Earths mantle lies below the crust and above the core. Scientists can

     Earth’s mantle lies below the crust and above the core. Scientists cannot examine the mantle directly but have inferred its structure by measuring how seismic waves from earthquakes are distorted as they traverse it. In laboratory experiments, researchers have also studied mineral formation at the high pressures and temperatures in the mantle. These measurements and experiments indicate that the mantle consists of several concentric layers, with different minerals predominating at different depths. As pressures and temperatures increase toward the core, they force the elements to rearrange into new crystal structures, forming different minerals.

     Until 2004, scientists believed that the lower mantle was fairly uniformly composed of a dense form of magnesium silicate belonging to a family of crystals called perovskite that extended all the way down to the boundary between the mantle and the core. But seismic measurements revealed the lower mantle’s bottommost layer to be substantially denser than the rest. Since magnesium silicate perovskite has a tightly packed geometry that seemed to maximize the mass per unit volume, the higher pressures in this bottommost layer were not expected to change the perovskite’s geometry. Thus, scientists assumed that a greater abundance of heavy elements within the crystal must explain the higher density. However, this assumption was also problematic, because convection should stir the lower mantle, mixing the layers and producing a uniform distribution of elements.

     In 2004, geophysicists managed to expose magnesium silicate to the extreme pressures and temperatures of this bottommost layer, producing an unexpected new, denser crystalline structure they called postperovskite. Because postperovskite would conduct heat away from Earth’s core more rapidly than perovskite would, scientists hypothesize that the early Earth’s core must have been hotter than previously thought. This suggests that the inner core cooled enough to solidify only about one billion years ago. The solid inner core strengthens Earth’s magnetic field. This field shields the surface from radiation from space, which can cause genetic mutations and would be especially dangerous for life on land. Thus, the growing intensity of the magnetic field about one billion years ago may have first allowed life to expand from the seas onto dry land. So postperovskite's discovery may help explain the timing of developments in life’s evolution.

1. As evidence that postperovskite is present in Earth’s mantle, the passage mentions the

A. timing of developments in the evolution of life on Earth
B. different minerals predominating at different depths in the core
C. results of exposing magnesium silicate to high pressures and temperatures
D. indirectly measured temperature of Earth’s inner core
E. dangers that genetic mutations pose for life on Earth

2. According to the passage, the scientists’ hypothesis that the early Earth’s core must have been hotter than previously thought is based on

A. measurements of seismic waves from earthquakes that establish a layer of Earth’s inner core has been losing heat
B. samples of billion-year-old magnesium silicate crystals whose density is similar to that of postperovskite
C. a theoretical model demonstrating the range of temperatures in which postperovskite can be converted to perovskite
D. the presence of fossils showing that organisms were living on dry land before one billion years ago
E. differences between the heat-conduction properties of perovskite and those of postperovskite in the lower mantle

3. Based on the passage, the implications of the discovery of postperovskite most likely challenged many geophysicists' beliefs about

A. why Earth’s inner core strengthens Earth’s magnetic field
B. when the elements in Earth’s inner core first became solid
C. which elements are present in Earth’s core and mantle
D. whether crystalline magnesium silicate can form in Earth’s lower mantle
E. what causes magnesium silicate crystals to have a tightly packed geometry

4. Based on the passage, which of the following statements about perovskite is most likely true?

A. It could be transformed into postperovskite as it sinks through the mantle.
B. It may have a more tightly packed crystalline geometry than postperovskite does.
C. It has been directly observed to distort slightly when seismic waves traverse it.
D. It contains heavy elements in greater quantities than postperovskite does.
E. It might comprise several concentric layers of different minerals below the mantle.