Why is the Mantle Hot?
The Earth's mantle is hot due to a combination of factors: residual heat from planetary formation and ongoing radioactive decay within the mantle and core.
The Earth formed billions of years ago from the accretion of dust and gas. This process released enormous amounts of gravitational energy, converting it into heat. Much of this initial heat remains trapped within the planet's interior, contributing significantly to the mantle's high temperature. This initial heat source is comparable in importance to the heat generated by radioactive decay. [Source: Reddit r/askastronomy; Earth Observatory of Singapore FAQ]
Radioactive Decay
Radioactive isotopes, such as uranium, thorium, and potassium, are present within the Earth's core and mantle. The decay of these elements continuously generates heat through a process known as radioactive decay. This ongoing heat production is a substantial and ongoing contributor to the mantle's temperature. [Source: SERC Carleton; Reddit r/askastronomy; Earth Observatory of Singapore FAQ]
Inefficient Heat Loss
The large size of the Earth contributes to its internal heat retention. A low surface area to volume ratio makes heat loss from the planet's interior less efficient. The heat generated internally has difficulty escaping, thus maintaining the high temperatures of the mantle. [Source: SERC Carleton]
Summary
The Earth's mantle's high temperature is a result of the lingering heat from its formation and the continuous heat generation from radioactive decay within its core and mantle. The planet's size also plays a role by hindering efficient heat dissipation.
