bottle until the gas inside blows the cap off, and you can understand something of how a volcano erupts. If the magma is runny, and the gas can escape easily, the eruption is gentle. But if the magma is thick, the gas has to build up more pressure to escape. Then the eruption is explosive.

　　When a volcano erupts, the molten rock that flows from it is called lava. The lava may run for some distance, or it may pile up near the volcano, making the mountain larger.

　　During an explosive eruption, pieces of rock are thrown from the volcano. Scientists call these pyroclasts (火山碎屑). The smallest pyroclasts are called ash. Ash is often thrown into the atmosphere by the force of the eruption. Larger rocks, about walnut (胡桃) size, are called lapilli, Italian for "little stones". The largest pyroclasts are called blocks if they are solid, or bombs if they are molten.

　　Volcanoes have been a major force in the formation of our planet for thousands and thousands of years. Today geologists continually observe active volcanoes for signs of eruption. Using modern instruments, they are usually able to issue a warning before the volcano erupts.

　　5．The author explains how the volcano erupts by .

　　A．making a comparison　 B．telling a story

　　C．using figures D．asking questions

　　解析：选A　写作手法题。根据第三段"Imagine shaking a soda bottle until the gas inside blows the cap off, and you can understand something of how a volcano erupts."可知，作者在说明火山怎样喷发时打了一个比方。

　　6．Whether the eruption is explosive depends on .

　　A．the temperature of the crust

　　B．whether the magma is thick

　　C．whether the magma is near the mantle

　　D．the number of gas bubbles inside the magma

　　解析：选B　细节理解题。根据第三段末三句"If the magma is runny ... Then the eruption is explosive."可知，火山喷发是否是爆发性的，取决于岩浆是否黏稠。其中的runny的意思是"松软的，有流动倾向的"。

　　7．We learn from Paragraph 5 that .

　　A．ash is smaller than pyroclasts

　　B．bombs are about walnut size

　　C．blocks are larger than lapilli

　　D．lapilli exist in the atmosphere

解析：选C　细节理解题。根据第五段"The largest pyroclasts are called blocks ..."可知