Why do some volcanoes explode and others just ooze lava?

Mostly because of how much silica and dissolved gas the magma carries. Magma is molten rock under the ground, often mixed with crystals and gas bubbles. It forms where the mantle or crust melts: at subduction zones, rifts, mid-ocean ridges and hotspots. What it does at the surface depends on its chemistry.

Silica is the key ingredient. Each silicon atom grabs four oxygen atoms and forms a tiny pyramid. When there is a lot of silica, those pyramids link up into chains and clumps. The magma turns thick and sticky. Low-silica magma, the kind that builds Hawaii's gentle shield volcanoes, is about as runny as ketchup. It erupts at 1,100 to 1,200 °C and simply flows away. High-silica magma can be ten million times stickier, even though it erupts cooler.

Stickiness is what makes a volcano explode. Deep down, magma holds gases like water vapour and carbon dioxide, the way a closed soda bottle holds its fizz. As magma rises, the pressure drops and the gas forms bubbles. In runny magma, the bubbles slip out fairly gently. In sticky magma, they stay trapped. Pressure builds until the magma shatters and blasts ash and rock into the sky.

That's why so many explosive volcanoes sit above subduction zones. There, a sinking ocean plate releases water into the hot mantle above it. The water lowers the melting point, so new magma forms. That magma is loaded with dissolved water and gas. It feeds the volcanic arcs of the Pacific Ring of Fire, where some of the planet's most violent eruptions happen.