Where is one example of where a Volcanic Neck exists on earth

Puerto rico and Cabezon Peak

Explanation:

sixty percent of all active volcanoes occur at the boundaries between tectonic plates. Most volcanoes are found along a belt, called the “Ring of Fire” that encircles the Pacific Ocean. Some volcanoes, like those that form the Hawaiian Islands, occur in the interior of plates at areas called “hot spots.”

Hope this helped!

if you mean hydrogen and oxygen then water is the product