Our Earth has looked radically different in the past compared to today. Supercontinents emerged and broke apart. In their wake, they profoundly changed the course of life, geology, and climate on Earth. They caused sea levels to rise and fall, volcanoes to spew fire, and massive mountains to emerge. Before we explore Pannotia, Gondwana, and Pangea, we must first understand the concept of plate tectonics.
In order to understand how supercontinents form and break apart, we must first understand plate tectonics. The concept of plate tectonics being accepted by the scientific community is actually fairly recent. Alfred Wegener prototyped the idea with his idea of ‘continental drift’. Continental drift is the idea that continents float across the ocean floor. Wegener thought this was true because of how South America fits neatly into Africa.
While his idea of the mechanism that moved the continents was wrong, Wegener’s belief that continents move throughout geologic time is absolutely right. Surprisingly, records indicate that map makers from as early as the 1500s suspected that continents had moved over time.
The currently accepted mechanism for the movement of continents is the concept of plate tectonics. Plate tectonics is the idea that the crust of the Earth is made up of a bunch of oddly shaped puzzle-like pieces. These puzzle pieces, or tectonic plates, sit on top of the Earth’s mantle, which is the molten center of the Earth. This molten center has currents, sort of like our ocean. These currents of the super-hot, molten mantle move the plates on top of them. These plates collide with one another and separate from one another based on this mantle convection.
A simplified map of Earth’s modern tectonic plates. Red lines indicate plates that are colliding with one another whereas green lines signify plates that are moving away from one another. Map by Hugh Rance via Wikimedia.
Geologists still argue over how many tectonic plates there are. Some people think there are as few as a dozen, while others think there are over 150. The Pacific plate is the largest, which covers about 20% of the Earth underneath the Pacific ocean. The smallest plate may be smaller than the city of Salt Lake City, at 278 square kilometers!
The two main types of tectonic collisions, subduction and uplift, are important to know before delving into supercontinents.
Subduction occurs when two plates move towards each other and one plate slides underneath the other. Typically, an oceanic plate will move below a mostly land, or continental, plate. This is because the thicker continental crust is more buoyant atop the Earth’s mantle compared to the thinner oceanic plate.
Think of it this way. If a huge container cargo ship collided with a small boat, which one would go under the other? The cargo ship would plow overtop the small boat, sending it underwater. The huge force of buoyancy of the cargo ship keeps it floating above the water while the small boat, with its relatively small buoyancy, sinks. The thick continental plates plow over the thin oceanic plates for similar reasons.
As the oceanic plates subduct into the mantle, they form subduction zones. These zones often form coastal mountain ranges, inland volcanic ranges, and earthquakes. The coastal ranges of the western U.S. compared to the volcanic Cascades, which are slightly inland, are an example of this crustal action.
Uplift usually happens when two continental plates collide with each other. Since two continental plates have relatively similar buoyancy, one doesn’t slide underneath the other. Instead, the two landmasses crash on top of each other, piling on rocks that eventually form mountains.
The Himalayas, the largest mountain chain in the world, are a result of such uplift. The plate motion of the Indian subcontinent has been colliding with the Eurasian plate for the last 10 million years. This collision created the Himalayas. These mountains continue to grow because the Indian plate continues to move north into the Eurasian plate.
Subduction and uplift are the major events that lead to the formation of supercontinents. When enough plates collide together via subduction or uplift, a supercontinent is formed.
The three most recent supercontinents were Pangea, Gondwana, and Pannotia. Geologists think there were other supercontinents before these three, which are called Nuna (or Columbia), Rodinia, and Ur.
One definition of a supercontinent is a single landmass that contains at least 75% of all land on Earth. By comparison, the current African-Eurasian landmass contains about 57% of the land on our planet.
The supercontinent cycle, first proposed by Damien Nance in the 1980s, is a cycle no one organism will ever be able to experience. This cycle happens over the course of hundreds of millions of years. Beginning with a supercontinent, the tectonic plates will eventually begin to drift apart. This drift causes the supercontinent to break up into smaller continents. Eventually the landmasses begin to connect on some other side of the Earth to create a new supercontinent.
We are currently in the part of the cycle between supercontinents. The most recent supercontinent was Pangea, which began to break up about 175 million years ago. This cycle doesn’t happen on regular time intervals. Instead, it is rather random. Surprisingly, the supercontinent cycle has incredibly important implications for the Earth’s climate, biodiversity, sea levels, and general geography (more on these later).
Pannotia was a short-lived supercontinent that was centered around the south pole. Modern-day Africa was at the center of Pannotia. Apparently, the scientific jury is still out on whether Pannotia actually existed. Pannotia existed during the very end of the Neoproterozoic period. Life during this period was pretty basic. Shelled organisms didn’t exist but some kinds of worms did. We don’t have too many fossils from this period and prior to it.
The climate on Pannotia was likely pretty hostile. After the breakup of Rodinia (the supercontinent before Pannotia), Earth experienced a massive glaciation that caused a mass exctinction. The tectonic activity that eventually led to Pannotia also created tons of supervolcanoes that erupted. These eruptions sent cooling sediments and gasses into the atmosphere, causing drastic climate change that supercooled our planet. The icecaps may have covered the entire planet, even in the tropics. This precambrian era couldn’t have been different from the explosion of life that followed.
When Gondwana broke apart from Pannotia, Pannotia ceased to be a supercontinent.
