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.

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