19.3: A Very Different Archean

Figure \(\PageIndex{1}\): Blueschist cobble (with white vein) on Kayak Beach, Angel Island, California. (Callan Bentley photo)

There are several things that occurred in the Archean that did not occur afterwards. Perhaps the most famous example is komatiite, the ultramafic lava that does not erupt out of today’s volcanoes. Another example, TTG rocks are only in high abundance in cratonic rocks dated from the Archean and are rarely formed today. A third example has to do with certain metamorphic rocks that are not present in Archean rocks. These include UHPM (ultra-high pressure metamorphic rocks) and blueschists. Both of these are only formed with the lower temperature/higher pressure conditions in subduction. Two conditions must be true to form these rocks: buoyant continents and strong plates, which could explain the lack of blueschists and UHPM rocks before the late Archean is the evidence used, according to Brown and Johnson (2018). Additionally, as mentioned earlier, these melting broken slabs may be the source for the TTG suite of rocks. Finally, various geochemical isotope signatures (Dhuime et al., 2012, Tush et al., 2020) show that looking at rocks before 3 billion years ago and after 3 billion years ago, processes were just different. Isotopes of hafnium, oxygen, and tungsten show dramatic changes occurred in mantle mixing processes around 3 billion years ago. Oceanic basalts also show concentrations of trace elements that shift around this time (Condie 2015). These are significant enough to convince many scientists that the true start of modern tectonics occurred at this time. At a recent conference on the origins and evolution of plate tectonics, a majority of the researchers put the start of plate tectonics at 3 billion years ago. Some are still convinced that it could have started as early as 4.2 billion years ago, others as recent as 1 billion years ago. It should be noted that there are qualifiers, where some scientists say a different tectonic paradigm existed earlier, whereas only modern plate tectonics, like we know, started later. Evidence for this includes kimberlites, rocks that bring up diamonds that may owe their origin to modern tectonics. While some are old, many more are found within the last billion years of the rock record (Stern et al., 2016). Also, since the oldest blueschist is 780 million years old in China, the oldest UHPM rock is 620 million years old in Mali, it could mean a much more recent start to modern tectonics. Ophiolites are similar in their record, and are a clear indicator of modern tectonics. There are a few ophiolites that are much older, close to 2 billion years old, but the vast majority are less than 1 billion years old. Of course, the incompleteness of the rock record, especially as one ventures further into the past, should be considered. It is certainly possible that the lack of these rocks going back in time is due to destruction via the rock cycle and tectonics.

Figure \(\PageIndex{2}\): Archean Cratons (light grey) and locations of Earth’s oldest rocks on a world map. (By Jonathan O’Neil, with permission, from: UOttawa Early Earth Blog)

Whenever it started, it is clear that by the end of the Archean, most of the cratons on Earth had formed, though the growth of continental cores like cratons has been shown to be episodic (McCulloch and Bennett, 1994), even in the Archean, meaning some sort of process must have periodically added felsic crust. The Wilson Cycle can help explain this pattern today and in the recent past. It is not as useful for a time before tectonics, nor at explaining that most cratons are dated to ~3 billion years old and older. One last item to consider: most of the cores of all modern cratons are Archean and subsequent rifting has mostly gone around them, not through them, presumably due to their strength. Were the processes that occurred before 2.5 billion years ago so fundamentally different, in that the only time in Earth history where cratons could form (known as ‘cratonization’), was in the Archean? Some difference in how the Earth constructs its surface, from before the Archean to after, appears to be true. If true, it would be a significant exception to the idea of uniformitarianism.

Did I Get It? – Starting Tectonics and Making Continents Quiz

Exercise \(\PageIndex{1}\)

Which tectonic process makes continents today, and thus, it is important to know when it started to understand continental formation?

a. Subduction

b. Rifting

c. Seafloor spreading

d. Collision

Answer

a. Subduction

Exercise \(\PageIndex{2}\)

Early "tectonics" focused on which kind of movement?

a. Circular (convective)

b. Equant (square)

c. Vertical (up/down)

d. Lateral (side to side)

Answer

c. Vertical (up/down)

Exercise \(\PageIndex{3}\)

Which rock type, found only in recent rocks, can only form via the cold temperatures of subduction?

a. Ophiolite

b. TTGs

c. Kimberlites

d. Blueschist

Answer

d. Blueschist