Geology Reference
In-Depth Information
more active tectonic processes at plate boundaries. Except for the greenstone and
granite-gneiss formations, very little evidence of these early, extremely active tectonics
remains due to the continued subduction and recycling of earth's crustal plates over the
millions of years since the Precambrian ended.
Originating with Orogens: Supercontinents
of the Proterozoic Eon
During the Proterozoic eon (between 2.5 billion and 542 million years ago), the young
continents moved around the earth crashing into each other. Landmasses grew, rocks
were deformed, and elevations changed through a sequence of
orogenies,
or mountain-
building episodes.
The long, linear rock formations that are deformed by each continental colli-
sion are called
orogens.
By mapping orogens, geologists have constructed a his-
tory of some of the events that created modern continents.
Over a period of almost 2 billion years, plate movements continued, building early con-
tinents such as
Laurentia,
which contained pieces of modern Greenland and Scotland at-
tached to the craton of North America. Laurentia was formed by the collisions of plates
(orogenies)
and by
accretion,
or the addition of rock material from the crustal plates that
it collided with.
Laurentia was a large landmass, but it was not a supercontinent. A
supercontinent
is a
single landmass consisting of at least two preexisting landmasses. While Laurentia con-
tained pieces of more than two modern continents, it was only one of the Precambrian
continents. (Numerous smaller land masses including Baltica, Siberia, and others were
also present.)
By approximately 1 billion years ago, all the Precambrian continents had crashed to-
gether and formed the first supercontinent,
Rodinia.
The evidence for the formation of
Rodinia is found in rocks of a substantial mountain-building event called the
Grenville
Orogeny
dated to between 1.3 and 1 billion years ago. The rocks of the Grenville Orogeny
are found on the cratons of all the modern continents (illustrated in Figure 18-1). The
Grenville Orogeny is responsible for accreting large portions of continental material
onto Laurentia as it formed Rodinia. In fact, when Laurentia was part of Rodinia, 75 per-
