Under Increasing Pressure

　　Since liquid rock is less dense than solid rock, magma begins to rise through the Earth’s crust. It forces its way up, melting surrounding rock and increasing the amount of magma.

　　Magma only stops rising when the pressure from the rock layer above it becomes too great. It gathers below the Earth’s surface in a “magma chamber”.

　　When the pressure increases in the chamber, the crust finally gives way and magma spews out onto the Earth’s surface forming a volcano. When it reaches the surface, magma becomes known as “l(fā)ava”.

　　Volcanic eruptions vary in intensity and appearance depending on two factors:

　　the amount of gas contained in the magma

　　its viscosity—how runny it is

　　In general, the explosive eruptions come from high gas levels and high viscosity in the magma. Whilst lava floes result from magmas with low gas levels and low viscosity.

　　Mount Etna

　　Over millions of years, lava and ash from eruptions might build up to form a mountainous volcano like Mt Etna in Italy.

　　Hurricanes

　　By definition, a hurricane is fierce rotating storm with an intense centre of low pressure that only happens in the tropics. In south-east Asia they’re known as typhoons and in the Indian Ocean, cyclones.

　　They cause high winds, huge waves, and heavy flooding. In 1998, Hurricane Gilbert produced 160mph winds, killing 318 people, and devastating Jamaica. A tropical storm can only be classified as a hurricane if it sustains wind speeds above 73mph or force 12 on the Beaufort Scale. Each year about 50 tropical storms reach hurricane status.

　　One of the most powerful of all weather systems, hurricanes are powered by the heat energy released by the condensation of water vapour. However, the conditions have to be exact for a hurricane to form, with the sea’s surface temperature being above 26.5°C.

　　How Hurricanes Happen

　　Air above warm tropical water rises quickly as it is heated by the sea. As the air rises it rotates or spins creating an area of low pressure, known as the eye of the storm. The eye can be clearly seen on satellite pictures, and is usually eerily calm.

　　The hurricane only moves slowly at speeds of 20-25mph bringing torrential rain and thunderstorms and very strong winds. However, they also cause flooding on low lying coastlines with a phenomenon known as a “storm surge”.

　　Storm Surge

　　This is caused by the intense low pressure at the eye of a hurricane, combining with the effect of strong winds. The sea rises 1cm for every millibar of pressure - if the pressure is 930 millibar, the sea surge will be about 80 cm. Hurricanes can raise the seas surface by as much as 4m.

　　The hurricane winds push the surge along in front of its path. When this surge hits low-lying coasts, the effects can be devastating. In addition to the sea surge, flooding can also result from torrential rain falling from the storm clouds.

　　Once it reaches the mainland, a hurricane may cause widespread damage for a few days, but with no warm water to supply heat, they quickly die out.

　　1. Earthquakes are caused by the motion of tectonic plates that make up the Earth’s surface.

　　2. The Earth is made up of four main layers: the core, the mantle, the crust and the magma.

　　3. It is the convection that make hot liquid rise to displace cooler liquid, creating a current.

　　4. The earthquake at the epicenter is weakest.

　　5. Both of volcanoes and earthquakes form at weak-points in the Earth’s crust when tectonic plates collide,.

　　6. Magma will fall through the Earth’s crust if the liquid rock is less dense than solid rock.

　　7. Mt Etna in Italy produced 160mph winds and killed 296 people in 2000.

　　8. A hurricane is defined as a fierce rotating storm with an intense centre of low pressure that_______.

　　9. Torrential rain, __________ are always brought by the hurricane.

　　10. If there is no warm water to supply heat, the hurricane will______.

　　相關鏈接：江濤英語：2008年6月英語四級寫作預測

　　　　　　　名師江濤：預測6月21日英語四六級真題

　　　　　　　2008年6月21日英語四六級試題答案發(fā)布預告

更多信息請訪問：考試吧四六級欄目

＞＞＞來四六級論壇看看吧

＞＞＞四六級博客圈，你開博了嗎？