四级复习资料大全阅读(29)

　　Earthquakes are caused by the motion of tectonic platesindividual sections that make up the Earths surface like panels on a football. Immense strain accumulates along fault lines where adjacent plates meet. When the rock separating the plates gives way, sudden seismic ground-shaking movement occurs.

　　Of course, if we want to know why earthquakes happen, we need to dig a little deeper.

　　Centre of the Earth

　　The Earth is made up of three main layers: The core is at the centre of the Earth. The mantle is a mobile semi-molten layer around the core. The outer-shell of the Earth is called the crust. Scientists call this the lithosphereits the part were on now.

　　The crust is made up of 12 individual tectonic plates. Below the sea, they can measure three to six miles thick and under land this increases to 20-44 miles . Below the crust, radiation from the Earths core heats the semi-molten mantle to temperatures of over 5000C.

　　All fluids when heatedeven molten rockare affected by a process called convection. That makes hot liquid rise to displace cooler liquid, creating a current. Tectonic plates effectively float on the mantle, like croutons in a bowl of super-heated soup. But these plates and constantly move due to the convection current.

　　Of course, they creep along very slowlyroughly the same speed your fingernails grow. Even at this sub snail-pace, the effects can be devastating. The combined annual force of earthquakes is equal to 100,000 times the power of the atomic bombs that flattened Hiroshima.

　　The point where the seismic activity occurs is the epicentre, where the earthquake is strongest. But it doesnt always end there; seismic waves travel out from the epicentre, sometimes creating widespread destruction as they pass.

　　Volcanoes

　　Without doubt, erupting volcanoes are the most awesome and terrifying sights in nature. In fact, their untamed destructive firepower has shaped and influenced many ancient cultures from Pompeii, to Japan.

　　Its estimated that one in ten of the Worlds population live within danger range of volcanoes. According to the Smithsonian Institute, there are 1511 active volcanoes across the globe, and many more dormant ones that could recharge at any moment.

　　When tectonic plates collide，like earthquakes, volcanoes form at weak-points in the Earths crust, known as fault-lines.

　　When two tectonic plates collide, the event can provide the catalyst for volcanic activity. As one section slides on top of the other, the one beneath is pushed down into the mantle. Water trapped in the crust can cause reactions within the mantle. Why? Well, its thought that when the water mixes with the mantle, it lowers the mantles melting point, and the solid mantle melts to form a liquid, known as magma.

　　Under Increasing Pressure

　　Since liquid rock is less dense than solid rock, magma begins to rise through the Earths 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 Earths surface in a magma chamber.

　　When the pressure increases in the chamber, the crust finally gives way and magma spews out onto the Earths surface forming a volcano. When it reaches the surface, magma becomes known as lava.

　　Volcanic eruptions vary in intensity and appearance depending on two factors: the amount of gas contained in the magma. its viscosityhow 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 theyre 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 seas surface temperature being above 26.5C.

　　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 Earths 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 Earths crust when tectonic plates collide,.

　　6. Magma will fall through the Earths 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______.