Which theory involves magma rising from the lower mantle to drive plate movement?

The theory that involves magma rising from the lower mantle to drive plate movement is called "Mantle Convection". This theory is a fundamental component of plate tectonics. According to the mantle convection theory, the Earth's mantle behaves like a fluid over geologic time scales and convects due to the heat emitted by the Earth's core and the heat produced by radioactive decay within the mantle itself. The heat causes the rocks in the mantle to expand and become less dense. This creates buoyant forces that can cause the hotter, less dense magma to rise towards the surface (this is known as upwelling).

When this magma reaches near the Earth's lithosphere, it spreads out, carrying the plates above it away from the spreading center. This process is part of what drives the movement of tectonic plates on the surface. The cooler, denser parts of the mantle eventually sink back into the deeper mantle, in a process known as subduction. This creates a sort of conveyor belt effect beneath the Earth's surface, which contributes to the movement of tectonic plates above it.

Extra: The concept of mantle convection is a key part of understanding how tectonic plates are able to move over Earth's surface. Tectonic plates are large slabs of rock that make up the Earth's lithosphere (the outermost shell of the planet). The Earth's lithosphere is divided into several major and minor tectonic plates that float atop the semi-fluid, convecting mantle layer known as the asthenosphere.

The boundaries where these plates meet are called plate boundaries and can be of several types: divergent (where plates move apart), convergent (where plates come together), and transform (where plates slide past one another). Divergent boundaries are often the sites of upwelling magma, which can lead to volcanic activity and the formation of new crust as the plates move apart. Convergent boundaries can result in one plate being thrust beneath another into the mantle in a process known as subduction, which can cause earthquakes and volcanic eruptions as well. Transform boundaries can also result in earthquakes due to the sheer force of the plates grinding past each other.

Mantle convection is the current understanding of the driving force behind this plate movement, though additional forces such as slab pull (the weight of a descending plate helping to pull the trailing lithosphere into a subduction zone) and ridge push (the gravitational force that causes a raised tectonic plate at a mid-ocean ridge to push away from the ridge and move toward a subduction zone) also play a role in plate tectonics.