Hi, I’m Ali Sealander with the University of Texas at Dallas
and I’m here to tell you about how the oldest continents formed.
Let's start with the basics.
The Earth’s crust is divided into two types: oceanic and continental crust. The oceanic crust
makes up around 70% of the Earth’s surface, it’s thin – between 4 to 6 kilometers thick, or about 2.5 – 4 miles,
and relatively young – no more than 200 million years old. The rocks here are born in oceanic
spreading ridges and then return to the mantle in the subduction zones at convergent boundaries.
Continental crust makes up around 30% of the Earth’s surface, and is about 40km thick,
or about 25 miles. Because of their low density, continents are more buoyant than oceanic crust,
Which means that they don’t get recycled back into the mantle at subduction zones.
The continents are composed of
rocks that formed over the past 4 billion years. But with time comes erosion which wears down
the continents. That eroded rock is carried by rivers to the sea. Plate tectonics does a good job
of subducting the oceanic crust and the overlying sediments, returning that material to the mantle.
Thus, much of the earliest crust that formed has been recycled and lost from the rock record.
As a result, we don’t know how much continental crust formed in the first 2 billion years of
Earth’s existence. But some old crust has survived. All continents contain some crust
that is 2.5 billion years or older, way from way back in the Archean eon.
These areas of old continental crust are called cratons, from the German word kratogen meaning “born strong”.
In contrast to the faulted and folded regions that surround them, cratons are undeformed because they
are underlain by a strong mantle keel made of thick and buoyant, long-lived mantle lithosphere.
Archean cratons preserve an important record of crust formation and growth
through Earth’s history and host important ore deposits.
So how do we get these cratons?
A new paper in the January 2023 issue of GSA Today looks at the story preserved
in the oldest rocks on Earth. Author Carol Frost and her fellow co-authors
explore how these Archean cratons formed using the craton in Wyoming as an example.
In Wyoming Archean rocks are exposed in mountain uplifts that show vertical,
three-dimensional sections of Archean crust for geoscientists to study. The new paper suggests
that the Wyoming craton, like most cratons, results from three stages of evolution – An
initial stage where a thick basaltic or mafic crust forms,
a middle stage characterized by a
crust made of granitic rocks called TTGs made by
partially melting the initial mafic crust, and the underlying mantle
and a final stage with rock assemblages that formed from plate tectonic processes.
We will start with the youngest Archean rocks because they are better preserved
than rocks of the earlier stages, so our interpretations are more confident. We will
then work back in time and take a look at what happened in the second and first stage.
The third and most recent stage led to the final stabilization of the Wyoming craton.
It is characterized by rock assemblages formed by plate tectonic processes, most
notably when plates collide. Rocks in this stage include examples of continental magmatic arcs,
high-pressure continent-continent collisional zones, granites formed by partial melting of
aluminum-rich sedimentary rocks like shale, and lastly we will see examples of accreted terranes.
These features are indistinguishable from younger examples formed by modern tectonic processes.
Let’s look at one example of each of these four features.
Continental magmatic arcs form on continental crust above subduction zones.
Voluminous continental arc batholiths composed of gabbro to diorite to granite first appear
in the Wyoming craton in the Bighorn Mountains and Beartooth Mountains, then in the Wind River
Mountains, where granitic rocks form magnificent scenery. The compositions of these continental arc
batholiths and their formation from a combination of mantle and crustal magma sources indicates that
they are analogous to modern continental arcs like the Andes of South America.
High-pressure metamorphic rocks form where continents collide, like in the modern Himalayas.
One of the best examples of Archean rocks formed by continent-continent
collision is preserved in the northern Teton Range, where two distinct gneisses
with contrasting geologic histories were juxtaposed 2.68 billion years ago along
shear zones that put high pressure metamorphic rocks against lower grade metamorphic rocks.
Some granites form by melting aluminum-rich sediments like
shales and these can be recognized because they contain aluminum-rich minerals such as garnet.
Such granites are characteristic of collision zones like the Himalayas. In the Wyoming craton,
they are found in the northern Teton Range and also in the east. (SLIDE 22)
Collision of smaller blocks to form large crustal tracts is
called “terrane accretion” and is typical of modern plate-tectonics. In the Wyoming craton,
metamorphosed igneous rocks of oceanic affinity and immature metasedimentary rocks occur along
the southern margin of the Wyoming craton, accreting to it 2.65-2.63 billion years ago.
