#continental drift

[Two spacecraft, a Gemini capsule and an Agena Target Vehicle with a loose tether connecting them, are in space over the Mediterranean with Italy in the background]

Spacecraft: Houston, the view is superb. We can see the continents spread out below us, right where they've been since the earth formed.

[Caption beneath panel:]

It's easy to think that there is some common sense when it comes to defining a continent and their number, but it turns out that this is a matter that's fairly multifaceted. I've been curious about this myself many times before and it's something I've discussed with friends, and they've brought a number of perspectives to the table. So for this post, I'd like to break down the definition of continent and the number of proposed continents and see whether we can make sense of them in a way that can be mutually-agreed. And if we can't agree, why not?

First of all, who came up with the concept of continents? As Josephine Quinn explained in her 2024 book How the World Made the West, "Ionian Greek scholars invented the continents". The Ancient Greeks of the 6th Century BC split their world into Europe and Asia based upon the liquid-divisions of the Mediterranean Sea through the west coast of Anatolia and on to the Black Sea. The question of Africa was conflicting for a time, as some scholars recognized the achievements of the Phoenicians under Necho II of Egypt in circumnavigating Africa, while others subsumed Africa into Asia. Whatever their dimensions, to the Greeks these landmasses had political and cultural connotations from the very beginning: as the wars with Persia dawned, they believed that the different continents produced different temperaments and conditions, resulting in conflicting views about the nature of "Europeans" and "Asians" (Quinn 2024).

Of course, as the centuries rolled by and people began to make longer journeys overseas, they became more familiar with their homelands. And with the European discoveries of "New Worlds", of course, played a roll in shaping opinions. However, as late as the 18th Century, geographers were still in discourse about the nature of continents: some questioned the traditional ideas of antiquity in subdividing the "Old World" into distinct landmasses, and others preferred a looser definition that included sizable islands. In any case, many geographers and nonspecialists in the 21st Century still refer to "Europe", "Asia", and "Africa", so the shadows of the Ionians have remained over us to this day.

In their landmark 1997 book The Myth of Continents, Martin W. Lewis & Karen E. Wigen argued comprehensively that this long history of continental discourse simply reflects cultural conventions. Whatever scientific arguments the ancients had about the nature and number of continents was never really scientific to begin with, riddled as they were in ethnocentric bias and the western need to label and categorize.

That said, is there a way to define and delineate continents from a scientific perspective that focuses solely on geology and not culture?

When looking upon the planet Earth, geologists recognize that there is a distinction between land and sea (and I'm not talking about the dividing-line being the water): the key is in the rocks of the crust.

Geologic age of oceanic crust (FCrameri, CC BY-SA 4.0) - Larger Image

The ocean floor consists mainly of basaltic and gabbroic rocks. These igneous rocks form a series of layers going down about 4-6 miles (7-10 kilometers) deep and are composed of dark-colored mafic minerals like iron and magnesium. As a rule, basaltic rocks form the upper layers while gabbroic rocks form the lower layers that neighbor the top of the mantle.

The lands of the continents consist mainly of a foundation of granitic rocks: also igneous and composed of light-colored felsic minerals like aluminum and silicon. Due to the rock cycle, many areas of the continental crust have been frequently recycled and changed and so consist of sedimentary or metamorphic rocks, and can vary in height from 12 miles (20 km) to 44 miles (km) in thickness. As well, sea-level changes can expose or submerge this continental crust, and the true scale of these granite-pedestals can be seen in the continental shelves of the oceans today.

So this could be one piece of criteria to work with: a continent consists of a distinct granitic crust base.

But we must remember that the Earth's crust is not one uniform layer. Since the formation of Alfred Wegener's hypothesis of continental drift and the subsequent modern scientific consensus of plate tectonics by the late 1960s, geologists have recognized that the lithosphere is cracked like an egg into constantly moving plates. Tectonic plates essentially float above the mantle due to being comprised of lighter minerals. They move through a combination of convection currents in the mantle, gravitational forces, and the rising of magma at mid-ocean ridges, which causes these plates to divide, collide, and shape the Earth's crust.

Earth's Tectonic Plates (M. Bitton, CC BY-SA 3.0) - Larger Image

The map shown above is a generally accepted representation of the Earth's tectonic plates. As you look, you can make out certain plates that correspond to conventionally-recognized continents, but as you continue looking you also find that things aren't straightforward. Europe is not considered a distinct plate but rather belongs to a larger Eurasian plate. Yet, this Eurasian plate doesn't correspond to how people would subsume Europe & Asia: there are separate Arabian, Indian, and Philippine plates, and part of Siberia is included in the North American Plate! As well, there are many tectonic plates that are composed mainly of the oceanic basaltic crust and not the continental granitic crust we're looking for.

That's not even the half of it. For the last decade or so there has been a growing body of research into plate tectonics, and many researchers have argued strongly for a rethink of how we look at the Earth's crust. One 2022 paper by Derrick Hasterok and colleagues proposed that there are 16 major plates and 54 "microplates", while another 2023 paper by Janpieter van Dijk culminated in a division of the Earth into 1,180 plates (see below).

