TAILIEUCHUNG - The official guide to the toefl ibt third edition part 38

For the Speaking section, test takers wear noise-cancelling headphones and speak into a microphone. Responses are digitally recorded and sent to ETS's Online Scoring Network. | TOEFL iBT Practice Test 1 GEOLOGY AND LANDSCAPE Most people consider the landscape to be unchanging but Earth is a dynamic body and its surface is continually altering slowly on the human time scale but relatively rapidly when compared to the great age of Earth about 4 500 billion years . There are two principal influences that shape the terrain constructive processes such as uplift which create new landscape features and destructive forces such as erosion which gradually wear away exposed landforms. Hills and mountains are often regarded as the epitome of permanence successfully resisting the destructive forces of nature but in fact they tend to be relatively short-lived in geological terms. As a general rule the higher a mountain is the more recently it was formed for example the high mountains of the Himalayas are only about 50 million years old. Lower mountains tend to be older and are often the eroded relics of much higher mountain chains. About 400 million years ago when the present-day continents of North America and Europe were joined the Caledonian mountain chain was the same size as the modern Himalayas. Today however the relics of the Caledonian orogeny mountain-building period exist as the comparatively low mountains of Greenland the northern Appalachians in the United States the Scottish Highlands and the Norwegian coastal plateau. The Earth s crust is thought to be divided into huge movable segments called plates which float on a soft plastic layer of rock. Some mountains were formed as a result of these plates crashing into each other and forcing up the rock at the plate margins. In this process sedimentary rocks that originally formed on the seabed may be folded upwards to altitudes of more than 26 000 feet. Other mountains may be raised by earthquakes which fracture the Earth s crust and can displace enough rock to produce block mountains. A third type of mountain may be formed as a result of volcanic activity which occurs in regions of active fold

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