It is an adverb phrase (tells where).
If the phrase describes (modifies) a noun or pronoun, it's an adjective phrase. If the phrase describes a verb, adjective, or adverb, it's an adverb phrase.
head-on is an adjective
Adverb phrase
An infinitive phrase. An infinitive phrase is a noun phrase with an infinitive as its head. Unlike the other noun phrases, however, an infinitive phrase can also function as an adjective or an adverb.
adverb phrase
If the phrase describes (modifies) a noun or pronoun, it's an adjective phrase. If the phrase describes a verb, adjective, or adverb, it's an adverb phrase.
The word 'above' is both an adverb and a preposition. In the phrase 'above her head', the word is a preposition; the noun 'head' is the object of the preposition.
adverb phrase
Adverb Phrase
an adjective phrase acts like an adjective and modifies the noun or pronoun in the sentence. an adverb phrase acts like an adverb and modifies the verb, adjective, or adverb in the sentence.
adjective phrase
adverb
adverb phrase
head-on is an adjective
adverb phrase
It is an adverb phrase, although in forms such as "The period after a supernova is marked by stellar collapse" it seems to be an adjective phrase.
This is a sentence (or clause), not a phrase. The adjective is dumb, and the adverb is very, modifying dumb. So "very dumb" is the adjective phrase.