rhyolite. The most finely grained igneous rock is volcanic glass or obsidian, which has no crystals. Some researchers believe this is because of rapid cooling; others believe it is because of a lack of water vapor and other gases in the lava. The black obsidian cliffs of Yellowstone National Park are the result of a lava flow of basalt running head on into a glacier. Some of the glacier melted on contact, but suddenly there also appeared a huge black mass of glassy stone.The word "finely" in the last paragraph is closest in meaning to_____________.

Xem chi tiết 563 lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.Any rock that has cooled and solidified from a molten state is an igneous rock. Therefore, if the Earth began as a superheated sphere in space, all the rocks making up its crust may well have been igneous and thus the ancestors of all other rocks. Even today, approximately 95 percent of the entire crust is igneous. Periodically, molten material wells out of the Earth's interior to invade the surface layers or to flow onto the surface itself. This material cools into a wide variety of igneous rocks. In the molten state, it is called magma as it pushes into the crust and lava when it runs out onto the surface.All magma consists basically of a variety of silicate minerals (high in silicon-oxygen compounds), but the chemical composition of any given flow may differ radically from that of any other. The resulting igneous rocks will reflect these differences. Igneous rocks also vary in texture as well as chemistry. Granite, for instance, is a coarse-grained igneous rock whose individual mineral crystals have formed to a size easily seen by the naked eye. A slow rate of cooling has allowed the crystals to reach this size. Normally, slow cooling occurs when the crust is invaded by magma that remains buried well below the surface. Granite may be found on the surface of the contemporary landscape, but from its coarse texture we know that it must have formed through slow cooling at a great depth and later been laid bare by erosion. Igneous rocks with this coarse-grained texture that formed at depth are called plutonic.On the other hand, if the same magma flows onto the surface and is quickly cooled by the atmosphere, the resulting rock will be fine-grained and appear quite different from granite, although the chemical composition will be identical. This kind of rock is called rhyolite. The most finely grained igneous rock is volcanic glass or obsidian, which has no crystals. Some researchers believe this is because of rapid cooling; others believe it is because of a lack of water vapor and other gases in the lava. The black obsidian cliffs of Yellowstone National Park are the result of a lava flow of basalt running head on into a glacier. Some of the glacier melted on contact, but suddenly there also appeared a huge black mass of glassy stone.Which of the following is produced when magma cools rapidly?

Xem chi tiết 579 lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.Any rock that has cooled and solidified from a molten state is an igneous rock. Therefore, if the Earth began as a superheated sphere in space, all the rocks making up its crust may well have been igneous and thus the ancestors of all other rocks. Even today, approximately 95 percent of the entire crust is igneous. Periodically, molten material wells out of the Earth's interior to invade the surface layers or to flow onto the surface itself. This material cools into a wide variety of igneous rocks. In the molten state, it is called magma as it pushes into the crust and lava when it runs out onto the surface.All magma consists basically of a variety of silicate minerals (high in silicon-oxygen compounds), but the chemical composition of any given flow may differ radically from that of any other. The resulting igneous rocks will reflect these differences. Igneous rocks also vary in texture as well as chemistry. Granite, for instance, is a coarse-grained igneous rock whose individual mineral crystals have formed to a size easily seen by the naked eye. A slow rate of cooling has allowed the crystals to reach this size. Normally, slow cooling occurs when the crust is invaded by magma that remains buried well below the surface. Granite may be found on the surface of the contemporary landscape, but from its coarse texture we know that it must have formed through slow cooling at a great depth and later been laid bare by erosion. Igneous rocks with this coarse-grained texture that formed at depth are called plutonic.On the other hand, if the same magma flows onto the surface and is quickly cooled by the atmosphere, the resulting rock will be fine-grained and appear quite different from granite, although the chemical composition will be identical. This kind of rock is called rhyolite. The most finely grained igneous rock is volcanic glass or obsidian, which has no crystals. Some researchers believe this is because of rapid cooling; others believe it is because of a lack of water vapor and other gases in the lava. The black obsidian cliffs of Yellowstone National Park are the result of a lava flow of basalt running head on into a glacier. Some of the glacier melted on contact, but suddenly there also appeared a huge black mass of glassy stone.Granite that has been found above ground has been_____________.

