Final answer:
The Mid-Ocean Ridge is the longest mountain chain in the world, but Mount Everest is the tallest peak on Earth and has a summit comprised of Ordovician limestone, meaning it was once underwater.
Step-by-step explanation:
The longest mountain chain in the world is the Mid-Ocean Ridge, which is mostly underwater. It stretches for about 65,000 kilometers (40,390 miles) in the world's oceans. The Mid-Ocean Ridge is also quite wide, with an average width of about 1,500 kilometers (930 miles). However, if we consider mountain chains above sea level, the Andes in South America are the longest continental mountain range. Its highest peak is Mount Aconcagua in Argentina, which rises to approximately 6,961 meters (22,838 feet) above sea level. Speaking specifically about the tallest peak on Earth, this title goes to Mount Everest, at 8,848 meters (29,029 feet). Everest's summit is made up of a layer of Ordovician limestone, indicating that the region was once under a deep sea.