Tectonics and climate of the Precambrian Geology 103

When Did the Solar System Form? 4.56 billion years ago How do we know? (evidence for formation) Meteorite photo by Carl Allen at Lunar samples to 4.6 Ga Meteorites Ga Earth – 3.9 (or 4.4 Ga) Lunar meteorite at

How Did We Get a Solar System? Huge cloud of cold, thinly dispersed interstellar gas and dust – threaded with magnetic fields that resist collapse – solar nebula theory of Swedenborg (1734), Kant (1755) and Laplace (1796). Hubble image at Image: LPI

Concentrations of dust and gas in the cloud; material starts to collect (gravity > magnetic forces) How Did We Get a Solar System? Hubble image at Image: LPI

How Did We Get a Solar System? Gravity concentrates most stuff near center Heat and pressure increase Collapses – central proto-sun rotates faster (probably got initial rotation from the cloud) Image: LPI

How Did We Get a Solar System? NASA artwork at Rotating, flattening, contracting disk - solar nebula! Equatorial Plane Orbit Direction

After ~10 million years, material in center of nebula hot enough to fuse H “...here comes the sun…” How Did We Get a Solar System? NASA/JPL-Caltech Image at

How Did We Get a Solar System? Hubble photo at Metallic elements (Mg, Si, Fe) condense into solids at high temps. Combined with O to make tiny grains Lower temp (H, He, CH4, H2O, N2, ice) - outer edges Planetary Compositions

How Did We Get a Solar System? Inner Planets: Hot – Silicate minerals, metals, no light elements, ice Begin to stick together with dust  clumps Image: LPI

How Did We Get a Solar System? Accretion - particles collide and stick together … or break apart … gravity not involved if small pieces Form planetesimals, up to a few km across Image: LPI

How Did We Get a Solar System? Gravitational accretion: planetesimals attract stuff Large protoplanets dominate, grow rapidly, clean up area ( takes ~10 to 25 My) Image: LPI

The Precambrian divisions are defined broadly by atmospheric changes Hadean: Lots of carbon dioxide, water vapor and methane Archean: Water vapor forms oceans, oxygen starts to be made by photosynthetic organisms Proterozoic: Significant oxygen in atmosphere, massive drop in carbon dioxide

Graphically…

Period of major accretion (~ my) { Period of heavy bombardment Present-day plate tectonics “begins” Some boundaries coincide with other events

Period of major accretion (~ my) { Period of heavy bombardment Present-day plate tectonics “begins” period of rapid crustal growth Archaen-Proterozoic transition To modern plate tectonics 1. Early plates became bigger and thicker 2. Continued recycling of oceanic crust formed large amounts of buoyant continental crust Continued partial melting/distillation Separation of Si and other elements from Mg and Fe Conversion of mafic material to felsic material through rock cycle 3. Decrease in heat production slowed mantle convection Drove system to larger convection cells Allowed larger plates to travel farther on the Earth’s surface and cool more Led to subduction rather than collision of plates Modern plate tectonics

The Witwatersrand (South Africa) goldfields

Evidence against the theory Not all gold deposits are the same age Clearly, some other mechanism deposits gold in this fashion – anoxic inland seas?

More evidence for atmospheric change in Archean Banded iron formations (BIFs) are interlayered alternating chert (jasper) and iron oxide Mostly found in Archean, some in Proterozoic, almost none in the Phanerozoic

Mechanism for generating BIFs

Since the Archaean Intensity of plate tectonics has varied over time Wilson cycles – 500 my cycles – Evidence of a supercontinent at my (Rodinia) – Pangea formed ~ 300 my Periods of rapid sea floor spreading (and vice versa) – Sea level rises because large amounts of shallow basalt form and don ’ t cool (and subside) much – High CO 2 release – released at spreading centers when new crust forms and subducting crust has sediment on it including calcite which releases CO 2 when it melts

Meanwhile, plate tectonics settles down Archean rocks worldwide are of only two types: granite/gneiss complexes (a high-grade metamorphic rock) and intervening greenstones (metamorphosed basalt and some sedimentary rock) Superior province in North America is among the biggest in the world

What does a greenstone belt remind you of?

But still different than today’s plate tectonics Komatiites are ultramafic igneous rocks that are common in the Precambrian but unknown today Hotter mantle? Wetter mantle? Diamonds!

First continents form and stick around

So, by the Proterozoic… Division between Archean and Proterozoic is based on oxidizing conditions found in surface waters (1.8 by) Tectonics is more similar to today’s; evidence for rifting and subduction and terrane accretion

What evidence exists for Rodinia? Grenville orogeny rocks (sometimes called “mobile belts”), originally defined to explain Canadian shield rocks, were found to exist on many other continents All this mountain-building implies some large-scale tectonic event, like a supercontinent (name was suggested in the 1990s) Rodinia is constructed at 1.1 by, rifts apart by 0.85 by

The Grenville orogeny rocks Primarily marine sandstones and carbonates (limestones) No bioturbation Since then, these rocks have been metamorphosed, but the original rock is easily inferred

Conventional reconstruction Line up all the Grenville orogenic belts and create the supercontinent Note that Antarctica and the US (Laurentia) are quite separated

The SWEAT hypothesis Rodinia joined the southwest (SW) US (West Texas, specifically) with eastern Antarctica (EAT) Shown through lead isotope measurements of similar age rocks that were part of a rift in both areas Key point: there was not just one zone of orogeny as in the conventional theory

Precambrian climate Positions of continents, especially existence of polar continents, determines when ice ages occur

Positive feedback If glaciers can build extensively to within 30° of the equator, the extensive ice will reflect a large portion of the Sun’s energy back into space, cooling the surface and allowing more glaciers to grow “Icehouse Earth” or “Snowball Earth” hypothesis (W. Brian Harland, Cambridge, 1964)

How to get out of the Icehouse Joe Kirschvink (Caltech, 1992) argued that volcanic activity and carbon dioxide production would not cease even during an Icehouse event, and nothing would “scrub” the carbon dioxide out of the atmosphere, enhancing the greenhouse effect

More detail about CO 2 scrubbing

Life alters as Rodinia breaks up Ediacaran fauna appears – first evidence of multicellular life No hard parts, preserved as molds Unclear if they are truly related to modern phyla, or represent extinct phyla Ediacaran period is a recognized division of the Proterozoic eon (630 – 542 my)