Episode 122 – Plate Tectonics

Listen to Episode 122 on PodBean, YouTube, Spotify, or wherever you feel like it!

The Earth is in motion beneath our feet, and this motion impacts nearly every feature and activity on the planet’s surface. And yet, we’ve only recently come to understand this central geological phenomenon. This episode, we discuss the scientific history and the deep history of Plate Tectonics.

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A Foundational Theory

Plate tectonics isn’t just any scientific theory, it’s a major unifying theory. Nearly every feature of the Earth’s surface – continents, oceans, trenches, mountains, earthquakes, volcanoes, etc. – are linked by the complex motions of the tectonic plates. And yet, this is a relatively recent revelation for the scientific community.

The lithosphere is separated into several major plates and many smaller ones (most of the small plates don’t appear on this map). Many plates include both oceanic crust (the thinner, denser crust that underlies oceans) and continental crust (the thicker, lighter crust that makes up continents). Image: USGS, Public Domain

The Earth’s lithosphere (that is, the crust and uppermost mantle) is not a single solid shell. It’s broken into pieces like a cracked egg. These pieces – plates – are constantly in motion, shifting slowly like gargantuan conveyor belts. As these giant chunks of crust slide together, apart, and past each other, oceans open and close, continents rift and collide, and all that tectonic activity generates a world’s worth of earthquakes and volcanoes.

There are three main ways that adjacent plates interact.
At divergent boundaries (left), where plates slide apart, molten rock wells up from the mantle and cools into new crust. On the surface, these are mid-ocean ridges, rift zones, and volcanoes.
At convergent boundaries (middle), where plates slide together, one plate usually sinks down to be recycled into the mantle. On the surface are trenches, mountain ranges, and volcanoes.
At transform boundaries (right), plates slide past each other.
Most plates have many boundaries along their edges. Over time these combined motions cause plates to grow, shrink, and rotate.
Images: domdomegg, CC BY 4.0

The motion of the plates is so slow (typically a few centimeters per year) that it’s no surprise it took scientists a while to realize they moved at all. German scientist Alfred Wegener popularized the idea of Continental Drift in the early 1900s – he noticed the matching shape and geology of shorelines on opposite sides of the Atlantic Ocean and suggested the continents were once joined together. But it took several decades and a lot more evidence before the theory of Plate Tectonics as we know it came to be accepted.

Among the most important clues to Plate Tectonics was magnetism!
Surveys in the mid-1900s revealed magnetic striping across the ocean floor, symmetrical on either side of the vast mountain ranges called mid-ocean ridges. Through Earth history, the magnetic field of the planet has flipped back and forth, and the new crust spreading from these ridges records it in both directions.
Image: USGS, Public Domain
Another major sign of Seafloor Spreading is the age of the ocean floor.
Crust is youngest at mid-ocean ridges and older farther away, since new crust is formed at those ridges and spreads out from there.
Image: Muller et al. 2008, CC BY 3.0
A map of the world’s earthquakes in 2016.
Where plates grind against each other, earthquakes result. These tremors help scientist locate the borders of the plates. At subduction zones, earthquake foci even form a line descending into the Earth, following the sinking crust as it grinds its way down.
Image: Phoenix7777, CC BY-SA 4.0

Tectonics in Time

For billions of years, the motion of the plates has slowly shaped Earth’s surface. As we peer into the past, we can follow geologic evidence to reconstruct the planet’s surface as it once looked. Today’s continents feature huge mountains, the sites of previous continental collisions; vast oceans, which began as rifts that tore continents apart; and coastlines that share fossils, mountain ranges, and geologic layers despite being separated by oceans today. Certain rocks and minerals in ancient sediments can identify subduction zones and rift zones of the past, and magnetic signatures in ancient rocks can reveal if a continent used to be nearer or farther from the North or South pole.

With our modern understanding of plate tectonics, we can put together surprisingly detailed maps of the past, especially for the past 600 million years or so. The farther back in time we go, the less evidence remains, and the harder this task becomes.
Images by C. R. Scotese, PALEOMAP Project (www.scotese.com)

Plate tectonics is a big deal for paleontology, since life has always reacted to the shifting surface. As continents separate and conjoin, as oceans open and close, plants and animals gain or lose opportunities for dispersing around the globe. Mountains and trenches can form impassable barriers or exploitable habitats, while the island chains formed above subduction zones or hot spots can be crucibles for evolutionary diversity. And, of course, the shapes of oceans and continents can impact water and air currents, and thus the climates of the past.

Learn More

This Dynamic Earth: the Story of Plate Tectonics – learn lots more details with this USGS module
Plates on the Move – educational tectonics game from the AMNH
Paleolatitude calculator – find your own paleolatitude!

Bradley 2011. Secular trends in the geologic record and the supercontinent cycle. A detailed overview of how we reconstruct continental movement in the past (technical)

Explore some of the big questions about plate tectonics:
When and how did plate tectonics begin on Earth?
Does plate tectonics exist elsewhere in the universe? Investigations of Mars, Venus, Europa, Titan, and exoplanets.

If you enjoyed this topic and want more like it, check out these related episodes:

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