THE WORLD IN THE MAKING 25 



Without much question the depression and surrounding rim 

 mark the spot where some huge meteoric mass has plunged into 

 the^earth. Lunar ''craters" likely have had a similar origin. 



So according to the planetesimal hypothesis our earth can 

 trace her history back to the time when she was one of the larger 

 knots of matter in the arm of a spiral nebula (Fig. 21). Because 

 of the original size of this knot and the abundant meteoric 

 material her changing orbit brought her into, she grew apace as 

 falling planetesimals were added to her bulk, and now she is 

 the fifth largest of the planets in our solar system. Jupiter, the 

 largest, is more than thirteen hundred times the size of our earth. 

 Saturn, Neptune, and Uranus are also larger. Venus, Mars, and 

 Mercury are smaller. 



As the earth thus grew in size it also became hot. The 

 succession of blows delivered by the hail of planetesimals gen- 

 erated heat, as the anvil becomes hot under repeated hammer 

 strokes or a nailhead is heated when the nail is being driven into 

 hard wood. This heat was superficial and quickly radiated. 

 The central portion of the earth was, however, constantly being 

 pressed upon by the overlying accumulating mass. As the earth 

 grew constantly by accession the central portion was condensed 

 more and more. We are all familiar with the fact that heat 

 makes things expand. The mercury in the thermometer bulb 

 expands more than the glass and so rises in the tube with 

 increased heat. The opposite of this is also true : as substances 

 condense heat is liberated; so the earth's interior became heated. 

 The condensation of the central mass also caused molecule to 

 rub against molecule as crushing went on and so added to the 

 central heat. The pressure was so great near the center that 

 the rock material could not actually melt. But nearer the 

 surface, especially where up-arching of the crust reheved the 

 pressure temporarily, the rock material melted, oozed up toward 

 the surface, infiltrated between the piled-up chunks and dust, 

 cementing these into a solid mass as the molten material cooled. 

 Thus formed, likely, the early igneous rocks. 



