Many rocks, such as granite, are made up of grains of various minerals which differ in color and in their capacity to absorb heat, and which therefore contract and expand in different ratios.  In heating and cooling these grains crowd against their neighbors and tear loose from them, so that finally the rock disintegrates into sand.

The conditions for the destructive action of heat and cold are most fully met in arid regions when vegetation is wanting for lack of sufficient rain.  The soil not being held together by the roots of plants is blown away over large areas, leaving the rocks bare to the blazing sun in a cloudless sky.  The air is dry, and the heat received by the earth by day is therefore rapidly radiated at night into space.  There is a sharp and sudden fall of temperature after sunset, and the rocks, strongly heated by day, are now chilled perhaps even to the freezing point.

In the Sahara the thermometer has been known to fall 131 degrees F. within a few hours.  In the light air of the Pamir plateau in central Asia a rise of 90 degrees F. has been recorded from seven o’clock in the morning to one o’clock in the afternoon.  On the mountains of southwestern Texas there are frequently heard crackling noises as the rocks of that arid region throw off scales from a fraction of an inch to four inches in thickness, and loud reports are made as huge bowlders split apart.  Desert pebbles weakened by long exposure to heat and cold have been shivered to fine sharp-pointed fragments on being placed in sand heated to 180 degrees F. Beds half a foot thick, forming the floor of limestone quarries in Wisconsin, have been known to buckle and arch and break to fragments under the heat of the summer sun.

Frost. By this term is meant the freezing and thawing of water contained in the pores and crevices of rocks.  All rocks are more or less porous and all contain more or less water in their pores.  Workers in stone call this “quarry water,” and speak of a stone as “green” before the quarry water has dried out.  Water also seeps along joints and bedding planes and gathers in all seams and crevices.  Water expands in freezing, ten cubic inches of water freezing to about eleven cubic inches of ice.  As water freezes in the rifts and pores of rocks it expands with the irresistible force illustrated in the freezing and breaking of water pipes in winter.  The first rift in the rock, perhaps too narrow to be seen, is widened little by little by the wedges of successive frosts, and finally the rock is broken into detached blocks, and these into angular chip-stone by the same process.

It is on mountain tops and in high latitudes that the effects of frost are most plainly seen.  “Every summit” says Whymper, “amongst the rock summits upon which I have stood has been nothing but a piled-up heap of fragments” (Fig. 7).  In Iceland, in Spitsbergen, in Kamchatka, and in other frigid lands large areas are thickly strewn with sharp-edged fragments into which the rock has been shattered by frost.