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ESO 4 B&G 8

The Earth's External Energy

Key Information

Vocabulary

Geological agent	Any agent able to alter the surface of our planet.
External geological agent	Geological agents powered by the Sun or the Earth's gravity: the water flows (streams, torrents, rivers, glaciers), the sea, the wind, the atmospheric phenomena (rainfall, thunderstorms, cyclones), the temperature variations or the living beings.
Weathering / Erosion	Weathering is the decomposition of rocks, soils and their minerals through the action of some of the external geological agents. It is called erosion when the decomposing agent also transports the resulting fragments away.
Clasts	They are the resulting rock grains of the weathering process. They are classified upon their size as follows: clay (< 0.004 mm), silt (< 0.06 mm), sand (< 2 mm), gravel (< 6 cm), cobbles (< 25 cm) and boulders (> 25 cm).
Frost shattering	Or freeze-thaw weathering. It's a kind of physical weathering usually produced by the expansion of the water filling the cracks in the rocks of the mountain areas where the daily temperature variation is around 0°C. The nightly pieces (wedges) of ice thus formed exert a pressure in the cracks called ice-wedging.
Gully	Landform that resembles a series of sharp and very short channels carved by intermittent running water, usually by streams, and typically formed on deforestated hillsides.
Ravine	Deep, narrow and short channel with steep sides, carved by intermittent running water, usually by torrents. Bigger than gullies and smaller than valleys.
Canyon / Gorge	An extreme type of V-shaped valley: narrow, deep and with very steep sides (cliffs), carved by running water, usually by rivers. If the sides are stepped, reflecting alternating rock resistances, it is called a canyon; otherwise it is a gorge.

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