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Carbonation is the process in which atmospheric carbon dioxide leads to solution weathering. Carbonation occurs on rocks which contain calcium carbonate such as limestone and chalk. Carbonation takes place when rain combines with carbon dioxide to form a weak carbonic acid which reacts with calcium carbonate (the limestone) and forms calcium bicarbonate.

This process speeds up with a decrease in temperature and therefore is a large feature of glacial weathering as carbon dioxide is more soluble at lower as opposed to higher temperatures.

The reactions as follows:

Within the weathering environment chemical oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Fe²⁺ (iron) and combination with oxygen and water to form Fe³⁺ hydroxides and oxides such as goethite, limonite, and hematite. This gives the affected rocks a reddish-brown coloration on the surface which crumbles easily and weakens the rock.

This process is better known as 'rusting'.

The US state of New Hampshire is mourning the loss of its most famous landmark - a mountainside formation shaped like a face.

The renowned Old Man of the Mountain - a natural stone profile resembling the features of an old man - has collapsed into a pile of rubble.

The ancient edifice was considered New Hampshire's state symbol and appeared on countless souvenirs and even the state's quarter coin.

Police said the Old Man likely succumbed to the forces of nature.

The landmark, which was situated in New Hampshire's White Mountains, was reported missing by workers on Saturday.

"It's a strange and unreal feeling that something that has been there all your life is gone," said Amy Cyrs, one of the first people to notice it had disappeared.

The 40-foot-high (12-metre) profile, which was made out of five granite ledges, was created by geological changes that began 200 million years ago

The image was immortalised in Nathaniel Hawthorne's story "The Great Stone Face", read by generations of US schoolchildren.

The feature helped draw hundreds of thousands of tourists to the area every year and was one of the most photographed sites in the state.

"This is a symbol of our state. It's a sad feeling of emptiness," said Richard McLeod, director of state parks.

Now a giant mountainside scar is all that remains of the Old Man.

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