50 Fun Facts About Plate Tectonics

Plate tectonics describes the movement of pieces of which layer of the Earth?

This rigid outer shell, crust plus upper mantle, rides on the hotter, more ductile asthenosphere beneath.

How many 'major' tectonic plates are usually counted?

The Indo-Australian plate is sometimes split in two, which is why the count wobbles.

What are the three types of plate boundary?

Where two plates meet decides whether crust is destroyed, created or simply slides past.

Plates typically move at speeds up to roughly how much per year?

The Mid-Atlantic Ridge spreads at fingernail-growth pace; the Nazca plate races along at hair-growth pace, about 160 mm a year.

What is the process called in which one plate is carried down beneath another into the mantle?

It shrinks Earth's surface, which is balanced by new crust forming at spreading ridges, a tectonic 'conveyor belt'.

Which is the only planet known to have active plate tectonics?

Some moons and neighbouring planets show signs of past or icy tectonic activity, but nothing like Earth's.

Which of Jupiter's moons shows signs of ice crustal plates moving and interacting?

Its icy shell behaves in some ways like Earth's rocky plates.

Which meteorologist proposed continental drift in 1912?

He expanded the idea in his 1915 book The Origin of Continents and Oceans, but died in 1930 before being vindicated.

Why was Wegener's continental drift not widely accepted for decades?

Critics like Harold Jeffreys could not see how continental rock could plough through the denser oceanic crust.

What name is given to the single landmass Wegener argued the present continents once formed?

Wegener likened the continents to icebergs of light sial floating on a sea of denser sima.

Which mammal-like reptile found across now-separate southern continents showed they were once joined?

The fossil plants Glossopteris and Gangamopteris told the same story across the southern continents.

Which South African geologist's 1937 book Our Wandering Continents went beyond Wegener on Gondwana?

Field geologists in the southern hemisphere found the evidence for joined continents far more obvious than northern critics did.

Which English geologist proposed in 1928 that convection currents in the mantle might drive the plates?

He had suggested in 1920 that plate junctions might lie beneath the sea, but such ideas were long ignored.

Who coined the term 'seafloor spreading'?

Priority for the idea belongs to Harry Hess, who circulated an unpublished manuscript in 1960.

Harry Hess, the Princeton geologist behind seafloor spreading, held what rank in the Naval Reserve?

He concluded the Atlantic was expanding while the Pacific was shrinking.

Bruce Heezen's 1960 paper describing a worldwide rift system was based on his work with which cartographer?

Heezen at first backed Warren Carey's 'expanding Earth' idea to explain where all the new crust went.

What did magnetometers reveal about the ocean floor in the 1950s and 60s?

Alternating bands of normal and reversed polarity, symmetrical about the ridges, recorded Earth's field reversals as new crust formed.

The 1963 hypothesis linking magnetic stripes to seafloor spreading is named after which three scientists?

Lawrence Morley reached the idea independently of Fred Vine and Drummond Matthews.

Which magnetic mineral in seafloor basalt makes the magnetic striping possible?

Icelandic mariners had noticed the rock distorting compasses as early as the late 18th century.

Who added the third class of fault, where plates grind past one another, to the model in 1965?

His 1966 paper also introduced what became known as the Wilson Cycle of ocean opening and closing.

'Bullard's Fit', shown at the Royal Society in 1965, was a computer calculation of what?

The 1965 Royal Society symposium is regarded as the official start of the theory's acceptance.

In 1967, W. Jason Morgan proposed that Earth's surface consists of how many rigid plates?

Two months later Xavier Le Pichon published a six-plate model that marked the theory's final acceptance.

What was the theory initially called when it was first accepted in the mid-1960s?

The 50-year debate that ended in acceptance is now called the Plate Tectonics Revolution.

Which force is most widely thought to be the greatest driver of plate motion?

Cold, dense plates sinking into the mantle at trenches drag the rest along; 'ridge push' is really gravitational sliding.

The North American plate poses a puzzle for the sinking-slab theory of plate motion. Why?

