Earth is very old, and it can be difficult to visualize where key events in the history of life occurred against this backdrop o
f "deep time." One common way to represent geologic time uses the analogy of a one-hour countdown timer. The timer starts from the top at Earth’s origin (4.6 billion years ago) and moves clockwise to the present, making one full circle. In this analogy, one hour represents 4.6 billion years, one minute represents roughly 76.67 million years, and one second represents roughly 1.28 million years. (Open Hint 1 for an explanation of how these calculations are done.)
The age, density, and thickness of oceanic crust increases with distance from the mid-ocean ridge. The magnetism of mid-ocean ridges helped scientists first identify the process of seafloor spreading in the early 20th century.
The answer is B. The Doppler Effect is used to calculate the velocity (speed) of something. Astronomers use it to see how fast stars and galaxies are moving away from us.
