Final answer:
The movement of tectonic plates is described using terms for distances and time scales from millimeters to centimeters per year. Ancient tectonic activity is understood through the principle of uniformitarianism and the study of Earth's topography. The rate of plate movement over time can be significant when considered over vast geological timescales.
Step-by-step explanation:
The processes of plate tectonics occur at different scales of time and space, using various terms to describe distances and time scales. To express the slow movement of tectonic plates, geologists measure rates in millimeters to a few centimeters per year. For instance, tectonic plates move on average at a rate similar to the growth of human fingernails.
When discussing the distances tectonic plates cover over time, we can calculate that a plate moving at 4.0 cm/year would travel a very tiny fraction of that in one second (4.0 × 10⁻² meters/31,536,000 seconds). Over much larger timescales, such as a million years, this rate translates to 40 kilometers per million years.
Uniformitarianism is a principle used by geologists to understand ancient tectonic activity by studying current geological processes and the topography of Earth's surface. By analyzing structures formed by plate interactions, such as mountains, volcanoes, and earthquake zones, scientists can infer the history of plate movements and the forces that shaped the Earth's surface.