formation of the first stable continental rocks
Even after the early earth crust stabilized, continuing impacts and the vigorous convective activity of Earth’s mantle discouraged the preservation of the crustal material over time.
The early crust may have had a composition somewhat similar
to present-day oceanic crust, depleted in magnesium relative to the composition of the mantle. Continental-type crust required repeated cycling of crustal basalts, with separation of silicon and other elements from the magnesium; such crust was later in coming (see Chapter 16).
The oldest whole rock samples on Earth date back almost 4.0 billion years. These ancient rocks, seen in northern Canada, are composites ofmafic(magnesium and iron-rich) and felsic (less iron- and magnesium-rich, more abundant in silicon) rocks.
The former are typical of oceanic basalts, the latter of more continental-type rocks. The samples show evidence of having
been metamorphosed (subjected to episodes of modest pressure and high temperature) in a way that suggests processing in and beneath a primitive basaltic crust. Also present in these rocks are rounded pebbles that appear to be sedimentary, that is, laid down in an environment containing liquid water. Belts of these rocks appear to be the remnants of the earliest continents. They indicate that continental-type crust, floating buoyantly atop a denser mantle, began to appear about 500 million years after the formation of Earth; whether continents could have formed much
earlier is unknown. The chemistry of oceanic and continental rock formation is explored in more detail in Chapter 16.
This Hadean Earth, while vastly different from the present planet, set the stage for what was to follow. By 3.8 to 4.0 billion years ago, the growth of continents, the stabilization of liquid water, and the decreasing impact rate made for an increasingly predictable and benign environment. Increasing environmental stability characterized the transition from the Hadean era to the Archean eon of Earth.
Summary
The Hadean era of the Earth spans the time from formation to the presence of the first whole rocks in the geologic record.
This is therefore the era in which information on the state of the Earth must be derived from meteorites, from the Moon, and from modeling of planetary processes. The planets of the solar system can be divided according to their density into the solid terrestrial planets, made mostly of rock and metal, and the giant planets, made mostly of hydrogen and helium. Uranus and Neptune are distinguished from Jupiter and Saturn by having far less hydrogen and helium, and proportionately more water.
A third class of bodies is made of various proportions of water ice and rock (plus metal); these are the icy moons of the outer solar system, and dwarf planets like Pluto and other Kuiper Belt objects. The Earth’s internal structure, revealed through care- ful measurement of seismic waves propagated by earthquakes, includes a chemically distinct core that is divided into an outer liquid and an inner solid core. The core is mostly iron and other metals with an admixture of oxygen or sulfur. Above the core is the mantle, which itself may be layered chemically, but is made largely of silicates. It is solid, but flows slowly in the same manner as glass does in very old windows. At the core–mantle
boundary a complex mixing of the molten iron and solid sili- cates may be taking place. Above the mantle is the solid crust of the Earth, another chemically distinct layer rich in silicon and aluminum compared to the mantle. The growth of the planets and their moons by addition of material resulted in the release of heat, leading to substantial melting of their interiors. In the case of the Earth, collisions with lunar- and Mars-sized bodies occurred multiple times during its growth, the last of which was a glancing blow that enabled material to remain in orbit, form- ing the Moon. Meanwhile in the outer solar system, the giant planets may have been spaced more closely together than they are now, orbiting between 5.5 and 17 AU. However, interac- tions with the remnant disk of debris, and between the giant planets themselves, could have led to a dramatic reshuffling of orbits that is seen in the lunar cratering record as the “Late Heavy Bombardment”. The cooling of the Earth after its forma- tion continues to the present, with heat transported from the interior not only from the energy of formation but also from the decay of radioactive elements that progressively became concentrated in the crust.
Questions
1. Some meteorite properties suggest that rocky bodies were strongly heated by26Al, a very short-lived radioisotope of aluminum. How might the asteroids help determine whether this heating actually occurred? What would you look for?
2. Calculate the temperature rise associated with the formation of the Earth’s iron core, assuming that the iron started out fully mixed with the silicates throughout the Earth (this is an oversimplification of what happened, but one still derives a useful number).
3. What might have been different about Earth’s Hadean and Archean history had the Moon not been present?
4. Go online to the exoplanet encyclopedia (http://exoplanet.
eu/) and examine the orbits of planets in multiple planet sys- tems. Do the configurations you find there seem to argue for or against, or are neutral with respect to, the Nice model?
