... like I'm 5 years old
Imagine you're making a pot of tea. You put the water in, turn up the heat, and wait. As the water heats up, it starts to bubble and this is because of the heat. Now, apply this concept to the Earth. Volcanoes are like pots of tea for the Earth. The "water" in this case is molten rock or magma under the Earth's surface. When the pressure in the Earth builds up, it needs a place to escape, much like the steam from your boiling water. This escape is what we call a volcanic eruption.
Think of a volcano as the Earth's tea kettle. When the Earth gets too hot inside, it lets off some steam through a volcanic eruption!
... like I'm in College
Volcanoes erupt due to the movement of tectonic plates - huge slabs of the Earth's crust. When these plates move, they can cause cracks or faults to open up in the Earth's crust. The pressure from deep within the Earth forces molten rock, or magma, up through these cracks. As the magma rises, gases trapped within the magma are released. The release of these gases creates a great deal of pressure, which eventually leads to a volcanic eruption. The magma explodes from the volcano, cools, and forms a cone-shaped mountain around the opening.
To explain it with LEGO, gather a bunch of red, orange, and yellow bricks (to represent magma) and some brown and green bricks (to represent the Earth). Start by building a tall structure with your brown and green bricks, leaving a hole in the middle. This is your volcano.
Next, start filling up the hole with your red, orange, and yellow bricks, but remember to leave some space for the "eruption". Now, imagine the heat and pressure building up inside your LEGO volcano, just as it would inside a real one. When the pressure gets too much, the magma (your bright-colored bricks) will be forced up and out of the volcano.
This simple LEGO activity can help visualize the basic mechanics of a volcanic eruption. Just remember, in a real volcano, the process is much more violent and much, much hotter!
... like I'm an expert
Volcanic eruptions are highly complex phenomena and their mechanisms involve a range of geological processes. The key factor is the upwelling of magma from the Earth's mantle due to convection currents. This magma is less dense than the surrounding rock and thus, rises to the surface. As it ascends, the pressure decreases, allowing dissolved gases in the magma (mostly water, carbon dioxide, and sulfur dioxide) to exsolve and form bubbles.
The increase in volume leads to a further reduction in density, accelerating the ascent of the magma. When the pressure of the gas bubbles exceeds the weight of the overlying magma and the strength of the surrounding rocks, an eruption occurs. The style of the eruption (effusive or explosive) depends on the magma viscosity and the gas content.