Plate tectonics 101—what happens when plates move toward each other?
January 8, 2021
When plates move away from each other (divergent boundaries; see last post on December 31), new oceanic crust is created. Since Earth is not expanding, ocean crust must be destroyed elsewhere. This happens at convergent boundaries, where plates move toward each other. Here, oceanic crust is destroyed, or rather, it gets recycled back into Earth’s depths. The places where oceanic crust is descending back into Earth’s interior are called subduction zones. They are located underwater, where down-going plates create valleys in the seafloor called trenches—the deepest parts of the ocean. The greatest depth of 11 km (36,000 feet) is in the Marianas trench in the western Pacific.
You might ask—why does oceanic crust always lose? That is, why is it always the part of lithospheric plates that goes back into Earth’s interior, whereas continental crust stays at Earth’s surface? The answer is density. Oceanic crust is made of igneous rock with minerals rich in iron and magnesium that are nearly twice as dense as the rock in continental crust, with minerals more rich in silica and aluminum. You may wish to view my post about Yellowstone National Park. At the bottom of that post is a diagram to show the different types of igneous rocks. Continental crust is mostly made of silica-rich rocks like granite, whereas oceanic crust is mostly made of silica-poor rocks like basalt and gabbro. The denser rocks of oceanic crust is also why the ocean basins are deep; in contrast, the less-dense rocks of the continents “float” at higher levels over the asthenosphere.
Let’s look at earthquake and volcano data that provide important evidence for the locations of convergent plate boundaries.
How were these subduction zones discovered? Coincidentally, the two largest earthquakes ever recorded by instruments occurred during the 1960s, when geoscientists were in the process of figuring out the basics of plate tectonics. The largest occurred first (1960 Chilean earthquake), but it was the second largest (1964 Alaskan earthquake) that led to the initial discovery of the subduction process.
After studying evidence from the Alaskan earthquake, George Plafker traveled to Chile, since he realized the Chilean earthquake just a few years earlier was also probably the result of a “mega-thrust” earthquake in what we now recognize as the subduction zone where the Nazca (oceanic) plate is descending beneath the South American (continental) plate. He carefully mapped the coastal region and found a pattern similar to what had been observed in Alaska—seaward areas where the land had been uplifted (red area in the map above) and landward areas where the land had sunk (i.e., subsided).
From the evidence presented above, we can see that convergent plate boundaries are the most dangerous types—they cause the largest earthquakes, they create active volcanoes, and they generate tsunamis that can travel for long distances away from the plate boundaries. But they also create some of Earth’s most beautiful landscapes. For example, you may wish to visit some of my previous blog posts about the Cascadia subduction zone and its associated chain of Cascade volcanoes: Crater Lake, South Sister volcano, Lassen Volcanic National Park, and Mt. St. Helens.
The third type of convergent plate boundary—the convergence of two continents—creates what could be considered the most spectacular landscapes of all. The Himalayas, the highest mountains on our planet, and the Alps in southern Europe are both the results of continental collision.
Continents collide when the oceanic part of a plate gets completely subducted and the continental part of a plate then encounters another continent, as shown on the left-side diagram. Because continental crust is less dense than oceanic crust, it is more buoyant and can’t be subducted. So, like two male elephant seals in battle, the two continents “face off” and collide with each other, in the process making mountains that rise to high elevations. The right-side map (from USGS.gov) shows how the sub-continent of India has moved northward during the past 71 million years to finally ram into the southern edge of the Eurasian plate to create the Himalayan Mountains. Seventy million years ago, India was part of a plate with oceanic crust on the northern part, as in the upper diagram. Once the oceanic crust was used up (i.e., subducted beneath southern Asia), the continental collision began, as in the lower diagram. That collision continues and the Himalayas continue to rise. Map from https://pubs.usgs.gov/gip/dynamic/himalaya.html.
For more information about the geology of the Alps, you may wish to visit my posts of a trek from Chamonix to Zermott in 2019, published in September 18, September 22, and September 27. I had expected to visit the highest mountains in November 2020, but hopefully it will be possible to trek in the Himalayas in 2021—stay tuned!