We have pictured the water of the oceans thus sinking down into the hot rocks. It will not always cause an explosion at once. The steam may not immediately become free, but will become absorbed in the hot rocks till the lava grows so fully saturated by the hot vapour that it swells and requires more space. When the tension becomes great enough the crust begins to shake and the paroxysm continues till the steam-saturated lava moves along the nearest break or "fault" or vent. When the underlying molten rock has thus obtained more space the agitation ceases till the tension again becomes too powerful for the crust to withstand, when another readjustment takes place. A familiar illustration of this process is seen in the lid of a tea-kettle when the steam pressure accumulates till it sets the lid quivering. As the steam escapes at the sides the agitation slowly dies down and the lid then remains quiet till the accumulating pressure again requires relief, when the shaking is renewed. Thus the process is periodic, and the period depends on the rapidity with which the steam is developed. In the case of earthquakes, as already remarked, the steam is not free, but absorbed in the molten rock, and when the agitation begins this gives a similar quivering motion to the block of the earth's crust overlying it, and ceases only when readjustment occurs--usually by the neighbouring "fault" slipping in some way so as to give more space to the swelling lava beneath. Of course, many of the cracks caused by this swelling are never seen; and the molten lava seldom reaches the surface except when through volcano vents or cracks in mountains that are near the sea-shore; but such outbreaks are probably more common in the deep sea.