Crust (geology)

The internal structure of Earth

In geology, the crust is the outermost solid shell of a rocky planet, dwarf planet, or natural satellite. It is usually distinguished from the underlying mantle by its chemical makeup; however, in the case of icy satellites, it may be distinguished based on its phase (solid crust vs. liquid mantle).

The crusts of Earth, Moon, Mercury, Venus, Mars, Io, and other planetary bodies formed via igneous processes, and were later modified by erosion, impact cratering, volcanism, and sedimentation.

Most terrestrial planets have fairly uniform crusts. Earth, however, has two distinct types: continental crust and oceanic crust. These two types have different chemical compositions and physical properties, and were formed by different geological processes.

Types of crust

Planetary geologists divide crust into three categories, based on how and when they formed.[1]

Primary crust / primordial crust

This is a planet's "original" crust. It forms from solidification of a magma ocean. Toward the end of planetary accretion, the terrestrial planets likely had surfaces that were magma oceans. As these cooled, they solidified into crust.[2] This crust was likely destroyed by large impacts and re-formed many times as the Era of Heavy Bombardment drew to a close.[3]

The nature of primary crust is still debated: its chemical, mineralogic, and physical properties are unknown, as are the igneous mechanisms that formed them. This is because it is difficult to study: none of Earth's primary crust has survived to today.[4] Earth's high rates of erosion and crustal recycling from plate tectonics has destroyed all rocks older than about 4 billion years, including whatever primary crust Earth once had.

However, geologists can glean information about primary crust by studying it on other terrestrial planets. Mercury's highlands might represent primary crust, though this is debated.[5] The anorthosite highlands of the Moon are primary crust, formed as plagioclase crystallized out of the Moon's initial magma ocean and floated to the top;[6] however, it is unlikely that Earth followed a similar pattern, as the Moon was a water-less system and Earth had water.[7] The Martian meteorite ALH84001 might represent primary crust of Mars; however, again, this is debated.[5] Like Earth, Venus lacks primary crust, as the entire planet has been repeatedly resurfaced and modified.[8]

Secondary crust

Secondary crust is formed by partial melting of silicate materials in the mantle, and so is usually basaltic in composition.[1]

This is the most common type of crust in the Solar System. Most of the surfaces of Mercury, Venus, Earth, and Mars comprise secondary crust, as do the lunar maria. On Earth, we see secondary crust forming primarily at mid-ocean spreading centers, where the adiabatic rise of mantle causes partial melting.

Tertiary crust

Tertiary crust is more chemically-modified than either primary or secondary. It can form in several ways:

  • Igneous processes: partial-melting of secondary crust, coupled with differentiation or dehydration[5]
  • Erosion and sedimentation: sediments derived from primary, secondary, or tertiary crust

The only known example of tertiary crust is the continental crust of the Earth. It is unknown whether other terrestrial planets can be said to have tertiary crust, though the evidence so far suggests that they do not. This is likely because plate tectonics is needed to create tertiary crust, and Earth is the only planet in our Solar System with plate tectonics.

Other Languages
Адыгэбзэ: ЩӀыщхьэфэ
Afrikaans: Aardkors
Ænglisc: Hrūse
aragonés: Crosta terrestre
azərbaycanca: Yer qabığı
Bân-lâm-gú: Tē-khak
Basa Banyumasan: Kerak bumi
башҡортса: Ер ҡабығы
беларуская: Зямная кара
беларуская (тарашкевіца)‎: Зямная кара
български: Земна кора
bosanski: Zemljina kora
čeština: Zemská kůra
Deutsch: Erdkruste
eesti: Maakoor
Ελληνικά: Γήινος φλοιός
Esperanto: Terkrusto
euskara: Lurrazal
فارسی: پوسته
հայերեն: Երկրակեղև
हिन्दी: भूपर्पटी
hrvatski: Zemljina kora
Bahasa Indonesia: Kerak bumi
interlingua: Crusta terrestre
íslenska: Jarðskorpa
Basa Jawa: Kerak bumi
Kiswahili: Ganda la dunia
Кыргызча: Жер кыртышы
latviešu: Zemes garoza
lietuvių: Žemės pluta
Limburgs: Aerdkoosj
magyar: Földkéreg
македонски: Земјина кора
മലയാളം: ഭൂവൽക്കം
მარგალური: დიხაუჩაშ ქერქი
Bahasa Melayu: Kerak bumi
Nederlands: Aardkorst
Nedersaksies: Eardkorst
日本語: 地殻
norsk nynorsk: Jordskorpe
پنجابی: چھلڑ
Plattdüütsch: Eerdköst
português: Crosta
русский: Земная кора
Simple English: Earth's crust
slovenčina: Zemská kôra
slovenščina: Skorja (geologija)
Soomaaliga: Qolof Dhul
српски / srpski: Земљина кора
srpskohrvatski / српскохрватски: Zemljina kora
Basa Sunda: Intip bumi
suomi: Maankuori
svenska: Jordskorpa
தமிழ்: புவியோடு
татарча/tatarça: Җир кабыгы
Türkçe: Yerkabuğu
українська: Земна кора
粵語: 地殼
中文: 地球地壳