<P> Ben Nevis is all that remains of a Devonian volcano that met a cataclysmic end in the Carboniferous period around 350 million years ago . Evidence near the summit shows light - coloured granite (which had cooled in subterranean chambers several kilometres beneath the surface) lies among dark basaltic lavas (that only form on the surface). The two lying side - by - side is evidence the huge volcano collapsed in on itself creating an explosion comparable to Thera (2nd millennium BC) or Krakatoa (1883). The mountain is now all that remains of the imploded inner dome of the volcano . Its form has been extensively shaped by glaciation . </P> <P> Research has shown igneous rock from the Devonian period (around 400 million years ago) intrudes into the surrounding metamorphic schists; the intrusions take the form of a series of concentric ring dikes . The innermost of these, known as the Inner Granite, constitutes the southern bulk of the mountain above Lochan Meall an t - Suidhe, and also the neighbouring ridge of Càrn Mòr Dearg; Meall an t - Suidhe forms part of the Outer Granite, which is redder in colour . The summit dome itself, together with the steep northern cliffs, is composed of andesite and basaltic lavas . </P> <P> Ben Nevis has a highland (alpine) maritime (oceanic) Polar climate (ET climate in Köppen's classification). Ben Nevis's altitude, maritime location and topography frequently lead to cool and cloudy weather conditions, which can pose a danger to ill - equipped walkers . According to the observations carried out at the summit observatory from 1883--1904, fog was present on the summit for almost 80 per cent of the time between November and January, and 55 per cent of the time in May and June . The average winter temperature was around − 5 ° C (23 ° F), and the mean monthly temperature for the year was − 0.5 ° C (31.1 ° F). In an average year the summit sees 261 gales, and receives 4,350 millimetres (171 in) of rainfall, compared to only 2,050 millimetres (81 in) in nearby Fort William, 840 millimetres (33 in) in Inverness and 580 millimetres (23 in) in London . Rainfall on Ben Nevis is about twice as high in the winter as it is in the spring and summer . Snow can be found on the mountain almost all year round, particularly in the gullies of the north face--with the higher reaches of Observatory Gully holding snow until September most years and sometimes until the new snows of the following season . </P> <Table> <Tr> <Th_colspan="14"> hide Climate data for Aonach Mòr Summit, elevation: 1,130 m or 3,710 ft (1981--2010) </Th> </Tr> <Tr> <Th> Month </Th> <Th> Jan </Th> <Th> Feb </Th> <Th> Mar </Th> <Th> Apr </Th> <Th> May </Th> <Th> Jun </Th> <Th> Jul </Th> <Th> Aug </Th> <Th> Sep </Th> <Th> Oct </Th> <Th> Nov </Th> <Th> Dec </Th> <Th> Year </Th> </Tr> <Tr> <Th> Average high ° C (° F) </Th> <Td> − 0.5 (31.1) </Td> <Td> − 0.9 (30.4) </Td> <Td> 0.1 (32.2) </Td> <Td> 2.2 (36) </Td> <Td> 5.6 (42.1) </Td> <Td> 7.6 (45.7) </Td> <Td> 9.7 (49.5) </Td> <Td> 9.5 (49.1) </Td> <Td> 7.4 (45.3) </Td> <Td> 4.3 (39.7) </Td> <Td> 1.3 (34.3) </Td> <Td> 0.5 (32.9) </Td> <Td> 3.9 (39) </Td> </Tr> <Tr> <Th> Daily mean ° C (° F) </Th> <Td> − 2.5 (27.5) </Td> <Td> − 3.0 (26.6) </Td> <Td> − 2.2 (28) </Td> <Td> − 0.4 (31.3) </Td> <Td> 2.9 (37.2) </Td> <Td> 5.0 (41) </Td> <Td> 7.3 (45.1) </Td> <Td> 7.4 (45.3) </Td> <Td> 5.3 (41.5) </Td> <Td> 2.3 (36.1) </Td> <Td> − 0.7 (30.7) </Td> <Td> − 1.8 (28.8) </Td> <Td> 1.6 (34.9) </Td> </Tr> <Tr> <Th> Average low ° C (° F) </Th> <Td> − 4.4 (24.1) </Td> <Td> − 5.1 (22.8) </Td> <Td> − 4.4 (24.1) </Td> <Td> − 2.9 (26.8) </Td> <Td> 0.1 (32.2) </Td> <Td> 2.3 (36.1) </Td> <Td> 4.9 (40.8) </Td> <Td> 5.2 (41.4) </Td> <Td> 3.1 (37.6) </Td> <Td> 0.2 (32.4) </Td> <Td> − 2.7 (27.1) </Td> <Td> − 4.1 (24.6) </Td> <Td> − 0.6 (30.9) </Td> </Tr> <Tr> <Td_colspan="14"> Source: Met Office </Td> </Tr> </Table>

Ehat is the tallest mountain in the world