<P> The pressure at the bottom of the mantle is ~ 136 G Pa (1.4 million atm). Pressure increases as depth increases, since the material beneath has to support the weight of all the material above it . The entire mantle, however, is thought to deform like a fluid on long timescales, with permanent plastic deformation accommodated by the movement of point, line, and / or planar defects through the solid crystals comprising the mantle . Estimates for the viscosity of the upper mantle range between 10 and 10 Pa s, depending on depth, temperature, composition, state of stress, and numerous other factors . Thus, the upper mantle can only flow very slowly . However, when large forces are applied to the uppermost mantle it can become weaker, and this effect is thought to be important in allowing the formation of tectonic plate boundaries . </P> <P> Exploration of the mantle is generally conducted at the seabed rather than on land because of the relative thinness of the oceanic crust as compared to the significantly thicker continental crust . </P> <P> The first attempt at mantle exploration, known as Project Mohole, was abandoned in 1966 after repeated failures and cost over-runs . The deepest penetration was approximately 180 m (590 ft). In 2005 an oceanic borehole reached 1,416 metres (4,646 ft) below the sea floor from the ocean drilling vessel JOIDES Resolution . </P> <P> On 5 March 2007, a team of scientists on board the RRS James Cook embarked on a voyage to an area of the Atlantic seafloor where the mantle lies exposed without any crust covering, midway between the Cape Verde Islands and the Caribbean Sea . The exposed site lies approximately three kilometres beneath the ocean surface and covers thousands of square kilometres . A relatively difficult attempt to retrieve samples from the Earth's mantle was scheduled for later in 2007 . The Chikyu Hakken mission attempted to use the Japanese vessel Chikyū to drill up to 7,000 m (23,000 ft) below the seabed . This is nearly three times as deep as preceding oceanic drillings . </P>

Which of these is not a primary component of earths mantle