This insight was one of the founding pillars of plate tectonics, and ophiolites have always played a central role in plate tectonic theory and the interpretation of ancient mountain belts.
This definition has been challenged recently because new studies of oceanic crust by the Integrated Ocean Drilling Program and other research cruises have shown that in situ ocean crust can be quite variable, and that in places volcanic rocks sit directly on peridotite tectonite, with no intervening gabbros.
Moores and Vine Thus, it became widely accepted that ophiolites represent oceanic crust that had been emplaced on land.
Their great significance relates to their occurrence within mountain belts such as the Alps or the Himalayas, where they document the existence of former ocean basins that have now been consumed by subduction.
[In Photos: The Clovis Culture and Stone Tools] But in 1974, a small wooden scallop trawler was dredging the seafloor, about 230 feet (70 meters) below the sea surface and nearly 60 miles (100 kilometers) off the coastline in the Chesapeake Bay.
The motivating force for seafloor spreading ridges is tectonic plate pull rather than magma pressure, although there is typically significant magma activity at spreading ridges.If so, the combination of the finds may suggest that people lived in North America, and possibly butchered the mastodon, thousands of years before people from the Clovis culture, who are widely thought to be the first settlers of North America and the ancestors of all living Native Americans.But that hypothesis is controversial, with one expert saying the finds are too far removed from their original setting to draw any conclusions from them.Deep-sea fishing Most researchers believe the first Americans crossed the Bering Strait from Siberia about 15,000 years ago and quickly colonized North America.Artifacts from these ancient settlers, dubbed the Clovis culture after one of their iconic archaeological sites in Clovis, New Mexico, have been found from Canada to the edges of North America.