English edit Carla Elliff
Before answering this question, we must understand some concepts and theories. So, grab a world map and observe. By looking at this kind of map we are under the impression that our continents are anchored for good to a certain point of the Earth’s surface, right? But, in fact, they are drifting around the world over time. During the time it takes you to read this post, the place where you are will have slowly but surely drifted.
Looking at our world map again we can see an impressive jigsaw formed by the coastlines of either side of the Atlantic Ocean, especially along Africa and South America. This suggests that if you carefully put together the borders of all continents, they can be regrouped into a single landmass, solving the jigsaw puzzle.
By using geological and paleontological (mainly fossils) information, the German meteorologist and geophysicist Alfred Wegener presented to the world in 1915 a new hypothesis to understand the history of Earth: the continental drift. According to Wegener, between 100 and 150 million years ago, a single landmass called Pangea broke up and its pieces, which are now our continents, moved apart, opening up new ocean basins between them (an important detail here is that if there used to be a single supercontinent, there was also a single ocean, which was called Panthalassa).
However, several researchers objected to Wegener’s affirmation that the granite crust of the continents could “plow” its way through the denser basalt crust of the oceans. At the time, some geophysicists calculated that continental drift was not possible. In addition, Wegener himself could not explain why the continents drifted. Thus, Wegener’s theory remained ignored for over half a century.
However, with the later discovery of the Mid-Atlantic Ridge mountain range, new similarities were observed between the shape of this feature and the margins of Atlantic continents. Moreover, the seafloor of the valley of this mountain ridge was shown to be composed of newly crystallized basalt. Thus, in the mid-1960s, geologists and geophysicists proposed a new hypothesis: new seafloor and crust are created continuously!
They emerge from the intrusion and extrusion of basalt at the crest of all mid-oceanic ridges. The newly formed crust moves laterally, giving way at the crest of the ridges for new basaltic crust to form. This process, known as seafloor spreading, causes the expansion of ocean basins and this is how the continents on each side of the basin move with the seafloor, thus explaining continental drift!
The continental drift hypothesis then became part of a larger theory: plate tectonics. This theory was formulated in the 1960s and is based on the idea that the planet’s surface is divided into a series of plates with borders that are defined by seismicity, which means the frequency/amount, magnitude/strength and distribution of earthquakes. These plates, known as tectonic or lithospheric plates, are like thin pancakes covering our planet because they are much wider than they are thick – their width can be 10 to 50 times greater than their thickness.
In all ocean basins there are plates that are moving apart forming mid-ocean ridges and new seafloor (seafloor spreading).
In turn, other plates, especially those around the Pacific Ocean are undergoing a process called subduction: when a pair of plates actively collide and one plate is “forced” to dive under the other towards the asthenosphere (Earth’s second layer, right below the lithosphere), where it melts and is incorporated into the magma. When the continents of two different plates meet at a subduction zone, they collide and squash the marine sediments between each other, lifting them up and creating huge mountain folds, like those found in the Himalayas.
Last but not least, when the lithosphere is neither created nor destroyed at the limit of plates, we have what is called a transform fault. In these cases, plates slide along the sides of each other.
Now we are ready to get back to our initial question: is the Pacific Ocean getting smaller? The answer is yes! Since there are very few subduction zones in the Atlantic, Indian and Arctic oceans, these basins are expanding due to seafloor spreading. On the other hand, most subduction zones are found at the margins of the Pacific Ocean. Since subduction rates are much greater than the rates of producing new seafloor at the mid-oceanic ridges, simply because there are more spreading regions than subduction zones, the result is that the Pacific Ocean is shrinking on a geological time scale.
Sources:
Garrison, T. Essentials of oceanography. 5a edição. Brooks/Cole, Cengage Learning, 464 p.
Pinet, P.R. 2014. Invitation To Oceanography. 7a edição. Jones & Bartlett Learning. 662 p.