Janpieter van Dijk's global tectonic map (CC BY-SA 4.0)

With such a more refined understanding of tectonic forces comes newer and newer interpretations of how geologists should look at the world. Much news was made about Zealandia in 2017, when Nick Mortimer and colleagues formerly designated an area of undersea continental crust - comparable in size to South Asia - as "Earth's hidden continent". And more recently, Luke Longley and colleagues argued that the geologic boundary separating the North American and Eurasian plates along Iceland have not fully separated yet (Longley, et al. 2024). This Rifted Oceanic Magmatic Plateau would essentially mean that North America and Eurasia still consist of a single continent connected along the northern Atlantic.

So it seems that one possible second criteria that a continent must be separated by the divergent or convergent boundaries which surround tectonic plates - as some geologists have proposed in the past - is far less stable.

The Mortimer, et al. 2017 paper is fascinating to me in that they also set about the task to try to understand continents scientifically and give a concise definition. After reviewing past criteria, the team agreed with the designations that continents should have a base of granitic felsic crust. They argue that the elevation of the land should play a key role, and this makes sense given that continental crust is substantially thicker than oceanic crust. And they push for a size minimum of >386,102 square miles (one million square kilometers). Tectonic divisions do not appear to play a big role in their schema, as Zealandia is not on a separate plate but borders the Australian and Pacific plates.

As hinted above, there are smaller subdivisions of the Earth's crust, including microcontinents which have broken away from larger landmasses (think Madagascar), but these I suppose can be considered as distinct from continents in the way that plutoids and other dwarf planets are different from true planets.

All this said, there is one dimension that could very well throw a wrench into this whole discussion: deep time.

Continents have a history going back at least 4.4 billion years (Hazen 2012), when evidence from zircon crystals hints that mineral evolution had produced the first granitic crust. Interestingly, one hypothesis for the origin of continents is that as the early basalt crust cooled in the newborn oceans, it pressed the lower basaltic layers against the upper mantle, which melted and changed the mineral composition into granite. These rocks, being lighter in composition, floated and pushed up to the surface, eventually forming small islands of land which eventually collided into cratons.

Cratons are considered the grand progenitors of the continents and are the oldest surviving geologic structures on Earth, calculated in billions of years. They can be considerably large and much of the remaining continental land area of the Earth essentially merged around them, slowly forming the landmasses we recognize today.

The North American Craton or Laurentia (U.S. Geological Survey, Public Domain)

Because of plate tectonics, our perception of stable continents shatters as we wind back the clock. Land bridges connect landmasses. Supercontinents form and break apart. Microcontinents abound and then collide into continents, becoming subsumed. It's only for the past 100 million years or so that the outlines of the modern continents can become discernible, beyond that, boundaries are blurred. One wonders how humans would've divided the planet's continents had we first emerged in, say, the Ordovician Period of 460 million years ago?

With this in mind, we run into a similar problem in biology: how to define species. When Linnaeus devised his System of Nature in the 18th Century, he had only living animals and plants to rely on when he coined the species-level in taxonomy. Since that time, every living thing has been given a binomial name. Humans are Homo sapiens, gray wolves are Canis lupus, etc, etc. These were essentially fixed categories, as the early naturalists of the time were mainly creationists. But once scientists understood evolution, deciphered its mechanisms, and recognized the volumes of history in deep time, suddenly species were no longer fixed but constantly changing entities. Wind back the clock and Homo sapiens eventually submerges into Homo erectus or some related form; go back even further and there are no longer hundreds of primates or rodents or whales but earlier placental mammals that belonged to their own unknown species.

This revolutionary shift in thinking has transformed how biologists classify species. They no longer rely on a simple system of comparing morphology but could use any of 16-32 different definitions, and they can now refer back to the fossil record or compare genomic sequences for clues about how living forms changed over time. Animals and plants that we once defined as concrete taxa have been split into multiple species and, conversely, many have been lumped into a single species.

Maybe we should be looking at continents in this way? Instead of creating a list of criteria and comparing the different living landmasses together, we should be thinking temporally, and considering multiple definitions for continent. The big side-effect of this would be that "continent" is a fluid, ever-changing category much like species. No one would have a single answer, because deep time renders these divisions as near-meaningless in the grand scheme of things. That's not to say that plate boundaries or cratons or granitic vs. basaltic crusts don't matter or can't tell us anything, it's just that this is another way the natural world rejects attempts at being subdued by rigid classification.

What is a continent? How many continents are there? Maybe these are the wrong questions to ask and the wrong ways to think, and it would be best to shake off the last few shackles those Ionian Greeks left us over 2,500 years ago.

What do you think?

Book Citations

Philip Eales, et al - The Science of the Earth (DK, 2022)

Robert M. Hazen - The Story of Earth (Penguin Books, 2012)

Josephine Quinn - How the World Made the West (Random House, 2024)

Martin W. Lewis & Karen E. Wigen - The Myth of Continents (University of California Press, 1997)

Paper Citations

Janpieter van Dijk, 2023. The new global tectonic map - Analyses and implications (Terra Nova)

Derrick Hasterok, et al. 2022. New Maps of Global Geologic Provinces and Tectonic Plates (Earth-Science Reviews)

Luke Longley, et al. 2024. The David Strait proto-microcontinent: The role of plate tectonic reorganization in continental cleaving (Gondwana Research)