3.6: The Beginning of Plate Tectonics and the Continents

Introduction

Understanding the origin of Earth's first rocks is fairly straightforward, even though no direct evidence of their existence has ever been found. At the start, Earth was basically a hot ball of magma with the densest (metallic) material sinking to form the core and lighter (silicate) material floating upwards and cooling enough to form the first crust, most likely a very mafic (high in iron and magnesium and relatively low in silica) veneer on a magma ocean. Earth may have resembled modern Venus with a crust of mafic volcanic rock, no plate tectonics, and volcanism driving the constructive processes shaping the surface.

As for the start of plate tectonics, the story is not so straightforward. We do know that modern subduction zones didn't exist in the earliest chapters of Earth's story because subduction-zone rocks, such as blueschist (a metamorphic rock covered elsewhere in this text), are missing from the rock record of the Archean and earlier. Today, blueschists form under high pressure and relatively low-temperature conditions along subduction zones. Using uniformitarianism, we can infer that a lack of old blueschists indicates that the tectonic conditions needed to form them didn't yet exist. Consequently, determining the transition from more "Venus-like" conditions to the type of planetary geology we have today, with distinct slabs of interacting continental and oceanic crust, is not so straightforward.

Subduction and Continents

The problem with determining the start of plate tectonics is that we don't know when the lithosphere, with distinct continental and ocean crusts, was established.

Figure \(\PageIndex{1}\): A cross-sectional view of Earth. From top to bottom are the lithosphere, asthenosphere, mesosphere, and inner and outer cores. Image from Volcan26, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

This is crucial because an outer layer composed of distinct continental and oceanic crust enables subduction, a key component of plate tectonics. In fact, subduction may be the most important component in making plate boundaries and driving plate tectonics. Once subduction begins, pieces of crust (terranes) could accrete (smash together like globs of play-dough) to build continents through collisions.

Watch this video to learn about how accretion works.

Figure \(\PageIndex{2}\): Cartoon showing the general process of accretion in growing continents. (Graphic: BZenith via Wikimedia.)

In contrast, where the ocean crust is stretched, it can eventually rift apart into two pieces, allowing seafloor spreading.

So, how did plate tectonics start (before there was distinct continental material)? And how can continental crust be made without plate tectonics? In modern settings, making the felsic rock that composes the continental crust components of plates involves the differentiation of partially melted mantle material through the subduction process and partial melting. This is summarized in the illustration below, where (1) water released from the subducting plate reduces the melting temperature of the overriding plate. This causes partial melting (2) of the ultramafic (very high in metals) mantle and generation of basaltic (high in metals) magma, which rises to the base of the continental crust where it stalls out (3) due to it being more dense. The heat from this magma partially melts the continental crust (4), making intermediate-felsic (lower in metals; higher in silica) magma that rises and further differentiates as it incorporates the continental crust and mixes with other plutons (5 & 6).

Figure \(\PageIndex{3}\): Relation of mafic magma to felsic magma differentiation by Dee Trent.

We arrive at a chicken-and-the-egg scenario, where we can't be certain which came first: distinct masses of continental crust or plate tectonics. There likely was a gradual transition from slabs of ultramafic-mafic (high in metals) crust to distinct plates composed of oceanic and continental crust, with some subduction-like process, or "proto-subduction," as the first step.

Proto-Subduction

Even without plate tectonics as we know it today, it's possible to make some felsic (low in metals; high in silica) material, or at least material that is more felsic than the average crust at the time, in various ways. Perhaps that is all that is needed; a slight increase in felsic material in a section of the crust might have been enough to make it buoyant, start the more complex processes described above, and make full-fledged continents.

The internal heat of the early Earth was much higher than today (as much as 300 degrees Celsius), so there was higher heat flow out of the planet and, thus, a warmer lithosphere. Modeling indicates that this heat would have weakened the lithosphere and made it more prone to breaking. A trigger, such as the extra heat associated with rising mantle plumes (Greya et al., 2015), the "lava" in a lava lamp, or even a very large impact (e.g., O’Neil et al., 2019), may have weakened the lithosphere enough to initiate a subduction-like process.

Figure \(\PageIndex{4}\): Hypothetical core-mantle differentiation processes: Percolation, diking, and diapirism. After Rubie et al. (2015). (CC BY-SA 4.0; By AlexInMetal – Own work, https://commons.wikimedia.org/w/inde...curid=76607390)

What was this earliest form of tectonics like, and what is the evidence for it? The oldest observable features that can answer this question are called greenstone belts. These are linear or branching zones of distinctive low-grade metasedimentary (metamorphosed sedimentary rocks) and metavolcanic (metamorphosed volcanic rocks) rocks. Notable greenstone belts on Earth include examples from the oldest parts of the shields (“continental nuclei”) of Australia, South Africa, Europe, Siberia, Brazil, Antarctica, India, Greenland, China, and north-central North America. All are included in an otherwise fairly homogenous felsic crust of similarly ancient age.

