3.1 Theory of plate tectonics and continental drift

Plate tectonics and continental drift revolutionized our understanding of Earth's dynamic surface. These theories explain how Earth's lithosphere is divided into moving plates, driven by convection currents in the mantle below.

Evidence from coastline shapes, rock formations, and fossils supports the idea that continents were once joined. Seafloor spreading at mid-ocean ridges creates new crust, while subduction zones recycle old crust back into the mantle.

Plate Tectonic Theory

Lithosphere and Asthenosphere

Earth's outermost solid layer is the lithosphere, which includes both the crust and the rigid uppermost portion of the mantle. It averages about 100 km in thickness, though it's thinner beneath oceans (~7–10 km of crust) and thicker beneath continents (~35–70 km of crust).

Below the lithosphere sits the asthenosphere, a zone of the upper mantle that extends down to roughly 660 km depth. The asthenosphere is not fully liquid, but it's hot enough to be partially molten and deform plastically over long timescales. Think of it as solid rock that flows very slowly, almost like extremely thick putty.

This distinction matters because the rigid lithosphere is broken into several large tectonic plates that "float" and move on top of the weaker, flowing asthenosphere. That relative motion between plates is what drives earthquakes, volcanism, and mountain building.

Convection Currents and Mantle Plumes

Convection currents in the mantle are a primary driving force behind plate motion. The process works like this:

1. Heat from Earth's core and radioactive decay warms mantle material near the base.
2. This hot, less dense material rises toward the surface.
3. Near the top, it spreads laterally, cools, and becomes denser.
4. The cooled material sinks back down, completing a circular loop called a convection cell.

These convection cells transfer heat from Earth's deep interior to the surface and exert drag on the overlying lithospheric plates. Two other forces also contribute to plate motion: ridge push (gravity pushing newly formed crust away from an elevated mid-ocean ridge) and slab pull (the weight of a dense, subducting plate dragging the rest of the plate behind it). Most geophysicists consider slab pull the single strongest driver.

Mantle plumes are a separate phenomenon. These are narrow, localized columns of unusually hot material that rise from deep in the mantle, possibly from near the core-mantle boundary. They produce volcanic activity at hotspots like Hawaii and Iceland. Unlike most volcanism, mantle plumes are not tied to plate boundaries and can occur in the middle of a plate.

Continental Drift

Alfred Wegener's Theory

In 1912, German meteorologist Alfred Wegener proposed the theory of continental drift. He noticed that the coastlines of South America and Africa fit together remarkably well, like pieces of a jigsaw puzzle. But his argument went far beyond shapes.

Wegener compiled evidence from multiple fields: matching rock formations on opposite sides of the Atlantic, identical fossil species found on continents now separated by thousands of kilometers of ocean, and glacial deposits in regions that are now tropical. All of this pointed to one conclusion: the continents were once joined in a single supercontinent he called Pangaea (meaning "all lands"), which began breaking apart roughly 200 million years ago.

Wegener's theory was largely rejected during his lifetime because he couldn't explain how continents moved. He proposed that continents plowed through oceanic crust, but physicists showed the crust was far too strong for that. It wasn't until the 1960s, when seafloor spreading and mantle convection were better understood, that his core idea gained acceptance within the framework of plate tectonics.

Evidence for Continental Drift

• Paleomagnetism provides some of the strongest evidence. When rocks form, iron-bearing minerals align with Earth's magnetic field, recording the orientation and latitude of the magnetic poles at that time. Paleomagnetic data from rocks on different continents show that those continents have shifted position relative to each other and to the poles over geologic time.
• Fossil evidence ties distant continents together. The seed fern Glossopteris has been found in South America, Africa, India, Australia, and Antarctica. The freshwater reptile Mesosaurus appears only in southern Africa and eastern South America. Since neither organism could have crossed wide oceans, these landmasses must have once been connected.
• Matching rock formations reinforce the connection. The Appalachian Mountains in eastern North America align in age, structure, and rock type with the Caledonian Mountains in Scotland and Scandinavia, suggesting they formed as a single mountain belt before the Atlantic Ocean opened.
• Glacial evidence adds another layer. Wegener found glacial striations and tillite deposits (sediment left by glaciers) in places like southern Africa, India, and Australia that are now tropical or subtropical. When you reconstruct Pangaea, these scattered glacial traces cluster together near the South Pole, exactly where you'd expect an ice sheet to form.

Seafloor Spreading

Convection Currents and Seafloor Spreading

Seafloor spreading is the process by which new oceanic crust forms at mid-ocean ridges and moves outward from the ridge axis. Here's how it works:

1. Convection currents (along with ridge push and slab pull) cause two lithospheric plates to diverge at a mid-ocean ridge.
2. As the plates pull apart, hot mantle material (asthenosphere) rises into the gap.
3. This material partially melts due to the drop in pressure, producing basaltic magma that cools and solidifies into new oceanic crust.
4. The new crust is continuously pushed outward as more material wells up behind it.

Harry Hess proposed this model in 1962, and it was soon confirmed by two key observations. First, ocean floor drilling showed that sediment thickness and crustal age both increase with distance from the ridge, exactly as the model predicts. Second, magnetometers towed behind ships revealed symmetric magnetic striping on either side of mid-ocean ridges. As new crust forms, it records Earth's magnetic field direction at that time. Because Earth's field periodically reverses polarity, the result is alternating bands of normal and reversed magnetization, mirrored on both sides of the ridge.

This process explains why oceanic crust is geologically young. The oldest oceanic crust is less than 200 million years old, because it's constantly being created at ridges and destroyed at subduction zones. Continental crust, by contrast, can be up to 4 billion years old since it's too buoyant to subduct.

The rate of seafloor spreading varies by ridge. The Mid-Atlantic Ridge spreads at about 2.5 cm per year (a slow ridge), while the East Pacific Rise spreads at up to 15 cm per year (a fast ridge). The global average is roughly 5 cm per year, about the speed at which your fingernails grow.

Mantle Plumes and Hotspots

Mantle plumes can interact with mid-ocean ridges, as in the case of Iceland, where a plume sits directly beneath the Mid-Atlantic Ridge. This produces unusually vigorous volcanism and elevated topography compared to a typical ridge segment.

When a lithospheric plate moves over a stationary mantle plume, the result is a chain of volcanoes that records the plate's direction and speed of travel:

• The youngest, most active volcano sits directly above the plume.
• Progressively older and more eroded volcanoes trail off in the direction the plate has traveled.
• The Hawaiian Island chain is the classic example. Kilauea, on the Big Island, is the currently active volcano. Older, extinct volcanoes and submerged seamounts extend to the northwest, tracing the Pacific Plate's motion over millions of years. A notable bend in the chain (at the Emperor Seamounts) records a shift in the Pacific Plate's direction of motion around 47 million years ago.

Paleomagnetism recorded in these volcanic rocks can also be used to reconstruct past plate positions, since the magnetic signature captures where the plate was located when each volcano formed.