When did plate tectonics begin?
Earth is the only known planet with moving plates. Estimates of when they started range over three billion years.
The question
Earth's surface is broken into rigid plates that slowly move, a few centimetres a year. Where they pull apart, new ocean floor forms; where they collide, one sinks beneath the other into the mantle, a process called subduction, building mountains and volcanoes. This machinery recycles the crust, controls the long-term climate by moving carbon between the surface and the interior, and creates the continents we live on.
No other known planet has it. Venus, similar in size, has a single rigid lid. Mars stopped long ago. And on Earth, it seems unlikely that plates were always moving: the young planet was much hotter, and hot rock behaves differently. So when did plate tectonics begin? Researchers' answers range from more than four billion years ago to less than one billion.
What we know
- Plate tectonics as a theory was established in the 1960s. Soon after, geologists began asking whether it had always operated. In 1966 J. Tuzo Wilson showed that an ancient ocean had opened and closed where the Atlantic is now, the first sign that the process had a long history.
- The oldest known mineral grains, zircons from the Jack Hills in Western Australia, are about 4.4 billion years old. Their chemistry suggests that liquid water and some kind of continental crust existed very early. Whether that crust was made by plates is disputed.
- Rock samples older than about 3.5 billion years are rare and heavily altered. Most of the early record has been destroyed — by plate tectonics itself, among other things.
- Many typical features of modern subduction appear only late. Blueschists and ultra-high-pressure rocks, formed when cold crust is pushed deep and brought back up quickly, are almost entirely younger than about 800 million years.
- Other signatures, such as changes in the chemistry of continental crust and in the way continents grew, shift noticeably around 3.2–2.5 billion years ago.
- The early mantle was perhaps 200 degrees Celsius hotter than today. Hotter mantle makes thicker, more buoyant ocean crust, which may sink less easily.
Main hypotheses
Very early
Plates were moving more than four billion years ago, in some form, almost as soon as there was a solid crust. Supporters point to the chemistry of the oldest zircons and argue that only plate tectonics can efficiently cool a planet.
Gradual start in the Archean
Before plate tectonics there was a "stagnant lid" or a "squishy lid": a crust broken and recycled by hot plumes, drips and occasional, short-lived subduction. Global, continuous plate motion emerged gradually around 3.2–2.5 billion years ago, as the mantle cooled.
Late
Modern-style plate tectonics, with deep, cold subduction, began only about one billion years ago or later, as shown by the timing of blueschists and similar rocks. Earlier, the Earth worked in a different way.
Not one start
Perhaps plate tectonics started and stopped several times, or was active in some regions and not others, before becoming permanent. Then asking for a single date would be the wrong question.
Where understanding ends
The difficulty is that the evidence is a record written by the very process we are trying to date, and that process erases its own past. Ocean floor older than about 200 million years has almost entirely been subducted. What remains from the first billions of years are fragments, often reheated and deformed, from which a global picture must be reconstructed.
And the indicators disagree because they measure different things. A sign of subduction in one place doesn't prove global plates; the absence of modern rock types may only mean the hotter Earth produced different ones. Each hypothesis can be supported by some evidence, depending on what is taken to define "plate tectonics".
Underneath lies a question of physics: we still don't fully understand why plates exist on Earth at all. Models of how a hot planet's lid breaks into plates depend sensitively on the strength of rock, water content and temperature, all of which are uncertain for the early Earth.
What would settle it
- An agreed definition of plate tectonics that can be recognised in old rocks, and a set of indicators that all point to the same time.
- More and better-preserved ancient rocks, and new chemical methods that can tell plate-driven crust from crust made in other ways.
- Physical models of the early Earth that predict which kind of tectonics should appear when, and match the rock record.
- Comparisons with other planets, including rocky exoplanets, which may show how common plate tectonics is and what it requires.
Sources
- Wilson, J. T. (1966). Did the Atlantic close and then re-open? Nature, 211(5050), 676–681.
- Wilde, S. A., Valley, J. W., Peck, W. H., & Graham, C. M. (2001). Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature, 409(6817), 175–178.
- Stern, R. J. (2005). Evidence from ophiolites, blueschists, and ultrahigh-pressure metamorphic terranes that the modern episode of subduction tectonics began in Neoproterozoic time. Geology, 33(7), 557–560.
- Harrison, T. M. (2009). The Hadean crust: evidence from >4 Ga zircons. Annual Review of Earth and Planetary Sciences, 37, 479–505.
- Korenaga, J. (2013). Initiation and evolution of plate tectonics on Earth: theories and observations. Annual Review of Earth and Planetary Sciences, 41, 117–151.
- Cawood, P. A., et al. (2018). Geological archive of the onset of plate tectonics. Philosophical Transactions of the Royal Society A, 376(2132), 20170405.