**2. History prior to the 1950s**

Long before seismology became a recognized discipline early in the 20th century, many scientists and philosophers had speculated on the nature of the interior of Earth. One of the earliest of these was Athanasius Kircher (1602–1680) whose view of Earth (**Figure 1**) was published in 1665 and entitled: Mundus Subterraneus [4].

Over the subsequent three centuries as more detailed information became available from improved scientific methods, especially in seismology, our view of the interior of Earth has evolved, but major features have remained unchanged (**Figure 2**).

Harvard University faculty dominated investigations of the problems related to mineral physics in the first half of the 20th century. The first was Percy Bridgman whose investigations of the properties if matter under high pressure earned him the Nobel Prize in Physics in 1946. Among the graduate students of Bridgman was Francis Birch, who focused on high-pressure studies relevant to geophysical problems. In his classical paper in 1952 [5], he demonstrated that the mantle is predominately composed of silicate minerals and that the upper mantle and lower mantle regions, each essentially homogeneous but of somewhat differing compositions, are separated by a thin transition zone associated with silicate phase transitions (see green band at 670 km in **Figure 3**). Birch also concluded that the inner and outer core are alloys of crystalline and molten iron, respectively, which contrasted with the prevailing views at the time. Although a few refinements have become necessary in light of subsequent research, the essential details of this model are still valid. For this paper and other contributions, Birch is widely acknowledged as the "father of mineral physics" (though he never used that term).

**Figure 1.** *View of earth in 1665 by Athanasius Kircher (1602–1680).*

### **Figure 2.**

*Views of the interior of the earth from 1665 to 1990. Courtesy of Ed Garnero.*

**Figure 3.** *Internal structure and composition of the Earth's interior.*
