Answer: Adverb
Step-by-step explanation:
To show us the lava flow is underlined and it is a infinite verbal phrase in serve of adverb. Why he did that? Tho show us quickly what lava flow looks like.
Verb phrases are showing us how the rest of the sentence that depends of verb is providing or describing information, action or idea in the sentence. Verbal phrases are used for more effective and informative sentence which is causing clear and direct writing or speech.