We are making our way through the Phanerozoic Eon, and we just learned about the first era within,
the Paleozoic Era. Let’s pick things back up right as this era was coming to a close.
Trouble was brewing in Siberia around 250 million years ago. As convection currents
within the Earth’s mantle changed, a mantle plume was forming. Mantle plumes are hot,
rising areas of Earth’s mantle and are responsible for some of the largest volcanic eruptions.
The Siberian Traps erupted around 4 million cubic kilometers of lava within only two million years,
which is comparable to the entire Mediterranean Sea being filled with lava.
Not only did this volcanism spew large amounts of CO2 into the atmosphere, but the volcanoes
erupted in the vicinity of large deposits of coal and carbonate rocks. The heat from
the volcanoes burned these carbonaceous rocks, which released even more CO2 into the atmosphere.
This huge outpouring of greenhouse gas into the atmosphere caused significant global warming,
which along with other factors led to widespread ocean anoxia and acidification, causing the
extinction of over 90% of all species. This was the largest mass extinction event of all time and
is known as the “Great Dying”. Only time will tell if an event of such magnitude will be repeated.
After the end-Permian mass extinction, life immediately began to recover and evolution
kicked into high gear, taking advantage of increased living space and reduction of predators.
Thus began the Mesozoic Era, which is known as the age of the dinosaurs,
since they evolved and subsequently took over Earth during this time.
In addition to the evolution of dinosaurs, which occurred around 240 million years ago,
mammals also evolved during the Mesozoic, but around 30 million years later.
This evolutionary head start helped the dinosaurs dominate the mammals throughout their existence.
For a long time, dinosaurs were imagined to be enormous, lumbering creatures,
but recent discoveries suggest that they may have been warm-blooded, meaning they could raise their
body temperature in cold weather, allowing then to be more active than any other animal at that time,
at least until the evolution of mammals. The most impactful tectonic event of the
Mesozoic Era was the breaking apart of Pangea, which began around 180 million years ago. The axis
along which Pangea broke apart would go on to become the Atlantic Ocean, as the crust was
stretched and thinned down to below sea level due to the divergent motions of the mantle below. The
Atlantic Ocean continues to widen to this day as new oceanic crust is created along its axis, where
hot, upwelling mantle rock partially melts, and solidifies near the surface of the ocean floor.
Along the western coast of North America, which was also the western coast of Pangea,
convergent forces dominated, and a new subduction zone was created around 200 million years ago.
This would begin a theme of continuing cycles of mountain building and volcanism
that have also lasted until present day. This convergent plate boundary was responsible
for uplifting the Rocky Mountains, which began forming around 80 million years ago.
As we wrap up the Mesozoic Era, some extraterrestrial trouble is on the way that
will change the Earth’s biosphere forever. The Mesozoic Era ended with another mass
extinction event, but this one was not of an Earthly origin. An asteroid with a mass
equivalent to about 50 million Titanics slammed into Earth near the present-day Yucatan Peninsula.
The impact itself was equivalent to a 100 million megaton bomb. The thermal pulse from this impact
instantly killed anything nearby and the shockwave created hurricane force winds extending out about
1500 kilometers from the blast site. It was a seismic event equivalent to a magnitude 10
earthquake. Because the asteroid impacted the ocean, tsunamis 100 to 300 meters high radiated
across the Atlantic, crashing along coastlines, and penetrating up to 100 kilometers inland.
Molten fragments of Earth were ejected into the atmosphere upon impact that,
upon returning to the surface, would heat Earth’s surface to hundreds of degrees Fahrenheit,
devastating terrestrial plants and animals, but sparing most marine life due to the high heat
capacity of water. The combination of these events obliterated life proximal to the impact site,
but the real worldwide devastation would come from long term consequences.
Dust particles were blown into the atmosphere, where they remained for months.
This blocked off sunlight, causing both perpetual darkness and a global winter. This several month
period of darkness would have likely killed off anything that relied on photosynthesis for energy,
which would then travel down the food chain as herbivores ran out of food.
However, the longest-term effect of the impact was the global warming associated with CO2 released
from vaporized evaporite rocks, which precipitated from the evaporation of ocean water. These rocks
were primarily composed of calcium carbonate, the source for the CO2, and calcium sulfate, which
vaporized and reacted with precipitation to form sulfuric acid, which rained down upon the Earth.
The released CO2 was the main long-term problem, raising Earth’s temperature by around 7 degrees
Celsius for a few hundred thousand years. In addition, the rapid transition from
impact-induced cooling to warming would have been very difficult for most types of
life to deal with. As a result of the events just described, around 80% of all species went extinct,
including all dinosaurs. Though the loss was great, events at the end of the Cretaceous
Period opened the door for a renewal of life and a mammal-dominated world in the Cenozoic Era.
Let’s move forward and wrap up our journey with this third and final era.