Determining the anchor (reference) of a nonrestrictive apposition [closed]

Solution 1:

It's a restrictive relative clause. Paring the sentence down:

The word tephra, from the Greek word meaning ash, has come into use among geologists to describe the assortment of fragments, ranging from blocks of material to dust, that is ejected into the air during a volcanic eruption.

The word tephra ... has come into use among geologists to describe the assortment of fragments ... that is ejected into the air during a volcanic eruption.

The word tephra describes the assortment of fragments that is ejected into the air during a volcanic eruption.

It's a specific type of assortment (of fragments) that is restricted to/by the definition in the clause. That introduces a restrictive clause; which a restrictive or a non-restrictive clause.