The African, Eurasian and Antarctic plates pose the same puzzle.

Which two continents' matching coastlines were noticed as early as 1596?

Abraham Ortelius, Antonio Snider-Pellegrini and Eduard Suess all remarked on the fit before Wegener.

Old textbooks called oceanic crust 'sima' and continental crust 'sial'. What do the terms stand for?

Oceanic crust is denser because it has less silicon and more heavy elements, so it sits below sea level.

Roughly how thick is the rigid outer shell under the continents, versus 100 km under oceans?

It varies a lot between basins, mountain ranges and the stable cratonic cores of continents.

What are ophiolites?

They failed to be subducted and now mark the sites of vanished oceans within existing plates.

The Mid-Atlantic Ridge is an example of which boundary type?

Ocean-to-ocean rifting there creates new ocean basin, small volcanoes and shallow earthquakes.

The San Andreas Fault in California is the classic example of which boundary type?

Its motion is dextral, meaning the far side moves to the observer's right.

Curving chains of volcanic islands such as the Aleutians and Marianas form at which kind of boundary?

The subducted plate heats up and magma rises to build the arc; a deep arc-shaped trench forms alongside.

Why is neither plate subducted when two continents collide, as with the Himalayas?

Instead the edges are compressed, folded and uplifted; the Alps formed the same way.

What covered the area where the Himalayas now stand, before the plates collided?

The world's tallest range is the product of two major plates colliding.

What do descending slabs release that makes the mantle above them melt and feed volcanoes?

The added water lowers the melting point of the overlying mantle rock, and the resulting magma rises.

Deep, inclined earthquake zones along trenches are named after which two seismologists?

They dip 40 to 60 degrees and reach several hundred kilometres down, tracing the descending plate.

The Worldwide Standardized Seismograph Network of the 1960s was set up to monitor compliance with what?

Its data let seismologists map earthquake zones precisely, a windfall for plate tectonics.

Which plate is essentially ringed by trenches where its neighbours dive beneath it?

Plates attached to subducting slabs move much faster than those welded to continents, like the Atlantic plates.

Which supercontinent, also called Nuna, formed around 2 billion years ago as the first recognised one?

Rodinia came later, forming about a billion years ago and breaking into eight continents around 600 million years ago.

The most recent supercontinent broke up into which two landmasses?

Laurasia became North America and Eurasia; Gondwana became the remaining continents.

Which is the fastest-moving of the plates mentioned in the key principles, at about 160 mm per year?

That is roughly the speed hair grows; the Mid-Atlantic Ridge spreads at fingernail speed.

Which of these is one of the eight largest minor plates?

The others are the Arabian, Caribbean, Cocos, Nazca, Philippine Sea, Scotia and Somali plates.

One leading explanation for Venus lacking plate tectonics is the absence of what?

Earth's water-soaked crust develops the weak shear zones plates need; Venus is too hot for that.

Magnetic striping was detected on which planet by an orbiting spacecraft in 1999?

The patterns failed a 'magnetic reversal test', so they probably were not made by seafloor spreading.

Stern and Gerya argued plate tectonics enables complex life because it regulates what?

Continental drift also explains why related species turn up on continents now far apart.

Proposals in the 2020s divide Earth's crust into roughly how many smaller pieces called terranes?

The scheme reflects how much the big plates have deformed and fragmented over time.

The word 'tectonics' derives from an Ancient Greek term meaning 'pertaining to' what?

Earth's plates are thought to have been slowly moving for three to four billion years.

Which 16th-century mapmaker was among the first to note that South America and Africa seemed to fit together?

Snider-Pellegrini, Suess, Mantovani and Frank Bursley Taylor also preceded Wegener, who first marshalled hard evidence.

Which Tasmanian geologist organised a landmark 1956 continental-drift symposium and championed an expanding Earth?

Keith Runcorn's 1956 paper on palaeomagnetism also showed the data supported continental drift that same year.

Which of these is a place where a continent is splitting apart and a new ocean basin may form?

The East African, West Antarctic and Rio Grande rifts are other examples of continents splitting apart.