at what type of plate boundary do plates move apart, resulting in the upwelling of material from the mantle to create new sea floor

you can have just about every answer you can think of

yes, because even though a theory has a  lot of evidence to support it, it is not always true

mass and weight are closely related because mass affects the weight of an object experiencing the effects of gravity. weight is the measure of the force of gravity on an objects's mass, while mass is the measure of how much matter there is in an object.