History of the Earth Part 3: Phanerozoic Eon – Mesozoic Era

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.