The short term effects of a volcano blast include the hazard of the magma itself. Whether lava flows, or air fall, or ash flows, the toxic gasses emitted also are hazardous. Then, in the longer term, there are some nutrients that go into soil such as P (Phosphorus), Na (Sodium), K (Potassium) that help the soil in the long. Another effect is COOLING, which comes when big eruptions put ash and gasses in the stratosphere. This cooling happens because the S (Sulfur) gases hydrolyze to form sulphuric acid aerosols, and these tiny droplets reflect incoming solar radiation. Is this reflection what keeps the earth cooler by increasing its overall reflectivity (less sunlight gets stored in atmosphere/oceans).

So, volcanic eruptions are good and bad!

On the other hand, natural ash in the atmosphere makes for very dangerous conditions for jet engines. A jet engine sucks in air, but if that air has tiny particles (ash), these particles can quickly FOUL the engine, which causes the loss of thrust, and hence the plane crashes.