The area in Greenland, also called the Isua Supracrustal Belt, is an Archean greenstone belt located in southwestern Greenland. (Archean is a geologic eon, occurring before the Proterozoic eon, which ended about 2.5 billion years ago.) The area contains, now, the oldest rock ever discovered, along with other 3.7 to 3.8 billion year old, well preserved metavolcanic, metasedimentary, and sedimentary rocks.
Metavolcanic rock is produced by volcanoes and later buried beneath other rock and subjected to extremely high pressures and temperatures until it eventually recrystallizes. Metasedimentary rock is sedimentary rock that has been subjected to metamorphism (the re-crystallization of rock due to extreme temperatures and pressures).
The rocks were found within something called sheeted dikes, which are vertical layers of previously molten rock that is compressed between other sheeted dikes, which are then trapped between pillow lavas (above them: lavas formed from underwater eruptions and shaped like pillows) and hard crystalline rocks (below them). The whole combination of rock is called ophiolite, which includes rocks from the oceanic crust and the upper mantle that have been lifted and is observable along with continental crustal rocks.
What is exciting for scientists about this find it the ability, now, to move back the movement of the Earth’s tectonic plates to an earlier time. With the Earth being formed about 4.5 billion years ago, scientists were not really sure whether plate tectonics occurred early in the first half of this history or later in the second half.
Plate tectonics is the geological theory that explains the large scale movements of the Earth’s crust.
This Greenland discovery, because these rocks seemed to have formed due to the processes that occur due to plate tectonics, shows clear indication that plate tectonics was occurring 3.8 billion years ago—thus, in the first half of the evolution of the Earth.