Figure \(\PageIndex{5}\): Map showing the distribution of Precambrian (Archean + Proterozoic) crust on Earth. (Callan Bentley graphic)

The greenstone belts are typically sandwiched between large bodies of cratonic crustal rocks such as granite and gneiss. Not all greenstone belts are Archean; some are also known from the Proterozoic.

The greenstone belts are interpreted as remnants of ancient ocean basins that were squeezed between ancient proto-continental terranes, in the same sense that, in modern plate tectonics, ophiolites mark the positions of former ocean basins that have since been uplifted. The process of partial melting repeatedly “distilled” the planet's silicate crust into small mobile blobs, roughly the size of modern U.S. counties or states, and once solidified, these resisted subduction due to their low density. They drifted about, approaching other similar granitic terranes, with the intervening ocean basin sagging out of the way ("keel" in the illustration below), straight down below. The more sagging occurred, the more room there was for sediments to accumulate atop the subsiding region, until the neighboring granitic blobs got close enough to compress the rocks between them. Once they collided, they often remained stuck together (accretion), forming a granite/greenstone belt/granite horizontal granite sequence. This geometry is called “dome and keel,” and it’s thought to represent a kind of vertical tectonics that dominated during the Archean. Below is a diagram showing the relative motion of the plutonic domes and the surface strata folded into the thin keels.

Figure \(\PageIndex{6}\): Block diagram showing the structure of a typical Archean craton, such as the Pilbara. Arrows show relative motion of the granite “domes” relative to the metavolcanic & metasedimentary “keels.” (Callan Bentley cartoon)

The idea is that the crust hadn’t yet organized itself into thick, laterally continuous slabs like plates. Instead, on a much smaller scale, the primordial crust formed through plutonic processes (pushing up from below) and surface processes (eruption of lava and production and deposition of sediment). The rising domes of granite moved vertically, and to accommodate this relative motion, the surface rocks sagged downward, perhaps even foundering (akin to a sinking ship) and “dripping” into the dynamic mantle.

Early models for the movement of material within Earth have focused on "vertical tectonics" due to the structure of greenstone belts, implying upward-moving mantle plumes and diapirs and downward-moving keels. The extra heat in the mantle likely drove a more active plume system across the planet, with each plume causing heating and weakening of the lithosphere - perhaps similar to the hot spots of today but far greater in concentration. This is known as "plume-lid tectonics" - the rising material from plumes was "trapped" by the young, hot, but continuous lithosphere. Another part of this plume-lid model involves a rock known as eclogite. Models predict a thicker oceanic crust during this time, much thicker than today’s. At a thickness of about 40 km, basalt metamorphoses into eclogite, a much denser rock. This dense lower part of the crust would “peel off” in a process known as delamination and "drip" downward. This proto-subduction is a possible mechanism for material cycling before the onset of plate tectonics. It would also trigger mountain building and volcanism, with the hot, less dense asthenosphere replacing the dense eclogite and driving uplift and the partial melting of the overlying crust (Foulger, G. R. (2011). Plates vs plumes: A geological controversy. John Wiley & Sons.).

With time, accretion and partial melting would result in larger masses of more felsic, buoyant crust; meanwhile, Earth's interior cooled, slowing the rate of rising plumes and diapirs and that frequently disrupted the early continental crust. As crustal pieces became more rigid and permanent, their interaction may have resulted in a slightly denser piece sinking below a less dense one, which in turn could initiate convection currents in the mantle. Eventually, Earth's surface would be defined by distinct oceanic and continental crust, with denser oceanic lithosphere subducting beneath less dense continental crust. Early on, though, subduction is thought to have been shallower than it is today, because higher core heat flow would have prevented oceanic lithospheric slabs from remaining intact deep into the mantle, as is the case today in modern plate tectonics.

The video below, The World Before Plate Tectonics, starts at 2:42. Watch until at least 4:20.

As is typically the case in geology, the younger the rocks, the more detailed their story is because they have had less time to "lose information". In North America, the sedimentary rock record tells a more precise story about the start of the plate tectonics cycle.

Key Terms

• diapir - blobs of magma that rise towards Earth's surface
• greenstone belt - a linear or branching zone of metamorphosed sedimentary and igneous rock within Precambrian cratons; the name comes from the presence of green minerals like chlorite, actinolite, and eclogite
• mantle plume - long-lived narrow conduits of magma rising from Earth's core to make hot spots
• uniformitarianism - the recognition that the processes operating on Earth today must have been operating on Earth in the past as well