Plate tectonics is a scientific theory that describes the large-scale motions of Earth's lithosphere, the hard outer shell of the planet. The model builds on the concepts of continental drift, developed during the first few decades of the 20th century. The geoscientific community accepted the theory of plate tectonics only after the concepts of seafloor spreading were developed in the late 1950s and early 1960s.

Tectonic plates are able to move because the Earth's lithosphere has a higher strength than the underlying asthenosphere (the upper portion of the mantle). Lateral density variations in the mantle result in convection and plate movement is thought to be driven by a combination of the motion of the seafloor away from the spreading ridge (due to variations in topography and density of the crust) and drag, also known as downward suction, at the subduction zones. Another explanation lies in the different forces generated by the rotation of the globe and the gravitational forces of the Sun and the Moon. The relative importance of each of these factors is unclear, but scientists are sure that each plays some part in the underlying movement.

On Earth, the lithosphere is broken up into seven or eight major tectonic plates (depending on how they are defined) and many minor plates. Where plates meet and their relative motion determine the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain building, and oceanic trench formation occur along these plate boundaries. On convergent boundaries, subduction carries plates into the mantle; the material lost is roughly balanced by the formation of new oceanic crust along divergent margins by seafloor spreading. In this way, the total surface of the globe remains the same - this prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories, which still have some supporters, proposed gradual shrinking or gradual expansion of the globe but more recent evidence suggests this is not the case. Regardless, it is clear that plate tectonics, while firmly entrenched as a theory in the field of geoscience, is still evolving and will continue to change in the years to come.

1. Which of the following situations is most analogous to the “conveyor belt principle” described in the 3rd paragraph?

A. The total amount in a savings account increases this year because interest earned on the total savings more than offsets the several withdrawals throughout the year.
B. The food stockpiles at a local charity remain the same over the past month because no one has requested any food supplies and there have been no new donations.
C. The number of crops on a farm goes up and down throughout the year as some crops are harvested and new crops are planted.
D. The price of a product remains the same because supply and demand remain balanced throughout the year.
E. Inventory in a retail store remains constant because items are automatically replaced when they are purchased by consumers.