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Earth's Proto-Lithosphere

 

In our last episode of Earth’s formation, the planet had differentiated into distinct layers based on their density. These layers were identified as a result of differences or discontinuities in the speed of seismic waves traveling through the Earth. In this section, we examine several lines of evidence that help us define processes during the formation of earth’s crust, better known as the lithosphere.

 

 

The lithosphere, a term introduced in 1914 by the American geologist Joseph Barrell, refers to the “strong” part of the Earth (lithos means stone in Greek) whereas the layer beneath it, the asthenosphere, refers to the “plastic” weaker layer underneath (from the Greek word asthenes or weak)

In modern-day usage, the lithosphere includes the Earth’s crust—which we separate into oceanic crust and continental crust—and those parts of the upper mantle that can support continents. Don Anderson at Cal Tech takes exception to “loose” definitions of the lithosphere; this part of the Earth’s crust should refer only to the tectonic (aka lithospheric) plates, the parts of the crust subject to displacement (a subject for subsequent chapters). Various layers immediately below the plates that do not move with the plates should not be called lithosphere, although they often are, according to Anderson. These parts would belong to the asthenosphere, the “weak layer” of the upper mantle. The idea that the lithosphere should be confined to the moving plates only makes sense to me. My point here is that you should always check how a person or paper are defining a particular word before you draw conclusions based on them.

Our story here concerns the separation of Earth’s crust into oceanic and continental parts. If you stop for a moment and think about it, doesn’t it seem peculiar that the surface of our Earth congealed into two different types of rock? The fact is that oceanic crust and continental crust are quite different with different properties. A brief review of those properties will help us better understand how they formed, according to scientists who study these things.

Here’s a summary in words:

  • oceanic crust is more dense and hence, heavier than continental crust
  • oceanic crust is composed primarily out of mafic materials whereas continental crust is primarily felsic minerals
  • oceanic crust is thinner than continental crust
  • oceanic crust forms “depressions” in the surface of the earth whereas continental crust “rises above” the surface of the earth (note: some books use the term “float” to refer to the manner in which continental crust rests within Earth’s mantle, but oceanic crust also “floats” in the mantle, so this description is less than useful.)
  • oceanic crust is generally more uniform in composition, although it does occur in two major forms named from their source: mid-ocean ridge basalts (MORBs...I just love that word) and ocean island basalts (OIBs)
  • continental crust contains most known elements and is highly diverse mineralogically; it exists typically in the form of cratons (the stable interior of continents undisturbed since the Precambrian; see also continental shields and platforms), orogenic belts (formed from compressional forces—continental and oceanic pieces “slamming” into each other; see also continental margins and extended crust)
  • oceanic crust formed initially from the cooling of ultramafic mantle into komatiites; continental crust formed subsequently from melting of descending slabs of sediment-covered oceanic crust whose light (or felsic) components rose to the surface in the form of tonalites and grandiorites
  • oceanic crust is “rapidly” recycled (approximately every 200 million years) whereas continental crust is long lived (up to 4.3 billion years);
  • because oceanic crust is much younger and less accessible than continental crust, much of what we know about the early earth is based on continental crust and fragments of oceanic crust, known as ophiolites, that have been accreted onto continental crust

One additional, non-traditional, type of “crust” deserves mention here. Since the 1990s, crustal features known as large igneous provinces (aka LIPs) have drawn considerable attention not only because they are unusual but because of a possible role in mass extinctions of organisms. LIPs consist of massive outpourings of basaltic magma on continents and the sea floor. One such LIP, the Central Atlantic Magmatic Province (CAMP), can be found on four continents—North America, Europe, Africa and South America—and it  may have preceded the formation of the Atlantic Ocean. While mostly basaltic (and, therefore, mafic), at least one felsic LIP has been described. The origin(s) of LIPs is contentious but given that they occur both within continents and on the sea floor and given their widespread occurrence, we may need to consider them as a separate form of crust.

Okay, that’s a lot of geology terminology for one sitting but the more you see these words and use them the more familiar they will get. Promise me you will use them again and again until they are like an old friend. Until then, let’s take a look at an illustration produced by the U.S. Geological Survey that highlights the modern day differences between oceanic and continental crust.

A map of the thickness of the continents—based on 560 seismic refraction measurements (the bending of waves, in this case, seismic waves)—demonstrates the major differences between oceanic and continental crust. Take a few moments and study this map. Where does the thickest crust occur? What forces may cause continental crust to thicken to more than 70 kilometers? Where does the thinnest crust occur? Why?

You may not be able to answer these questions at this moment but you can marvel in its beauty and its simple statement that oceanic and continental crust are different.

Given this description of the modern-day Earth’s crust, we can ask “How did Earth’s crust form?” What early processes gave Earth its shape? How did the ocean basins form providing a very convenient receptacle for the world ocean?

One of the earliest ideas about Earth’s early crust compared it to a drying apple. As the Earth cooled, it contracted and wrinkled. The indentations in the apple represented the ocean basins and the ridges in the apple represented the continents and mountains. Many a respectable 19th century geologist argued for this “contraction” theory (including one named Chamberlin—not a relative that I am aware of) but new ideas and new measurements of gravity began to sway opinion against contraction (and catastrophism and all the other variants of a “static” earth). In 1912, Alfred Wegener published his ideas about continental drift but deeply ingrained prejudices and philosophical disputes between American and European geologist delayed acceptance of his ideas until the 1960s. The theory of plate tectonics—which incorporates some of Wegener’s ideas along with those of many other scientists—provides a more consistent and far-reaching framework on which an understanding of Earth’s lithosphere can be based.