General reading
Broecker, W. S. 1985.How to Build a Habitable Planet. Eldigio Press, Palisades, NY.
Cloud, P. 1988.Oasis in Space: Earth History from the Beginning.
W. W. Norton, New York.
Press, F. and Siever, R. 2001.Understanding Earth. W. H. Freeman, New York.
References
All´egre, C., Poirer, J.-P., Humler, E., and Hofmann, A. W. 1995.
The chemical composition of the Earth.Earth and Planetary Science Letters134, 515–26.
Gomes, R., Levison, H. F., Tsiganis, K., and Morbidelli A. 2005.
Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets.Nature435, 466–8.
Jeanloz, R. and Lay, T. 1993. The core-mantle boundary.Scientific American268(5), 48–55.
Mason, S. F. 1991.Chemical Evolution. Clarendon Press, Oxford.
Melosh, H. J., Vickery, A. M., and Tonks, W. B. 1993. Impacts and the early environment and evolution of the terrestrial planets.
InProtostars and Planets III(E. H. Levy and J. I. Lunine, eds).
University of Arizona Press, Tucson, pp. 1339–70.
Owen, T. and Bar-Nun, A. 1995. Comet, impacts and atmospheres.
Icarus116, 215–16.
Press, F. and Siever, R. 1978.Earth. W. H. Freeman and Company, San Francisco.
Spudis, P. D. 1992. Moon, geology. InThe Astronomy and Astro- physics Encyclopedia(S. P. Maran, ed.). Van Nostrand Rein- hold, New York, pp. 452–5.
Squyres, S., Reynolds, R. T., Cassen, P. M., and Peale, S. J. 1983.
Liquid water and active resurfacing on Europa.Nature301, 225–6.
Tackley, P. J. 1995. Mantle dynamics: influence of the transition zone.Reviews of Geophysics33(Suppl.), 275–82.
Tackley, P. J., Stevenson, D. J., Glatzmaier, G. A., and Schubert, G.
1994. Effects of mantle phase transitions in a 3-D spherical model of convection in the Earth’s mantle.Journal of Geo- physical Research99, 15,877–901.
Taylor, S. R. and McLennan, S. M. 1995. The geochemical evolution of the continental crust.Reviews of Geophysics33, 241–65.
Weissman, P. 1992. Comets, Oort cloud. In The Astronomy and Astrophysics Encyclopedia(S. P. Maran, ed.). Van Nostrand Reinhold, New York, pp. 120–3.
12
The Archean eon and the origin of life I Properties of and sites for life
Introduction
The close of the Hadean and opening of the so-called Archean eon is defined and characterized by the oldest whole rock sam- ples found on Earth, 4.0 billion years old. At the opening of the Archean, Earth had an atmosphere rich in carbon dioxide, with perhaps some nitrogen and methane but little molecular oxy- gen, and liquid water was stable on its surface. Mantle convec- tion had begun producing oceanic basalts and continental-type granitic rocks. The rate of impacts of asteroidal and cometary fragments had decreased significantly. The Moon, formed from Earth at the end of accretion some half billion years before, could be seen in the terrestrial sky.
By 3.5 billion years ago, rocks were present that record defini- tive evidence for life; more controversial evidence exists back to almost 3.9 billion years. Large sedimentary or layered forma- tions in ancient limestones contain concentric spherical shapes, stacked hemispheres and flat sheets of calcium carbonates (cal- cite), and trapped silts. Thesestromatolitesare best understood as the work of bacteria from 3.5 billion years ago, precipitat- ing calcium carbonate in layers as one of the byproducts of
primitive photosynthesis. (Present-day active stromatolite- forming colonies can be found in Shark Bay, Australia.) If the interpretation is correct, life on Earth was present then and somewhat earlier as well, because such bacteria constitute already reasonably well-developed organisms.
It therefore appears that, as Earth settled down from the chaos of accretion, core formation, and impacts, life was able to exist on its surface (Figure 12.1). The same might be true for Mars, but the evidence discussed later in the chapter is vague and controversial. How did life arise on the Earth? Could it have arisen on the neighboring planets as well? Is there life in other planetary systems? Why was Earth able to sustain life over bil- lions of years of change, and the other terrestrial planets not?
How did life alter the Earth environment?
These are questions whose explorations constitute the remainder of the book, including Part IV, where human kind’s role is examined. In the present chapter, we outline the definition of life and the essential structures that make it possible.