Gondwana lasted for a particularly long time. It assembled hundreds of millions of years before Pangea. Gondwana formed a large part of the Pangean supercontinent and even persisted for tens of millions of years after Pangea broke up.
Gondwana was something of a miniature supercontinent. It didn’t contain all land on Earth, or even close to it, really. Nearly of Earth’s modern southern hemisphere landmasses were part of Gondwana. In addition, the Arabian peninsula, North Africa, and the Indian subcontinent were part of Gondwana. Since Gondwana didn’t approach 75% of Earth’s landmass, some geologists don’t consider it a supercontinent.
The assembly of Gondwana coincided with the Cambrian explosion. Before the Cambrian explosion, life was mainly single-celled or simple, multicellular organisms. Geologists begin to see all major animal groups emerge during the Cambrian explosion.
Gondwana’s assembly created the first massive mountain range on Earth. Remnants of this mountain range can be found in Brazil and northern Africa. These mountains were thought to be Himalayan in scale. The erosion of these colossal mountains sent essential nutrients and sediments into the sea. Scientists think that these very sediments provided the conditions necessary for the Cambrian explosion.
Pangea (sometimes spelled pangaea) was Earth’s most recent supercontinent. This supercontinent contained nearly all the land on Earth. This fact is reflected in Pangea’s name which means ‘all lands’ in Greek. In total, the single continent of Pangea took up about 1/3 of Earth’s surface. The other two-thirds of the Earth was a single ocean, named Panthalassa.
Pangea began forming with the creation of Laurussia. This large continent was created when Laurentia, the core of modern North America, collided with two other continents, Avalonia and Baltica. This collision and uplift created the northern Appalachian mountains. This new continent was named Euramerica. Eventually, Euramerica impacted the northwestern part of Gondwana, which created the southern Appalachian mountains. While they aren’t incredibly tall today, the Appalachians may have been as tall as the modern Himalayas back during the formation of Pangea.
In Pangea, modern Africa and South America were snuggled up against each other. North America butted in with Florida between South America and Africa. Eurasia was connected to the northern part of Africa. When modern borders are put to the Pangean continent, it doesn’t look too different from our world.
However, life on Pangea did look much different than it does today.
Pangea formed during the two final periods of the late Paleozoic era, the Carboniferous period and the permian period. Life created the amniotic egg during the Carboniferous. Amniotic is just a fancy way to say an egg with a hard shell. Until this time, eggs needed outside moisture to keep them from drying out (think fish eggs). The amniotic egg allowed animals, such as birds and reptiles, to lay eggs in drier areas. As the interior of Pangea dried out, this amniotic egg was crucial to the success of the dinosaurs.
The Paleozoic era ended about 250 million years ago with the largest mass extinction on Earth. This extinction killed about 96% of species. Yikes!
Dinosaurs emerged on Pangea about 250 million years ago during the Triassic period after that nasty extinction. They reigned through the breakup of the supercontinent until the mass extinction about 66 milllion years ago. Apparently, reptiles and dinosaurs liked to inhabit the drier parts of Pangea that had one rainy season. Mammals, on the other hand, lived in wetter places with two wet seasons. During this era, Earth was 38F( (20C) hotter than today! There was 5 to 20 times as much carbon dioxide in the atmosphere, which created a massive greenhouse effect. That extra heat was crucial for the huge, cold-blooded dinosaurs.
As Europe, Asia, and North America broke away from Gondwana, Pangea ceased to exist. When Africa, South America, and India left Gondwana about 150 million years ago, the landmass was no longer a supercontinent. Rifting between Australia, New Zealand, and Antarctica about 67 million years ago got rid of any remnants of Gondwana.
Paleogeography, or the study of ancient geography, is the Earth science that studies topics like supercontinents. While I won’t claim to understand their techniques, there are a few easy-to-understand ways they can prove past supercontinents.
First, the geologic record shows that the same types of igneous, basalt, or sedimentary rocks appear in places far from one another. The age of these rocks date back to the formation of their respective supercontinents. For example, the rocks that made those first huge Gondwana mountains are found both in Brazil and northern Africa and date to the same period. This is strong evidence that these two landmasses were in cahoots forming that mountain range. Scientists can use the radioactive decay rate of different elements to get fairly precise estimates of the date of rocks hundreds of millions of years old.
Second, fossils of the same species are found in places that are currently way far apart from each other. For example, the fossils of the same species of small, land-based reptiles have been found on Antarctica, India, and South Africa. Since these species couldn’t have traversed the Atlantic or Indian oceans, it serves as more evidence for Gondwana as a previous supercontinent.
One other way to prove the supercontinent’s existence is through paleomagnetism. When rocks form, their magnetic grains point towards Earth’s magnetic pole. By looking at the orientation of these grains and comparing them to the known location of Earth’s magnetic pole at the time, scientists can get an idea of where the rocks were formed. Think of this paleomagnetic study sort of like an ancient compass.
As the supercontinent cycle goes on, our current continents will inevitably form a new massive continent. While geographers and geologists can’t predict the shape of the continents and seafloor billions of years from now, we have a general idea of what the Earth will look like in the near future. There are few competing ideas of the next supercontinent. Aurica is one option. It is the idea that the next supercontinent will be hotter and centered around the equator. Amasia is a supercontinent that might form near the Arctic circle. Either way, it will be at least 200 million years before Earth has another supercontinent.
Featured image by Fama Clamosa via Wikimedia.
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