what is the cause of volcano erruption? why there are always earthquake everytime volcanos errupted?

Answer 1

When a volcano erupts, it is releasing
pressure from underground.
When the pressure is released, the shape
of the top layers change.
That movement is your "earthquake".
Volcanoes generally cause smaller 'quakes`.
The larger ones are caused by movements
further down.

Answer 2

volcanoes cause a great amount of pressure, causing earthquakes