BotEC: The Magnitude of the India-Asia Collision
The story of the Himalayas can be traced back to the breakup of the supercontinent, called Pangaea, about 200 million years ago, when India began its rapid movement northward towards Asia. Asia was a much smaller continent then. Then, between 45 and 55 million years ago, India and Asia collided. Before collision, India moved northward at about 7-10 centimeters per year. Remarkably, India's northward movement was slowed only a little after the collision—it continues to plow into Asia at a rate of 5-6 centimeters per year. We can measure the present northward movement of India into Asia using GPS measurements over a period of time. So, there is little doubt of the continued movement of India. There is much debate about how the northward march of India into Asia is being accommodated. Of course, the high Himalayan Plateau is a manifestation of this collision between two continents.
Now refer to a map of the USA. For the sake of comparison, if India were to have collided with North America near Washington, D.C. and continued to plow westward into North America at 6 centimeters per year for 55 million years, then where would the "front" of the collision zone be presently? Name a major city in the USA that is at the correct approximate distance from Washington, D.C.
References and Resources
This SERC page describes the use of Back of the Envelope Calculations
The Back of the Envelope : This page outlines one of the essays in the book "Programming Pearls" (ISBN 0-201-65788-0). The book is written for computer science faculty and students, but this portion speaks very well to back of the envelope calculations in general.