EARTH: VOLCANOES

The volcanoes:

    A terrible explosion occurs, the Earth trembles and the sky darkens: a volcano erupts, spitting through its crater of incandescent lava and releasing clouds of ashes and toxic fumes. The volcanoes then are chimneys or cracks on the Earth’s crust. They allow molten rock (magma) to rise from the burning interior of the Earth and spread to the surface. An active volcano may erupt continuously and eventually form a wide mountain with gentle slopes. Other volcanoes, known as dormant, rarely erupt, but if an eruption occurs, the violence of its explosion will be such that it can destroy its cone and flanks. Many mountains are ancient extinct volcanoes. There are currently nearly 1,500 active volcanoes on Earth, and many more under the oceans.

An erupting volcano: (from within);

    In the mantle, under the Earth’s crust, the rock is not as solid as on the surface but is a molten material called magma. Sometimes this magma is pushed to the surface and lodges under the crust in a magmatic chamber (cavity located under the volcano). As the magma accumulates, the pressure increases, until the volcano chimney, obstructed, opens in a terrible explosion. The crater then acts as a rifle barrel from which flows lava, rocks, ashes and steam.

All shapes and sizes!

    The shape of a volcano depends on the viscosity of its lava, the frequency and the amplitude of its eruptions.
Strato-volcanoes (dome-shaped) create cones formed by the layers of lava and ash they produce. The cracked volcanoes are flat and the lava oozes through long cracks in the crust. Calderas, such as Crater Lake in Oregon, USA (right), are surrounded by low-lying craters, likely caused by an explosion that collapsed the pre-existing mountain.

Hot Spot!

    Most volcanoes form when land plates collide or move away from each other. However, some, such as the HAwaii Islands, appear in the middle of a plate and result from a hot spot> in the Earth’s mantle. Like a blowtorch, it pierces the oceanic crust and forms a volcano . The eruption of the volcano gradually decreases when the moving plate moves away from the hot point> and a new volcano is formed.

The volcanic breath:

    In Iceland, scientists wear gas masks to control toxic gases that escape from a fumarole (small volcanic chimney). These places are regularly tested. An increase in gas pressure or a change in gas composition may indicate an impending eruption.

Mont sans egal!

    The graceful flanks of Mount Fuji, symbol of Japan, amount to more than 3700m. Its perfect cone is made up of successive layers of lava and ashes. Its last eruption dates from 1707. Since then, it has been dormant.

Volcanic paradise!

    More than 1,000 years ago, groundwater heated by volcanic activity spilled over the slopes of the famous Pamukkale Plateau in Turkey. The salts of the water crystallized to create a magical landscape of "frozen" falls, stalactites and ponds. Since time immemorial, people have bathed in its warm waters.