Science stories, particular ones about the way the Earth formed and such, are often quite fascinating. This one is no different; apparently the early Earth was covered in a ten kilometre thick crust of lava from a series of monumental volcanic eruptions.
by William Atkins
According to an Australian ancient-earth researcher, about two to three billion years ago most of the Earth was buried under six to nine miles (10 to 15 kilometers) of lava from a
series of volcanic explosions.
Australian geoscientist Geoffrey F. Davies authored the paper “Episodic layering of the early mantle by the ‘basalt barrier’ mechanism,” which appeared in the journal Earth and Planetary Science Letters online on October 5, 2008 and in print form on November 15, 2008.
Dr. Davies is associated with the Research School of Earth Science (RSES), Australian National University (ANU), in Canberra, Australia.
The results were compiled through computer models made to accurately simulate the Earth’s interior.
The modeling used by Davies showed that the volcanic activity occurred when extreme radioactive-generated heat from the deep interior of the Earth built up until it finally caused the volcanism to erupt on the surface of Earth.
This explosive activity happened about two to three billion years ago, and each episode lasted for approximately one hundred million years.
Thereafter, the volcanoes went dormant but eventually repeated the cycle after about 100,000 to 150,000 million years.
Each episode destroyed most of the microscopic life on Earth, but the life that remained, only the strongest species, was able to recover and survive during the dormancy of the volcanoes.
The episodes ended about three billion years ago, and are not expected to occur again, according to Davies.
Davies stated, “The effect is caused by basaltic oceanic crust that is carried deep into
the Earth as tectonic plates sink into the Earth’s mantle. The basaltic crust is a little denser than the mantle at most depths, but is lighter within a small depth range about one third of the way down.”
He added, “The foundered crust tends to accumulate at this depth and, if enough of it collects, can form a ‘basalt barrier’ that prevents deeper mantle material from rising to the surface where it can cool.”
Davies stated, “The deeper mantle is then slowly heated by radioactivity. Eventually it gets hot enough to break through the barrier and rises in a rush. It melts as it reaches the lower pressures near the top of the mantle and the melted rock erupts as lava flow. The lava flows could pile up to a thickness of ten to fifteen kilometres over much of the Earth.”
“The episodes happened about every hundred million years so the bacteria would have had plenty of time in between to recover. Eventually, as the Earth’s interior slowly cooled, the tectonic plates became thick and heavy enough to break through the basalt barrier and
prevent it from reforming, and no further episodes occurred.”
The research performed by Dr. Davies helps to advance the scientific theory about how the continents now present on modern Earth were first formed two to three billion years ago.
The abstract to his paper goes into further detail about the ancient volcanism that occurred on Earth.
Davies stated within his paper, “Subducted oceanic crust is buoyant between depths of 660 km [kilometers] and about 750 km, and this causes early episodes of layering and breakdown in numerical models of the dynamical evolution of the mantle from 4.5 Ga [geologic age] ago until the present.”
“During layered periods, the upper mantle cools and the lower mantle warms, reaching temperature differences of up to 300 C [Celsius], and oceanic crust is only a few kilometers thick, which would facilitate subduction and plate tectonics. Layering inhibits heat loss,
so that high average mantle temperatures persist.”
His abstract continues,“The layering breaks down roughly every 100–150 Ma [million years],
at which time hot, fertile lower mantle floods the upper mantle and there is a dramatic burst of magmatism lasting a few million years and manifest in the models as oceanic crustal thicknesses of tens of kilometers.”
“As radioactive heating declines and the mantle gradually cools, subducted plates eventually become thick and heavy enough to penetrate and disrupt the ‘basalt barrier’, and no further layering occurs after 1.6–1.8 Ga in the models.”
“The models yield mean residence times of mantle material (between passages through melting zones) consistent with the mantle lead-isotopic apparent age of about 1.8 Ga.”
“Accumulations of (denser) basaltic material at the base of the models preserve a remarkably clear record of the early magmatic pulses.”
And, in conclusion, These accumulations persist strongly into the present, probably because of high internal temperature and consequent low viscosity, even though they are unlikely to form under present conditions.”