The middle stage in the Archean evolution of the Wyoming craton is when voluminous granitic igneous
rocks formed by partial melting of an older mafic crust. As this diagram shows,
there are many kinds of granitic rocks and those of this episode are comprised of tonalites,
trondhjemites, and granodiorites, known as TTG for short. These rocks formed episodically over
600 million years to produce the continental nucleus of the Wyoming craton. The oldest
rock in the Wyoming craton is a 3.45 billion year old TTG found in the Granite Mountains.
This might be the oldest rock in the United States! Slightly younger TTG occur throughout
the central and northern Wyoming craton, with a major TTG episode at around 3.3-3.2 billion years ago.
Scientists think that TTG magmas form by melting hydrated mafic rocks ~35 km or ~20 miles deep
in the Earth, implying the presence of a thick mafic crust similar to modern oceanic plateaus
that would have formed during the initial stage of craton development. The geodynamic setting where
partial melting of both crustal and mantle sources would produce voluminous TTG remains unresolved,
with a variety of plume-based and horizontal plate-based tectonic scenarios proposed.
There are no rocks preserved from the first stage of the Wyoming craton, but we know something
about this stage because of a mineral called zircon. Zircons are tiny but important
crystals in the science of dating rocks, or geochronology. Once zircons are formed they
are hard to destroy and they also trap small amounts of radioactive uranium when they form.
Uranium decays to lead and we can use the ratio of uranium to lead to determine when the zircon
crystallized from a melt. Most zircon ages tell us the when the rock that contains them were formed.
But sometimes zircons are inherited by other rocks and are no longer within the rock they originally
formed in. The sturdy zircons may have persisted while their igneous host rock vanished. Inherited
zircons are found in some sedimentary rocks and in younger granites and preserve an important record
of rocks that no longer exist. Like an heirloom passed down through the generations. Inherited
zircons in the Wyoming craton rocks range in age from 3.2 to 4 billion years old and suggest that
the earliest stage was formed of thickened basaltic crust like that of Hawaii today.
This crust may have formed over a mantle plume. The thick pile of basaltic rocks
partially remelts to form small volumes of felsic melts that increased with time to form
the embryo of the craton. In the Wyoming craton, the ages of inherited zircon grains suggest the
first felsic melts crystallized at about 3.8 to 4 billion years ago, the age of the oldest zircons.
Maybe someday rocks of the first stage will be found in the Wyoming craton!
In summary, Archean crust of the Wyoming craton formed as a result of three stages
over about 1.5 billion years, very early in Earth history. We know least about the first stage,
which occurred from soon after the Earth formed at 4.6 billion years to around 3.5 billion years ago,
because we only have the testimony of zircons. During this stage, basaltic crust formed.
This early crust became thick and hot enough that the lower portions melted to create TTG melts.
These early TTGs were the seeds from which the Wyoming craton grew. The second stage occurred
between 3.5 and 2.9 billion years ago and is dominated by formation of vast volumes TTGs.
The oldest rocks in the Wyoming craton and maybe the United States formed during this stage.
Both stage 1 basaltic crust and the underlying mantle melt to produce TTG magmas.
The final stage in forming the Wyoming craton occurred between 2.6 and 2.8 billion years ago.
This stage was characterized by modern plate tectonic processes. Starting at around 2.8 billion years ago,
a number of rock assemblages characteristic of modern plate tectonic environments are preserved
in the Wyoming craton, including continental arc batholiths, continent-continent collision zones,
production of aluminum-rich granites from melting of metasedimentary rocks,
and accretion of exotic terranes. We hope you enjoyed learning about
the Wyoming craton. If you want to learn more, please read the article in the January 2023 issue of GSA Today.
It’s free to download, just google “GSA Today” and navigate to the January 2023 issue.
Or check out the description box below this
video for a link directly to the article
While you’re there you might want to check at some of the other articles that
are also free to download. And if you want to learn more about the Wyoming craton and other
cratons around the world, take a look at some of the references at the end of this video.
This video was made in collaboration with Carol Frost and Susan Swapp of the
University of Wyoming, and UT Dallas’s Geoscience Studio. Thanks for watching!