system
Given the following post, respond as if you're commenting on Reddit:
human
From my 12 yr old while watching the Nova Kilauea episode: If I threw a rock in the lava coming out of the volcano, how long would it take to melt? Or would it?

We were watching, he asked, and I got curious, too! How long would it take? 

Or is the lava cooled enough that it doesn't have enough heat to melt rock? 

Or does it depend on the type of rock?
gpt
This is a tough question. Geologists have argued about it for years!

First, do you mean lava or magma? Lava is magma that is erupted. The difference is important for two reasons...

1) Basaltic lava is generally the hottest kind of lava. It is about 1200 C maximum when it erupts. Even basaltic lava cannot melt rocks. When a basaltic lava meets solid rock, the rock wins. The rock cools the basaltic lava.

What evidence tells us this? Well, if you ever see a chunk of rock caught in basaltic lava, look closely at the solidified lava around the rock chunk. You may see that the crystals in the basaltic lava are smaller than the crystals away from the rock chunk. This is because the crystals near the rock chunk cooled faster and therefore had less time to grow.

But this isn't the only evidence. We can do calculations and they tell us that it takes a lot longer for a rock to heat up and melt than it does for lava to cool and solidify. Lava that is erupted solidifies fast. What is the reason for this difference? Lava cools by convection and solid things like rocks cool or heat up by conduction. Conduction is much slower than convection.

2) Basaltic MAGMA that has NOT erupted can melt rocks, but it cannot melt basaltic rocks. Basaltic magma can REACT with solid basalt, but it can't MELT it. Basaltic magma CAN melt many rocks other than basalt, like rhyolite/granite. Rhyolite has a lower melting temperature than basalt. In other words, basaltic magma does not have enough energy to melt other basalt. But it does have enough energy to melt rhyolite.


Here's some more information that might help you:
The reason that basaltic magma that has not erupted can melt rocks is that it stays warm for much longer than lava. Because conditions are warmer beneath the surface. But remember that even beneath the surface, basaltic magma cannot usually melt a chunk of solid basalt. It can react with it and dissolve it like an aspirin in water, but it can't melt it. This is because the energy it takes to melt something of the same composition is about equal to the energy the liquid has.

To calculate the time it takes for a rock to melt, you need to know the size of the rock.