When lava flows, hot springs, geysers, etc., are naturally cooled by the climate they interact with when they reach the surface, how is core temperature sustainable?
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There is no magma in the middle of the earth. The inner core is solid metal and the out core is liquid metal. The center is very warm, it is hotter the the surface of the sun, the reason it is solid is because of the high pressure
Around the core, you have the malte that is made of silicate rock. It is predominantly solid, but in geological time it behaves as a viscous fluid. The reason it is solid is the hight pressure.
Magma is molten or semi-molten material. It is material from the crust or the mantle. It is just below or in the crust.
There is two main part of why the center of the earth is warm.
One major part is that all the heat generated when the earth was formed and all materials collided still has not escaped. The crust works as a layer of insulation.
Large objects take a long time to cool because the heat escapes through the surface and the volume-to-surface ratio is smaller the larger the object is. A human has a volume of around 0.1 cubic meters and a surface area of 2 square meters. that is a ratio of 0.1/20= 0.005
The volume of a sphere is 4/3pi r^3 and the surface area is 4pir^2. So the area-to-volume ratio is 4/3pi r^3 /4pir^2 = r/3. Earth’s radius is 6371000m so a ratio of 2,123,666 This meant the volume compare to the surface is around 400 million times larger than a human. The rated heat is lost for the large objects is not comparable to the small object we are used to in everyday life
So cool down time for an earth-sized object just because of this is hundred of million to billion of years. It is not enough to explain what we observe but a major part of it.
The second part is the radioactive decay of material in Earth. The released energy will be converted to heat It is primary Uranium, thorium, and potassium that release energy that way. The amount of energy released today is equal to around 50% of the energy that escapes from earth’s internals to the surface. The half-life of that element is 700 million to 4.5 billion of years, which is why they still exist. Earth is also around 4.5 billion years old so the initial energy they release will be more than double, adding to that shorter-lived element that is gone today.
So there is no magma in the core of earth, it is close to the surface. The inner part of Earth is still hot, it is just not magma.
The reason the inner part is hot is the remaining energy from the formation of Earth that take an enormous time to escape a large object like a planet plus the heat that radioactive decay produced.
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