Byron:
I have been pondering your question about the source of deep earth heat like the geothermal hot springs at Alum Rock Park, San Jose, California with Penatencia Creek water shed. There are numerous causes for heat deep in the earth. I can share many examples with you. Many people that I came across in my daily life never given it quite some thought. I doubt the gated public schools near by with their 20 feet high, 6.1 m fence given much thought. They are too busy taking tests. Those remedial neighborhood outback schools have the highest fence, 20 feet high, 6.1 meter high. I am not sure it is meant to keep me out or in? I always wondered about school that way. That is not the topic here.
The heat from the geothermal is from seismic faults opening up heat and allowing aquifers to infiltrate water to the heat along with the sulfur minerals. The topic here becomes geothermal gradient or the gradual increase of heat as the depth becomes deeper and deeper into the crust, Moho, asthenosphere, upper mantle, lower mantle, and nickel iron core. The nickel iron core is why we have compasses, magnetism, and paramagnetic - which will stick to magnets, diamagnetic - which will not likely stick to magnets like Au, aurum, or gold, and ferromagnetic - the magnet itself. Here is the lists of the reasons why the inside of the earth gets hotter, no it is not Pluto, the God of the under world or Hades, and burn in hell with the devil and Satan. Here are the causes of the hot earth, lava.
The earth was formed around 4.5 billions years ago by the sun's gravitational field attracting matter and spinning and orbiting around the sun. This is residual heat left over 4.5 billion years ago in the lower mantle.
There is uranium decay as it formed 4.5 billion years ago along with all the other elements attracting each other by gravity and the pressure from the weight into an equal geometry, thus the earth is very sphere like. That is why my mother's house cannot see your house in San Jose, because the earth is curved. The radioactive isotopes decay inside the earth's upper and lower mantle with lava are:
potassium 40
uranium 238
uranium 235
thorium 232
These allow rocks like Sierrian granitoid rocks to be dated. The earlier earth has more isotopes decaying away into more stable and thus safer elements have increased temperature in the lower mantle and causing convection currents like a hot pie filling in the over causing the crust to break up into plates and have geomorphological features like volcanoes, rift zones and valley, trenches, and seismic activity like earthquakes. The weight of the rocks and molten rocks cause the enormous pressure and friction, heat. There were natural nuclear fission reactions boiling underground aquifer water like geothermal in Gabon, West Africa, Oklo Fossil Reactor, where the uranium 235 fission away to uranium238 around 2 billion years ago. The uranium mine is Oklo, Gabon, West Africa fission away to less uranium 235 and more uranium 238, neodymium 142 and Ru 99, ruthenium, and molybdenum. Molybdenum is useful for lubrication greases. Random chance there was a natural source of 3% to 4% uranium 235 present at Oklo, West Africa, Gabon. That amount uranium 235 along with natural well water to act as a neutron moderator have fission about 2 billion years ago. The water would get hot. The early humanoids have not been present to used that heat to cook with as a natural hot spring water 2 billion years ago. Reactors and geothermal have been with us before language have been invented. Oklo Fossil boiling water fission reactor was discovered recently in our time in the present Holocene Cenozoic by Francis Perrin, France.
Meteorite and comet impacts have heated the earth up into lava.
Conclusion:
The heat in the earth is a geothermal gradient:
early formation of the earth by gravity
impact from early meteorites and comets
isotope element decay of the early earth
gravity, mass, and pressure
magnetism of the nickel iron core
Earthquakes are caused by the strain on the rocks at plate boundaries by underneath convection mantle currents and heat.
The rock from the early super hot earth is the Komatiites from deep into the mantle with high melting points and consists of low concentration aluminum, silicon, and potassium and really high magnesium and gold, copper, and nickel are mixed into the Komatiites.
Komatiites are found in:
Kamabalda, Western Australia
Barberton Belt, South Africa
Karelian greenstone belt in Northwest, Russia
greenstone belts in Canada
Komatiites depict the intense heat of the early earth and thin crust in the Achaean, 2.5 billion years ago.
The earth's water, the oceans and the seas came from a comet that smashed into the earth giving water to us.
I learned all of this and I do not intend to be placed behind a 20 feet high fence class room. I studied hard to be free. I built recumbent bicycles to be freed from dependency of oil and gasoline and no need for war machines. Japan has less war machines because they rely upon 55 boiling water reactors (11 have been shut down and 1 lost) along with liquefied petroleum gas, methane from diagenesis from kerogens, organic chemical compounds in sediments. Japan invaded China for oil, coal, and gas in the 1930s. Where else could Japan get their energy, even Fukushima One failed? Hong Kong city is nuclear power, too. I get along with my mother and brother, because I use recumbent bicycle and not use their car much or asked money for gasoline. If I was more careful with consumer spending, then I would have been totally free and independent. The goal is to be free, not dependent upon someone or thing, peace comes from that.
Lawsen