There’s actually a massive range of temperature for lava: 600 Celsius (1,112 Fahrenheit) to 1,600 Celsius (2,912 Fahrenheit). By comparison, it is estimated at 6,000 Celsius (10,800 Fahrenheit) in the core.
That’s ridiculously hot, even on the surface.
Check out how long it stays hot:
Lava cools very quickly at first and forms a thin crust that insulates the interior of the lava flow. As a result, basaltic lava flows can form crusts that are thick enough to walk on in 10-15 minutes but the flow itself can take several months to cool! Because of the insulating properties of lava, it cools slower and slower over time.
Thick stacks of lava flows (30 m or 100 ft thick) can take years to cool completely. An extreme example is a lava flow that was erupted in 1959 and partly filled a pit crater (Kilauea Iki). The “ponded” flow was about 85 meters thick (about 280 ft thick). It was drilled in 1988, and there was still some mushy, not-quite-solid stuff down near the bottom, 29 years after it erupted!
So, in other words, you could have a child and they could have a child and that child could be in middle-school because a lavaflow completely solidifies.
Apparently, mankind has only interacted with magma in the earth just a couple of times. However, one of those times we managed to harness it for power:
The Iceland Deep Drilling Project, while drilling several 5,000m holes in an attempt to harness the heat in the volcanic bedrock below the surface of Iceland, struck a pocket of magma at 2,100m in 2009. Being only the third time in recorded history that magma had been reached, IDDP decided to invest in the hole, naming it IDDP-1.
A cemented steel case was constructed in the hole with a perforation at the bottom close to the magma. The high temperatures and pressure of the magma steam were used to generate 36MW of power, making IDDP-1 the world’s first magma-enhanced geothermal system.