The main purpose of “Mountain of Doom” is to, whereas the main purpose of “My Journey to Pompeii” is to .

Mount Doom is a fiction, My journey to Pompeii is a non-fiction.

Explanation:

Mount Doom is a fictional volcano in J.R.R. Tolkien's book. The book talks about the damage caused by the volcano and the struggle in a caravan. In the end the evil falls into the fire and destroyed.This book talks about Pompeii, the famous city destroyed by a volcano in 79 CE.  Mount Vesuvius, erupted and destroyed the whole city. The land was covered with ashes and volcanic debris. The city was buried under the ashes and later in the 16th century, it was rediscovered.  

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