Measurements of the sea floor show that the Eurasian and North American plates have moved \(60 \mathrm{~km}\) apart in the past \(3.3\) million years. How far apart (in millimeters) do they move in one year? (By comparison, your fingernails grow at a rate of about \(50 \mathrm{~mm} /\) year.)

The Earth is like a giant, spherical puzzle comprised of large pieces called 'tectonic plates.' The outer layer of our planet is known as the lithosphere, which includes the crust and the upper mantle. Think of the lithosphere as the hard, outer shell of the Earth, dividing into massive, irregularly-shaped plates that fit together. These plates glide over the more pliable layer beneath called the asthenosphere, a part of the upper mantle that can flow slowly over geological time.

Within the lithosphere are two types of crust: the thinner oceanic crust, which lies beneath our oceans, and the thicker continental crust, which forms the continents. Plate boundaries can be regions of intense geological activity as the plates can either move apart, slide past one another, or collide. This constant, though very slow, movement can result in earthquakes, volcanic eruptions, and the formation of mountain ranges. Understanding the composition and behavior of the lithosphere is essential in comprehending the notions of plate tectonics, such as the exercise where we calculate the movement between the Eurasian and North American plates.

Continental drift is a fascinating concept that explains how continents have moved over the Earth's surface through time. This theory, proposed by Alfred Wegener in the early 20th century, suggests that continents were once part of a single landmass called Pangea. Over millions of years, this massive continent slowly broke apart, and its pieces drifted to their current positions.

The evidence for this theory includes the jigsaw-like fit of some continents, the distribution of fossils, and the similarity of rock formations and mountain ranges that now lie oceans apart. Plate tectonics extends this idea by describing the movement of the lithospheric plates on which the continents rest. In the exercise, we observe this phenomenon in action, quantifying the annual drift between tectonic plates, which is also responsible for the gradual shifting of continents.

The geological time scale is a way for scientists to organize Earth's history. It breaks down the vast expanse of time — spanning over 4.5 billion years — into various segments that highlight significant events in Earth's geological and biological timeline. This scale is composed of eons, eras, periods, epochs, and ages which each represent particular phases in Earth's development, from the formation of the planet to the appearance of humans and beyond.

Understanding the scale is critical when we deal with events in Earth's history, like continental drift, which seem imperceptible on human timescales but become significant over millions of years. For example, in the textbook exercise, we refer to a period of 3.3 million years. On the geological time scale, this falls within the Quaternary period of the Cenozoic era, a blink of an eye in terms of Earth's history. Despite this short timespan geologically, we can measure the considerable distances plates have moved, offering a glimpse into the dynamic nature of our planet.