This is a bit of a tricky question because it plays on the way we usually define "solid" and "liquid". While it's true that magma is molten rock found in the Earth's crust and upper mantle, the mantle itself isn't exactly a solid in the way we think of a rock.

Here's the breakdown:

* The mantle is a solid but behaves like a very viscous fluid over long periods. This is due to a concept called "solid-state flow". The immense pressure and heat within the mantle cause the rock to behave in a way that's both solid and fluid-like. Imagine a block of ice - it's solid, but if you apply enough pressure and heat over time, it will deform and flow. The mantle behaves similarly.

* Magma is formed when parts of the mantle melt. This happens due to a decrease in pressure or an increase in temperature. The melting point of rock is lower at lower pressures, so when rock rises from deeper within the mantle, it can begin to melt. This molten rock is what we call magma.

In summary:

* The mantle is solid, but its extreme pressure and heat allow it to behave as a very viscous fluid over geological timescales.

* Magma is formed when portions of the mantle melt due to changes in pressure or temperature.

So, while the mantle itself is technically solid, its behavior is more complex than simply being a solid rock.