Crustal Evolution

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Crustal Evolution
The oldest known rock on Earth is about 4.1 billion years old. It is a
granodiorite from Canada’s Northwest Territories. It formed below the
surface as an igneous rock. Its chemistry is somewhere between that of
granite and diorite. Granodiorite is usually formed at subduction zones.
It is possible that plate tectonics was very active over 4 billion years ago.
There probably were many very small plates. Collisions of these plates
would have formed island arcs. As plates joined together, the first small
continents formed. This process is called accretion. Southwest Greenland
contains the oldest preserved crust of a continental landmass. It is
3.8 billion years old. It is Earth’s oldest surviving accretionary orogen.
Here, oceanic lithosphere was subducted, and new continental crust
was produced. However, it is highly fragmented and metamorphosed.
This makes it difficult to study.
Subduction of the young crust was very important for the early
development of continents. Continental collisions result in subduction that
melts the leading edges of plates. As a result, andesitic magma forms.
This type of magma has a lower density than surrounding crustal rocks.
Rising plumes of light, andesitic magma form circular or oval-shaped
batholiths in the crust. They make it not only less dense, but thicker.
This increases its buoyancy. This makes these parts of the crust difficult
to subduct under other plates. As a result, these parts of the crust are
not recycled into the mantle. They remain on the surface where they are
subjected to billions of years of erosion. The thicker crust is also better
insulated from the upper mantle and becomes more stable. These areas of
the crust form continental shields. Only a few locations on Earth contain
continental shields. Canada, Australia, and Africa all contain rocks older
than 3.7 billion years. Most other rocks from Earth’s deep geologic history
have either been metamorphosed, eroded, or melted entirely.
Geologically important rocks are found around the shields. These are
called greenstones. They occur as gigantic belts that can be several
thousands of kilometers in length. The name comes from the green
mica-like minerals they contain. Greenstone belts consist of a mix of
metamorphosed lava flows and sediments from chains of volcanic
islands. They often indicate the island arcs that formed between Earth’s
oldest colliding continents. Remember that subduction made the shields
less dense and harder to subduct. There is another important process
that occurs when continents collide. Great slices of the ocean floor get
plastered onto the edge of an overriding plate. The nucleus of a shield
grows around the edges by the addition of rocks. These rocks contain
dark minerals and basaltic compositions from the seafloor.

Greenstone belts provide evidence of the ocean floor billions of years ago.
Their mineral composition is controlled by the chemistry of magma. It also
depends on the temperature at which the magma cooled. In South Africa,
the Barberton Supergroup is one of the most unique greenstone belts
on Earth. The stack of rocks is 3.6 to 3.2 billion years old. It is over 18 km
thick. It has remained largely unchanged for 3 billion years. The lower 7 km
includes preserved lava flows from Earth’s primitive oceanic crust. They
contain pillow structures like those that form today on modern ocean
floors. Scientists have melted samples of the structures in the lab. From
this, they have determined that the structures formed at temperatures
between 1300°C and 1650°C. Today, basaltic lavas erupting on ocean

floors are rarely above 1200°C. This indicates that the surface of the early
geosphere was similar to today’s geosphere. However, the interior was
much hotter. Earth has cooled significantly over the past 3 billion years.
It continues to slowly lose heat.
Plumes of granodiorite are common in greenstone belts. They cause
shields to dome upward. Trapped between the rising domes, the
greenstones became strongly folded and metamorphosed. They
generate high-grade rocks such as gneiss. Ancient blocks of crust with
greenstone-granodiorite-granite rocks are called cratons. This helps to
tell the difference between them and later-formed continental crust.
For example, the core of southern Africa is called the Kaapvaal Craton.
Today, it is buried beneath younger rocks. This ancient continent once
contained parts of Western Australia, Madagascar, and India. At about
the same time, other cratons formed in other parts of Earth. They formed
in Antarctica, Brazil, Canada, Finland, Siberia, and Greenland.