| How might you explain the same type of rock and
rock formations being found on different continents? |
|Question Date: 2019-04-25|
Excellent question, Mia. One thing to keep in mind
is that continents haven't always been arranged
the way they are today. For example, eastern
South America was joined to what's now west Africa
as recently as 120 million years ago. Prior to
that time, a large lake system sat in the middle
of what today are these two separate continents.
These lakes slowly filled in with sediments, which
in turn became a sedimentary rock formation.
(Interestingly, this formation contains the bones
of extinct aquatic reptiles call
mesosaurs.) As South America and Africa
rifted apart from each other, this formation, and
its fossils, were divided between the two
landmasses. Imagine slicing a pepperoni pizza
in half. Some pepperoni pieces will be cut in two.
You can move around the two separated pizza halves
all you want, but those split pieces will match up
again once the two halves are reunited.
Going back to mesosaur fossils, since these
animals lived in fresh water, it would have been
impossible for them to cross an ocean when they
were alive. It makes more sense to conclude that
mesosaurs once lived across a large
supercontinent, which has since fragmented into
Much in the same way that there is only one of
you, each rock formation is unique, and
formed at an individual point in time and space.
Because the Earth's crust is broken into several
different pieces (tectonic plates), if you
were to move pieces of the earth's crust around,
you would move around whatever is on top of the
crust. So, if a rock formation must have been
formed at one singular location, but is now found
in two different locations, the crust MUST have
Dr. Tanya Atwater is a Professor Emeritus at UC
Santa Barbara, and she is one of the pioneers
of plate tectonic theory. She has some awesome
animations which show how the continents have
moved throughout time. You can watch one here: plate
I hope this has answered your question.
Different continents may have similar conditions,
which allow similar rocks and formations to form.
For instance, the Hawaiian Islands and Iceland
are both volcanic and both have a lot of basalt
from when they erupted; however, the basalts
have different chemistry because they came from
The processes that form rocks are the same on
any continent as on any other continent. Some
processes that form rocks are not even unique to
Earth - consider for example the Burns Formation
on Mars, which exogeologists at NASA explored
using the Spirit and Opportunity rovers!
Excellent question Mia! I study this very
question in my research at UCSB.
The quick answer to your question is that
the continents are constantly in motion,
slowly shifting around Earth's surface, so rocks
found on different continents today may have
been a single continent in the past.
If you've ever looked closely at the continent
shapes, you may have noticed how they seem to fit
together like a jig-saw puzzle. The easiest
link is between North and South Americas with
western Africa and Europe. The concept of
"continental drift" was posed by Alfred
Wegener in the early 1900's. He thought all the
continents once fit together into a single
supercontinent whose name in Greek means "all
earth", that eventually broke up and moved into
their current positions. Early geologists rejected
Wegener's hypothesis because he could not provide
a mechanism for how the continents could move.
Later, Marie Tharp and Bruce Heezen created the
first scientific map of the Atlantic Ocean
seafloor. Her research revealed continuous rift
valleys along the center axis of the Atlantic
Ocean, which lead to the currently accepted
theory of plate tectonics.
Plate tectonics theory became widely
accepted starting in the 1970's, and describes
Earth's surface as broken into numerous plates
that move around Earth's surface, either slamming
into another plate, destroyed under a neighboring
plates, or grinding past another plate. The
tectonic plates, or slabs of Earth's crust sitting
on top of mantle rocks, move as rigid crust on
flowing mantle rocks (crust on the mantle is like
a graham cracker on a marshmallow, both are solids
but the mantle rocks can be deformed plastically
like a marshmallow).
The fascinating part about plate tectonics is that
scientists now know that Earth's surface has
gone through several cycles of crust smashing
together into supercontinents and then breaking up
into smaller continents.
Therefore, the similar rock types and rock
formations can be seen of different continents
because those continents were once
connected. Earth's rocks record that history
of the continents shifting around through time.
features are part of a
in support of the theory of
The similarity in the rocks (including both
the types of rocks and features such as magnetic
"stripes"), their layering, fossils found within
them, and the fit of large-scale geological
features (e.g. coastlines and mountain ranges)
spread across several continents led Alfred
Wegener to posit that the continents were
joined together at some point in their past.
Since the continents are now separated, they
must be able to move. This is the
continental drift theory.
Plate tectonics is the
modern version and is an evolution of
continental drift theory. It considers that
the crust of the earth comprises several distinct
tectonic plates which essentially float on a
liquid-ish mantle of melted (but still very
viscous, i.e. thick and not easily flowing) rock.
Those plates can then move,
driven by a variety of forces.
[For more discussion of evidence for continental
drift/plate tectonics and the theories,
questions on ScienceLine.]
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