Heat is constantly being produced by the little bit of radioactivity in the earth.
I suspect that 90% molten is a significant exaggeration.
Tells you something about how brief our time on Earth is.
7920 miles dia of planet earth
Earth’s crust is 5 miles deep under oceans, and 25 miles under continents.
Volume of sphere = 4/3 x Pi x R**3
Volume below the rocky crust based on average diameter of
7920 - 30 = 7890 miles
Which then computes the earth’s crust as 1.132% of
earths’s total volume.
Which means 98.868% of earth is below the solid crust outer shell.