Question

is the group of words in bold a phrase or a clause?
parícutin, a volcano that emerged in a mexican cornfield, erupted continually for nine years; incredibly, parícutin was just one of over a thousand volcanoes that formed in that region.
phrase clause

Explanation:

A clause has a subject and a predicate (a verb and its related elements), while a phrase does not have both. The bolded group "that emerged in a Mexican cornfield" contains the subject "that" (referring to "volcano") and the predicate "emerged in a Mexican cornfield" (with the verb "emerged" and its modifiers). So it is a clause.

Answer:

clause