In the theory of plate tectonics, the Earth’s crust is divided into at least 12 major “plates” (slabs, jigsaw puzzle pieces, eggshells, however your mind best visualizes it) made up of both oceanic and continental crust. New oceanic crust forms at mid-ocean ridges and is “recycled” (subducted) at submarine trenches. As this happens, continental crust—riding on the same plate as the oceanic crust—is taken for a “ride” (it moves). Thus, the continents move about on our planet, occasionally coming together into giant supercontinents or splitting into ocean basins and regular continents.

To understand processes that formed Earth’s crust and gave rise to the ocean basins and continents that we see today, we look first to clues provided by the oldest rocks on Earth, the 30-40 cratons scattered about the planet (Bleeker, 2002?). The best examples can be found in northeastern North America (the Canadian shield), southwestern Greenland (the Isua region) and western Australia (Murchison District), among other sites. These ancient cratons—preserved for at least four billion years—have yielded tiny and highly resistant crystals known as zircons, not unlike the stones you find in jewelry stores. However, unlike the gems that you find at the mall, these zircons have remained unchanged throughout Earth’s history. How do we know that? Radioisotope dating.

The oldest zircons found so far come from the Erawondoo region of Western Australia, the Narryer Gneiss Complex in the Mt. Narryer-Jack Hills region (in case you were planning on visiting), as reported by Mojzsis et al (Nature, Jan 11, 2001). Uranium-lead isotope dating yielded ages of approximately 4.3 Ga for some zircons, even though the parent rocks in which they were found were only 3.75 Ga. (Examination of thousands of these zircons have confirmed these results so we can have a fair amount of confidence in the ages of these zircons.) The chemical makeup of zircons—such that they can survive crustal melting and recycling—is what makes them a valuable source of information on early Earth.

But scientists have higher goals than setting a new Guinness Book of World Records for the oldest age of rocks. By examining the oxygen isotopes of these zircons, Mojzsis and his colleagues were able to deduce that they formed from recycled continental crust and, most astonishingly, that liquid water was present during their formation. You will recall that the moon formed between 4.54 and 4.44 Ga; only 100-200 million years separate that event and the possible presence of a proto-ocean. Thus, they conclude that some type of plate tectonic continent-forming/subduction/recycling process was underway early in Earth’s history.

Further evidence for early subduction processes comes from laboratory experiments on melted rocks at high pressure. These experiments appear to indicate that oceanic crust began to subduct in the early Archaean and continental crust followed in the late Archaean. Foley at al summarize their arguments in their Nature 2003 paper on which the figure below is based. Their model suggests that early subduction was shallow but progressively deepened. As subduction deepened, the composition of the melt (resulting from subduction) changed and eventually led to the formation of continental crust as we know it today.

Direct evidence for an Archaean onset of plate tectonics comes, oddly enough, from “shipwrecked” oceanic crust. While it’s true that the oceanic crust that makes up the ocean basins is younger than ~200-220 million years old, we can find remnants of older sea floor where it has been shoved on top of continental crust and attached to the continents. These blocks of sea floor found on continents are called ophiolites, a term first used by Alexander Brogniart in 1814. Ophiolites have a rich geological history and have provided many insights—some controversial—on the formation of sea floor and processes that occur at plate boundaries.

On 11 May 2001, Timothy Kusky and his colleagues published in Nature their findings of a ~2.5 Ga ophiolite in the North China craton. Their conclusions were based on observations of the rocks in this area—called field mapping—and their placement with respect to each other. What they found were all the “parts” of an ophiolite arranged in the structure most commonly attributed to ophiolites. In the ophiolite model, deep ultramafic and mafic rocks are overlain by gabbro (a dark green to black rock type similar to basalt but formed in a different manner) which in turn are overlain by sheeted dikes (dikes are minerals that have been squeezed into the cracks of other rocks) topped by pillow lavas (lava that looks like your pillow) with sediments on top. If you take a moment to look at their figure, you can see that the interpretation of these maps requires a geologist’s eye; it takes careful study to see how the rock types fit into the model. Not being a geologist (but trusting the geologists who reviewed their paper—a wise or unwise choice—I’m going to assume their conclusions are correct).

The presence of a well-defined ophiolite at 2.5 Ga suggests that:

  • processes that create modern-day oceanic crust were operating
  • continental crust was formed and actively “participating” in plate tectonic processes
  • most likely, modern-day plate tectonics operated at this time

Nonetheless, the picture is incomplete. The presence of one (or even a few) Archean ophiolites can not be unambiguously interpreted to mean that plate tectonics was widespread. It’s possible that plate tectonic processes operated at smaller scales and spread globally. We do know that continental crust was present circa 4 Ga but it may have been confined to small “patches”, as Levin (6th Edition) suggests, roughly about the size of Oregon. These microcontinents, as they are called, may have subsequently grown through plate accretion or they may have joined together through crustal movements. Such ideas are speculative and warrant further study. Fortunately, some of these new studies—such as the one cited above—are providing much-needed geochemical constraints on what could and could not happen in the Archaean.


 

 

   
   
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