2. During Earth's very early history, our planet was whacked by meteorites large and small, and eventually grew to its current size. Earth probably went through a period in which it was molten from near the surface to its center.
Throughout its history, Earth has been hotter at its center than at its outer surface.
(a) In caves just below Earth's ground surface in continental crust, the temperature tends to be a constant
-15°C (-59°F). (That's why people use caves for wine storage.) The deepest mine on Earth is currently the Mponeg
gold mine just southwest of Johannesburg, South Africa, which reaches just over 4 km below the surface. At the
bottom of that mine, the rock temperature reaches 66°C (-151°F). Using the cave temperature to represent the
surface temperature of the crust at -0 km depth, what is the rate at which temperature changes in Earth between
O and 4 km-the near-surface geothermal gradient?
°C/km