Earth is a unique planet. It's the only planet in the solar system that has continents--high standing crusts that have a felsic bulk composition (rich in Si, low in Mg and Fe). When and how did the continental crust form on Earth? As a land species, we want to know the answers because we owe our existence to Earth continents. The growth and evolution of the continental crust are closely related with many geological and exospheric processes, including the evolution of life. It's been one of Earth scientists' main tasks to decipher the origin of Earth unique continents.
Earth continents have been restless since their formation. They have been "drifting" around over Earth history. Sometimes they assemble to form supercontinents; sometimes they break up into several pieces, like modern continents. The continents are separated by oceans and oceanic crust lying beneath, which is generated at mid-ocean ridges and subducted near continent margins. The movement of continents is accompanied by the open of new oceans and close of the old. This process is called "plate tectonics", the major engine that drives the mass exchange between Earth interior and surface. Plate tectonics, which makes Earth an active planet today, has never been observed on other terrestrial planets.
Continents and plate tectonics, both unique to our Earth, are actually linked to each other.
In our latest work, we found new evidence for when the present-day like continents started to form. About 3 billion years ago, the proto-continent is only a fraction of present-day continent by area. The surface of the proto-continent, or subaerial crust in the early Archean, has a mafic bulk composition (rich in Mg and Fe, low in Si), comparable to the surface of other terrestrial planets, like Venus and Mars. This mafic upper continental crust gradually transformed to a felsic and present-day like upper continental crust from 3 to 2.5 billion years ago, attended by a massive five-fold continental area expansion. Driving this fundamental transition is probably the initiation of global plate tectonics around 3 billion years ago, which transported water, a key component to generate felsic magma, to the sources.
Earth continents have been restless since their formation. They have been "drifting" around over Earth history. Sometimes they assemble to form supercontinents; sometimes they break up into several pieces, like modern continents. The continents are separated by oceans and oceanic crust lying beneath, which is generated at mid-ocean ridges and subducted near continent margins. The movement of continents is accompanied by the open of new oceans and close of the old. This process is called "plate tectonics", the major engine that drives the mass exchange between Earth interior and surface. Plate tectonics, which makes Earth an active planet today, has never been observed on other terrestrial planets.
Continents and plate tectonics, both unique to our Earth, are actually linked to each other.
In our latest work, we found new evidence for when the present-day like continents started to form. About 3 billion years ago, the proto-continent is only a fraction of present-day continent by area. The surface of the proto-continent, or subaerial crust in the early Archean, has a mafic bulk composition (rich in Mg and Fe, low in Si), comparable to the surface of other terrestrial planets, like Venus and Mars. This mafic upper continental crust gradually transformed to a felsic and present-day like upper continental crust from 3 to 2.5 billion years ago, attended by a massive five-fold continental area expansion. Driving this fundamental transition is probably the initiation of global plate tectonics around 3 billion years ago, which transported water, a key component to generate felsic magma, to the sources.
Fig. 1. A size-composition comparison between the proto-continents in the early Archean (>3 billion years ago) and present-day continents. The distribution and shapes of the early Archean proto-continents shown here are not real. The green color of the early Archean ocean indicates high Fe (II) contents in the seawater at that time. Fe (II) is an ion that makes a green solution.