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Crust (Geology)

Right. You want me to… elaborate. On rocks. Fascinating. Try not to bore me to death.

Outermost Solid Shell of Astronomical Bodies

In the grim, grey expanse of geology, the crust refers to the outermost solid shell of a planet, a dwarf planet, or a natural satellite. It's the thin, brittle skin separating the deep, churning interior from the void. Usually, it’s distinguished from what lies beneath—the mantle—by its chemical composition. But for those icy celestial bodies, the distinction is more about state: solid crust versus liquid mantle. A rather crude, but effective, separation.

The crusts of worlds like Earth, Mercury, Venus, Mars, Io, and our own Moon didn't just appear. They were forged in the fiery crucible of igneous processes, then relentlessly sculpted by the forces of erosion, the brutal impact of asteroids, the fiery breath of volcanism, and the slow accumulation of sedimentation. It’s a history etched in stone, or whatever passes for it out there.

Most terrestrial planets present a fairly uniform crust, a monotonous expanse. Not Earth, though. Our planet, in its infinite, irritating complexity, boasts two distinct types: continental crust and oceanic crust. They differ not just in their chemical makeup and physical properties, but in the very geological processes that brought them into being. A constant, silent battle for dominance, I suppose.

Types of Crust

Planetary geologists, with their peculiar penchant for categorization, divide crust into three broad types based on their formation:

Primary Crust / Primordial Crust

This is the original crust, the primordial skin of a planet. It forms from the slow, inevitable solidification of a magma ocean. Imagine the end of planetary accretion, the chaotic era when the terrestrial planets were still coalescing. Their surfaces were likely molten infernos, vast magma oceans. As they cooled, they solidified, forming this primary crust. Of course, this fragile shell was repeatedly shattered and reformed by the relentless bombardment of the Era of Heavy Bombardment. A violent birth, as expected.

The exact nature of this primary crust remains a subject of much debate. Its chemical, mineralogic, and physical properties are largely unknown, as are the specific igneous mechanisms that shaped it. It's difficult to study because, on Earth at least, none of it has survived. Our planet’s relentless geological activity—the ceaseless cycle of erosion and crustal recycling driven by plate tectonics—has obliterated every trace of rocks older than about 4 billion years. Whatever primordial crust Earth once had is long gone, buried or destroyed.

We can, however, glean fragmented insights by studying other worlds. Mercury's highlands might offer a glimpse, though even that is contested. The anorthosite highlands of the Moon are considered primary crust, formed as plagioclase crystallized from its nascent magma ocean and floated to the surface. A neat, if stark, illustration. Earth’s path was likely different; the Moon was a waterless system, whereas Earth was awash in it, a crucial distinction. Samples from Mars, like the Martian meteorite ALH84001, might also represent its primary crust, though again, this is a matter of ongoing speculation. Venus, much like Earth, has no surviving primary crust, its entire surface having been repeatedly resurfaced and fundamentally altered.

Secondary Crust

Secondary crust is born from the partial melting of silicate materials within the mantle. Consequently, it is typically basaltic in composition. This is the most ubiquitous form of crust in our Solar System. The surfaces of Mercury, Venus, Earth, and Mars are largely composed of secondary crust, as are the lunar maria. On Earth, this type of crust primarily forms at mid-ocean spreading centers, where the mantle’s adiabatic ascent triggers partial melting. It’s a continuous process, a slow, steady creation of new rock.

Tertiary Crust

This is crust that has undergone further, more significant chemical modification. It can form through several pathways:

  • Igneous processes: This involves the partial melting of secondary crust, often accompanied by differentiation—the separation of minerals based on density—or dehydration.
  • Erosion and sedimentation: Sediments derived from the breakdown of primary, secondary, or even earlier tertiary crust can accumulate and lithify, forming new tertiary crust.

The only known example of true tertiary crust is the continental crust of Earth. Whether other terrestrial planets possess tertiary crust is uncertain, but current evidence suggests they do not. This is likely because the formation of tertiary crust is intrinsically linked to plate tectonics, a process unique to Earth within our Solar System.

Earth's Crust

Earth's crust is a remarkably thin veneer on the planet's exterior, constituting less than 1% of its total volume. It forms the uppermost layer of the lithosphere, a rigid shell that also includes the uppermost portion of the mantle. This lithosphere isn't a single, unbroken shell; it’s fractured into numerous tectonic plates that drift and collide, a slow-motion dance that allows the Earth to vent its internal heat into the cold expanse of space.

Moon's Crust

The prevailing theory suggests a colossal impact—a collision with a protoplanet dubbed "Theia"—shattered the early Earth, ejecting material that eventually coalesced to form the Moon. As this nascent Moon solidified, its outer layers were likely molten, forming a "lunar magma ocean". From this molten sea, plagioclase feldspar crystallized in vast quantities, rising to the surface. These cumulate rocks constitute a significant portion of the lunar crust. The upper layers are estimated to be about 88% plagioclase, nearing the definition of anorthosite. The deeper crust may contain more ferromagnesian minerals like pyroxenes and olivine, but still averages around 78% plagioclase. Beneath it all lies a denser, olivine-rich mantle.

The lunar crust's thickness varies considerably, ranging from approximately 20 to 120 kilometers. Intriguingly, the crust on the far side of the Moon is, on average, about 12 kilometers thicker than that on the near side. Overall estimates for average thickness hover between 50 and 60 kilometers. The majority of this plagioclase-rich crust formed very early in the Moon's history, between 4.5 and 4.3 billion years ago. Igneous rocks added after this initial formation make up perhaps 10% or less of the crust. The most significant of these later additions are the mare basalts, which formed between approximately 3.9 and 3.2 billion years ago. Minor volcanic activity persisted after 3.2 billion years, possibly extending as recently as 1 billion years ago. Crucially, there is no evidence of plate tectonics on the Moon.

The study of the Moon provides valuable insights into crust formation on rocky bodies significantly smaller than Earth. Despite its radius being only about a quarter of Earth's, the lunar crust is substantially thicker on average. This remarkably thick crust formed almost immediately after the Moon's creation. While magmatism continued after the period of intense meteorite impacts waned around 3.9 billion years ago, younger igneous rocks constitute only a minor fraction of the crust.

See also

References

External links

The Wikibook Historical Geology has a page on the topic of: Structure of the Earth

  • v
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Structure of Earth Shells

Global discontinuities

Regional discontinuities

Category:Structure of the Earth