Xem chi tiết 0.9 K lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.Any rock that has cooled and solidified from a molten state is an igneous rock. Therefore, if the Earth began as a superheated sphere in space, all the rocks making up its crust may well have been igneous and thus the ancestors of all other rocks. Even today, approximately 95 percent of the entire crust is igneous. Periodically, molten material wells out of the Earth's interior to invade the surface layers or to flow onto the surface itself. This material cools into a wide variety of igneous rocks. In the molten state, it is called magma as it pushes into the crust and lava when it runs out onto the surface.All magma consists basically of a variety of silicate minerals (high in silicon-oxygen compounds), but the chemical composition of any given flow may differ radically from that of any other. The resulting igneous rocks will reflect these differences. Igneous rocks also vary in texture as well as chemistry. Granite, for instance, is a coarse-grained igneous rock whose individual mineral crystals have formed to a size easily seen by the naked eye. A slow rate of cooling has allowed the crystals to reach this size. Normally, slow cooling occurs when the crust is invaded by magma that remains buried well below the surface. Granite may be found on the surface of the contemporary landscape, but from its coarse texture we know that it must have formed through slow cooling at a great depth and later been laid bare by erosion. Igneous rocks with this coarse-grained texture that formed at depth are called plutonic.On the other hand, if the same magma flows onto the surface and is quickly cooled by the atmosphere, the resulting rock will be fine-grained and appear quite different from granite, although the chemical composition will be identical. This kind of rock is called rhyolite. The most finely grained igneous rock is volcanic glass or obsidian, which has no crystals. Some researchers believe this is because of rapid cooling; others believe it is because of a lack of water vapor and other gases in the lava. The black obsidian cliffs of Yellowstone National Park are the result of a lava flow of basalt running head on into a glacier. Some of the glacier melted on contact, but suddenly there also appeared a huge black mass of glassy stone.The word "it" in paragraph 2 refers to_____________

Xem chi tiết 1.6 K lượt xem 4 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.Any rock that has cooled and solidified from a molten state is an igneous rock. Therefore, if the Earth began as a superheated sphere in space, all the rocks making up its crust may well have been igneous and thus the ancestors of all other rocks. Even today, approximately 95 percent of the entire crust is igneous. Periodically, molten material wells out of the Earth's interior to invade the surface layers or to flow onto the surface itself. This material cools into a wide variety of igneous rocks. In the molten state, it is called magma as it pushes into the crust and lava when it runs out onto the surface.All magma consists basically of a variety of silicate minerals (high in silicon-oxygen compounds), but the chemical composition of any given flow may differ radically from that of any other. The resulting igneous rocks will reflect these differences. Igneous rocks also vary in texture as well as chemistry. Granite, for instance, is a coarse-grained igneous rock whose individual mineral crystals have formed to a size easily seen by the naked eye. A slow rate of cooling has allowed the crystals to reach this size. Normally, slow cooling occurs when the crust is invaded by magma that remains buried well below the surface. Granite may be found on the surface of the contemporary landscape, but from its coarse texture we know that it must have formed through slow cooling at a great depth and later been laid bare by erosion. Igneous rocks with this coarse-grained texture that formed at depth are called plutonic.On the other hand, if the same magma flows onto the surface and is quickly cooled by the atmosphere, the resulting rock will be fine-grained and appear quite different from granite, although the chemical composition will be identical. This kind of rock is called rhyolite. The most finely grained igneous rock is volcanic glass or obsidian, which has no crystals. Some researchers believe this is because of rapid cooling; others believe it is because of a lack of water vapor and other gases in the lava. The black obsidian cliffs of Yellowstone National Park are the result of a lava flow of basalt running head on into a glacier. Some of the glacier melted on contact, but suddenly there also appeared a huge black mass of glassy stone.The word "contemporary'' in paragraph 2 is closest in meaning to____________

Xem chi tiết 2.7 K lượt xem 4 năm trước