What is a continent?

A continental mess

I have found these days that I'm most interested in problems of the obvious or those we take for granted.  Let's take continents.  What is the definition of a continent? I recently submitted a special session to a conference. The session was focused on the origin of continents, from the deep lithospheric mantle to the crust.  Because we wanted to keep the title short, we decided to describe the session as the "origin and evolution of continents".  However, the organizing committee said that it was much better to use the word "continental lithosphere" because continent is defined in the dictionary as land, that is, all that stuff above sea level. Indeed, lithosphere includes everything from the crust to the mantle, but the reality is many earth scientists don't even know what lithosphere is (if you don't believe me, drop the lithosphere-bomb in a geochemistry conference and then go to a geophysics conference and compare notes).

Well, Houston, we've got a problem because to define continents as area above sea level is not exactly correct. The area above sea level is called "land", so "land" is sufficient. If you think I'm being unreasonable, let me just say that today's world, in which continents are mostly above land, is probably anomalous. Long ago, much of the same areas we call land today were below sea level, and yet they were still at a higher elevation than the ocean basins. When a continent goes below sea level because of sea level rise, dynamic topography or other processes, does it cease to be a continent?  Continents have gone up and down, without any change in their crustal thicknesses or compositions.  There is a reason why I am worked up on this.

The Earth is characterized by a bimodal hypsometry if you take away the ocean water. About 70% of the Earth is at ~4 km below sea level and most of the remaining part of the Earth is just above sea level, with little in between. The low lying regions we call oceanic crust and the high lying regions we call continental crust. It just so happens that today, the area underlain by oceanic crust is also filled with water and the area underlain by continental crust is mostly above land. However, in the past, much of the continental crust was submerged below sea level, but it is still continental crust. Continents should be defined by elevation, but not by whether they are above sea level!

So what then makes continents ride high and oceanic crust low?  There is a common perception that this is because oceanic crust is basaltic and continental crust is more granitic, with granitic rocks being slightly less dense than basaltic crust. But this is a really bad misperception. The difference in density is real, but hardly sufficient to explain the difference in elevation.  What really controls the relative difference in buoyancy between continental and oceanic crust is crustal thickness. Oceanic crust is 7 km thick roughly, whereas continental crust is mostly 30 km thick, enough to give 4 km a relief between continental crust and oceanic crust.

I cannot stress more the importance to focus on crustal thickness rather than composition.  Petrologists, many of them at least, define continental crust as being felsic, but this thinking has the effect of closing one's mind.  The continental crust has a hodge podge of all different types of lithologies, including basalts. If thickness is what matters in terms of elevation, then there is no reason why you can't have thick piles of basalt ride high and be called a continent. Flood basalts, like Ontong Java are 20 km thick and almost make it (more on this later) to the level of a continent in terms of thickness.  The real question one must ask is what makes continents thick when the dominant forms of magmatism - mid-ocean ridge and island arc volcanism - just can't seem to make crust thick enough to look like a continent, that is, riding 4 km above the seafloor.  In fact, because there is erosion and gravitational relaxation of the crust, when continents are formed, they must be even thicker than the average 30 km they are now.  Making continents requires orogenic processes, so if you want to understand continent formation, you need to understand orogeny.  That continental crust may on average be more felsic appears to be a by-product of whatever coupled orogenic-magmatic differentiation process operates.  But felsic alone does not guarantee the making of a continent.  Rock composition, nonetheless, may give one insight into how the continent formed.

It is my belief that any significant progress on understanding continent formation is going to come from the interface between petrology, tectonics, geodynamics and geomorphology because all of these processes interact with each other.

but what really interests me is the transition between the continent and the oceanic crust.  Most of the Earth's surface can be classified as continent (near sea level) and oceanic crust/seafloor (4 km deep or so).  but there are some transitional areas where the crust is only 20 km thick.  This happens in oceanic plateaus or in regions where rifting has been inefficient, leaving a thinned continental crust like in Zealandia, in between New Zealand and Australia.  Or take the Basin and Range province in North America, which we would all call continental right now.  However, it's crust is only 20 km thick in some areas. Its high elevations are being supported by hot asthenosphere below - thermal buoyancy, but once this hot asthenosphere dissipates, it will subside to below sea level. At that point, would we call it continental crust or oceanic crust, continent or seafloor?  You get the idea. A petrologist might call it continental crust, but in reality it is transitional crust because it falls in the transitional region of elevation.  In all likelihood, it would be composed of a mixture of granitic like rocks and mafic rocks associated with limited extension.

There are a number of these transitional areas, mostly regions which have undergone inefficient continental rifting.  But most ocean-continent margins are rather sharp, defined by a sharp break at the continental shelf, which typically is a wedge of clastic sediments draped over a sloping basement, which itself is defined by a rapid transition in crustal thickness.  On the Atlantic margin, this transition is sharp. But what does it look like in regions which have inefficiently rifted? And why do some rifts evolve to clean breaks and others don't?  There's also the scenario of dynamic subsidence which generates continental scale inundation without any changes in crustal thickness. In these cases, you won't get a sharp continental shelf break as one is still beneath continental crust, even though it is below sea level.

To me, the most important problem right now is that ocean-continent transition and how the morphology of crustal thickness dictates how sediments are deposited and stored.  If you're interested in sequence stratigraphy, it is time to pull in the deep Earth.  If you're interested in continent formation and continental rifting, time to look at the sediments as they also hold in them the record of how you generate that transition between continent and ocean.

But most of all, let's just drop the idea that continents must be above sea level.

If you'd like to read more on this, go to my paper here

http://www.tandfonline.com/doi/full/10.1080/00206814.2017.1340853