The Way of Science

Now it's time to describe the old model of the Earth's formation, which we will often refer to as the "shrinking apple" model.

1. The Old Model ("Shrinking Apple" Model)

Note: the two optional articles by Hallam and Hurley have excellent descriptions for this section, and for the following section. If you elect to read them, try to skip the excessive detail, which is beyond the level of this course. In Hurley's article, skip from the third column, p.59 to the first new paragraph on p. 62.

Prior to 1912, when Alfred Wegener first presented his ideas to a scientific meeting in Germany, one model for the Earth's formation and subsequent development held center stage. This explanation for the production, location and nature of the Earth's major surface features is frequently likened to a "shrinking apple", and it went like this.

The solar system formed from masses of dust and debris gradually accumulating into larger and larger masses. The attraction of particles for each other was, of course, due to gravity. (Note: so far, there is no significant difference between the "shrinking apple" model and modern views. The modern model supplies a date for this process, about 4.5 billion years ago. It's worth noting at this point that an American billion is not the same as the British billion, and thus you will commonly see this figure listed as "four thousand million".) As the proto-Earth continued to grow by this process of accretion, it became hotter and hotter from the pounding of more and larger chunks. Very early in the Earth's history, the heat becomes intense enough to melt the entire ball, and we have a mass of molten material. (So far, still so good; there is no fundamental difference yet between this portion of the "shrinking apple" model and the modern version.) In this molten state the materials of different densities migrated differentially toward the center of gravity. Lighter materials (silicon, aluminum) remained closer to the surface, and denser materials, like iron and other metals, moved toward the core. The resulting equilibrium based on density is called isostasy (also spelled "isostacy") from the Greek roots meaning "standing still." Again, there is still no significant disagreement with the modern scenario, although details differ. We know that the Earth's outer core is a liquid mass of metal which migrated there during this very early "iron catastrophe," as it is sometimes called. The inner core is solid, a consequence of the enormous pressure. Incidentally, you might think about how one obtains information about the core, or the mantle, when no direct access is even remotely possible. The deepest mines don't even come close to penetrating the crust, which is proportionally thinner than an eggshell. Isostasy can also be seen today, in areas that were heavily glaciated up to 10,000 years ago. "Isostatic rebound" is still occurring, as land that was pushed downward by the weight of miles-thick ice now slowly rises. Equilibrium is maintained. If you wish to keep a simple example for isostasy in mind, think of shaking a mixture of oil and water together, and then letting it sit without disturbance. The oil (less dense) rises, and the water (more dense) is pulled downward.

Let us return to the old model. Maintaining a molten state in the cold of space is a losing battle, and heat flowed out from the Earth. Clearly, the surface cooled before the interior portions did. In fact, the Earth is still very hot in its interior, the heat maintained by radioactive decay. More on that subject will come later in the semester. As the surface cooled, a "skin" or crust formed. Now there was a solid surface, crust, analogous to the apple's skin, over a liquid interior, analogous to the moist flesh of the fruit. As heat continued to be lost, volume decreased. It is true, of course, that most solids, liquids and gasses will lose volume (shrink) when cooled, and undoubtedly the Earth did also. However, the wrinkling of "shrinking apple" was supposedly responsible, directly or indirectly, for the majority of large-scale surface features. There are two problems here: first, and most important for this class, any wrinkling of the crust was very quickly erased by plate movement, as you will soon see. Second, the radioactivity mentioned earlier generates a great deal of heat, and so extensive shrinkage from cooling was doubted by a minority of specialists even early in the 20th century.