Well there are Different levels of viscosity of magma in the earth's crust, you just got to know where the magma is according to the tectonic plates.
We know what the magma is like under the earth by studying Volcanoes and how they erupt and how the magma flows. The majority of Stratovolcano's or Composite Volcano's; (The tall explosive ones like MT. ST Helen's) usually explode and have dormant periods because of the plate boundary that they sit on.
On a Subduction zone the ocean plate is force under the lighter continental plate. But with it ocean water that has been trapped in the rocks alongside allot of Limestone melt in the mantel. Releasing allot of gases, which under the immense pressure of the earth dissolve in the semi molten rock, this makes it very thick and slow to move. Hence Stratovolcano, further up.
But you also get runny water like rock that isn't very viscous at all, in fact you can safely stand 100M away from it! or even out WALK it! You can find this at Hawaii out of Mt. Kilauea. The magma is like this because Hawaii sits on a Hot spot.
Magma.
Lava or Magma is rock in its molten state.
Magma is the molten material beneath the surface (crust) of the earth. Magma is called lava when it emerges from the earth in its molten state. The volcano is the geologic structure at the end of a volcanic chimney, which is the duct that connects the magma pocket in the earth through the crust to the surface. The volcano is actually at the "output end" of that volcanic tube.
Magma chambers form when molten rock, or magma, accumulates in underground reservoirs beneath the Earth's surface. This accumulation occurs as magma rises from the mantle due to pressure and temperature differences, often generated by tectonic processes such as subduction or rifting. Over time, as magma collects and cools, it may crystallize into igneous rock, but some remains in a liquid state, creating a chamber that can feed volcanic eruptions. Additionally, the presence of fractures and porous rock can facilitate the movement and storage of magma within these chambers.
This is called the magma chamber.
Your question as phrased is unclear. I will proceed to answer both interpretations I have of your question. Note: When magma crystallizes, it forms igneous rocks. 1) What is the change of state when magma forms from igneous rock? - Melting. Rocks are the solid state of matter and magma is the liquid state (although it can be a very viscous liquid). Changing something from solid to liquid is melting. 2) What is the change of state when magma crystallizes to form igneous rocks? - Solidification/Freezing. Magma becomes too cold to continue in a liquid state, it solidifies and crystallizes into solid igneous rocks: freezing.
Magma.
There is no solvent in magma. Rocks turn to a liquid state because of super-heating, not because the solids were dissolved in any solvent.
When the force of magma pushing upwards equals the force of the rock pushing downwards, the system is said to be in a state of equilibrium. This balance prevents the magma from continuing to rise or the rock from collapsing further.
Lava or Magma is rock in its molten state.
Magma, if underground, and Lava, if above the ground
Magma is the molten material beneath the surface (crust) of the earth. Magma is called lava when it emerges from the earth in its molten state. The volcano is the geologic structure at the end of a volcanic chimney, which is the duct that connects the magma pocket in the earth through the crust to the surface. The volcano is actually at the "output end" of that volcanic tube.
When the magma cools it hardens, turning from sort of a liquid into a solid. Just like if you melted cheese it would eventually harden back to its regular solid state.
Supercooled magma is not a state of matter itself, but rather a term used to describe magma that has cooled below its usual crystallization temperature. It is still considered a liquid state of matter, but in a highly viscous and partially solidified form.
An acid magma or a granitic magma.
Solids become liquids. A phase change.
Magma is molten rock beneath the Earth's surface that can sometimes rise to the surface through volcanic activity. It consists of a mixture of liquid rock, crystals, and dissolved gases. As magma cools and solidifies, it forms igneous rocks.