WHEN an unstoppable force like the Indian subcontinent crashes into an immovable object like the Eurasian plate, the consequences include the tallest mountains in the world and a cadence of earthquakes like the magnitude 7.8 one that struck Nepal last month and a major aftershock in the same region last week.

Many of the geological questions about the collision remain unanswered. How did the Indian subcontinent get so quickly to where it is today? How big was India originally? Even the simplest of questions — when did India meet Eurasia, the tectonic plate that Europe and Asia sit on? — is up for debate, with researchers offering answers that differ by some 30 million years.

“It’s going to be hard to convince anyone,” said Oliver E. Jagoutz, a geologist at M.I.T. and part of a team that outlined its ideas about the collision in the journal Nature Geoscience.

Another mystery is why India is still moving at a quick pace — one and a half to two inches a year — driving the devastating earthquakes.

“That is one of the biggest problems that we have in plate tectonics,” said Douwe J.J. van Hinsbergen, a professor of earth sciences at Utrecht University in the Netherlands. “It may not seem much, but it’s the rate at which your fingernails grow.”

The geologists are like accident investigators trying to decipher what happened from the wreckage, pondering how rocks from the ocean floor ended up high in the Himalayas. Much of the evidence, namely the chunk of India that is now jammed under Tibet and the Himalayas, is out of reach.