who only by moving can balance

Beryllium has a short-lived radioisotope that is ONLY formed in the atmosphere by cosmogenic spallation -- the half-life is about 1.5 million years (original paper). So if you find beryllium-10 in arc lavas, and you can be sure it's part of the actual magma and not just a result of interaction with the atmosphere (and there are ways you can be sure of this), you know that the arc magmas must be melting material that's been close to the surface in the recent ("recent"; the Earth is ~4.5 billion years old, and also a Libra) past, i.e. it's melting sediments that have subducted fairly recently, that were deposited on the top of the oceanic crust and then dragged down with it.