I don't think this sentence is true: > Most of Earth’s internal heat was delivered by the giant collisions that built the planet, a large portion of which rema

I don't think this sentence is true: > Most of Earth’s internal heat was delivered by the giant collisions that built the planet, a large portion of which remains locked inside its crust. In 19th century, Lord Kelvin estimated the age of Earth using this method, arriving at somewhere between 20 and 400 million years[1], much shorter than 4.6 billion years currently accepted. It turns out radioactive decay keeps the Earth's inside warm for a much longer time. (I.e., assuming Kelvin's calculation was correct, without radioactivity the Earth's core would have frozen over within the first 400M years.) [1] http://en.wikipedia.org/wiki/Age_of_the_Earth#Early_calculations http://en.wikipedia.org/wiki/Age_of_the_Earth#Early_calculat...