Plate tectonics 101—what happens when plates move away from each other?
December 31, 2020
It’s winter in the midst of a pandemic, and travel is restricted. So it seems a good time to go back to some geologic basics, the knowledge of which are sometimes assumed in these blog posts. The next four posts will explore the essentials of plate tectonics—the underpinning of our modern geologic understanding. Plate tectonics refers to the processes that control the structure and properties of Earth’s crust and its evolution through time. The Earth is broken up into variously-sized lithospheric plates that comprise the solid, brittle outer layer of our planet; the plates move over the hot, weak asthenosphere beneath (see diagram below). As we will see, the science of geology, and our understanding of how Earth works, has been completely revolutionized during just the past 60 years—during many of our lifetimes!
As early as the 16th and 17th centuries, explorers observed that the continents on each side of the Atlantic Ocean seemed to fit together like jig-saw pieces. In the early 20th century, Alfred Wegener, a German geoscientist, proposed the idea of continental drift—that continents are moving around on Earth’s surface. His proposal was based not only on the fit of the continents, but also on his observation of similar fossils and mountain ranges now found on separated continents, and on evidence of glacial activity in rocks now found in tropical latitudes. He described a super-continent called Pangea, a name still applied to the amalgamation of the continents around 250 million years ago (see Late Jurassic reconstruction below). The problem was that no one could understand how the continents could “plow through” the ocean to move to different places, and the hypothesis was largely dismissed.
By the early 60’s, geoscientists realized that new oceanic crust was being created at MORs, which form the boundaries between tectonic plates moving apart from each other (i.e., diverging) in a process called seafloor spreading. As the plates diverge, magma (i.e., molten rock) rises to the seafloor and hardens into solid rock that preserves the ambient magnetic field. When the polarity of Earth’s magnetic field reverses, the newly-formed rock preserves the reversed polarity in the igneous oceanic crust, symmetrically on each side of the MOR. Imagine a conveyor belt moving outward from the MOR, carrying seafloor and any attached continents with it. On the global map above, notice that the North American plate, for example, contains both oceanic and continental crust because it extends from the MOR in the center of the Atlantic Ocean to the western coast of the U.S., where the boundaries are convergent and transform types (to be explored in next two posts).
The problem with divergent boundaries is that they are mostly hidden beneath ocean water and therefore difficult to observe. Although about 70% of Earth’s volcanism occurs at divergent boundaries, we are rarely able to see the eruptions. Because of the volcanic heat at the MORs, they form long, high mountain ranges on the seafloor; the typical depth at the crest of a MOR is 2,500 m (8,000 feet), whereas the average depth of the ocean is 4,000 m (13,000 feet). The seafloor is deeper away from the MORs because as the oceanic crust moves away from the ridge, the rock cools and contracts, thus sinking to greater depths. Imagine putting your cake in the oven (MOR) where it heats and rises, then pulling your cake out of the oven (crust moves away from the ridge) where it cools and sinks.
Since the 1970s, many new technologies have been developed for seafloor exploration—most are remotely-operated vehicles (ROVs) that scientists can safely operate from a ship or land-based location. For more information about these technologies, see MBARI’s site about their robotic vehicles: https://www.mbari.org/at-sea/vehicles/.
On-land exposures of oceanic crust are important locations for economically-important mineral deposits. Metal-rich seafloor sediments, with high amounts of iron, manganese, nickel, zinc, copper, cobalt, lead, and other metals have been discovered near MORs in the Atlantic, Pacific, and Indian Oceans. Geologists now understand the origin of these ore deposits—they are pieces of ocean crust subsequently pushed onto land.
Once geoscientists realized that Earth was being pulled apart at mid-ocean ridges, and that new oceanic crust was being created there, a new question arose: if Earth is not expanding, oceanic crust must also be destroyed somewhere. Looking at the global map above, we can now see why oceanic crust is so much younger than continental crust. All around the edge of the Pacific Ocean are black lines that are subduction zones where oceanic crust is being destroyed (the so-called “ring of fire”). We will explore this type of (convergent) plate boundary in the next post.