Earth crust

Earth layers, in a schematic drawing.

The earth's crust is the outermost zone of the concentric structure of the geosphere, the solid part of the Earth. It is comparatively thin, with a thickness that varies from 5 km, at the bottom oceanic, up to 70 km in the active mountainous areas of the continents.

It has been suggested that the first crust on Earth formed 4.4-4.55 billion years ago. The volumes of the earth's crust have not been constant but are believed to have increased over time. It is known that 2.5 billion years ago there was already a formidable mass of crust; before this it is assumed that there was a lot of recycling of crust to the mantle. The growth, that is, the increase in volume of the crust, is believed to have occurred episodically with two major events: one 2.5-2.7 billion years ago and another 1.7-1.9 billion years ago.

Most planets have fairly uniform crusts, however Earth 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 Earth's crust

1: Continental cortex.
2: Ocean.
3: Upper mantle.
4: Oceanic cortex.

There are two types of Earth's crust: oceanic crust and continental crust.

Geological Provinces of the Earth (USGS)

Oceanic cortex (depending on your age) 0-20 Ma 20-65 Ma ▪65 Ma

Continental cortex Ancient shields or cratons Platforms (sedimentary sheds) Orogenic chains Tecto-sedimentary basins Ignese provinces Thin cut (by cortical extension)

Oceanic crust

Oceanic crust covers about 55% of the planet's surface. It is thinner than the continental one and three levels are recognized in it. The lowest level, called level III, adjoins the mantle at the Mohorovičić discontinuity; It is formed by gabbros, basic plutonic rocks. Above the gabbros is level II, of basalts, volcanic rocks of the same basic composition as the gabbros; there is a thicker lower zone made up of dikes, while the more superficial one is based on cushioned basalts, formed by rapid solidification of lava in contact with ocean water. Level I settles on the basalts, formed by sediments, pelagic in the middle of the ocean and terrigenous in the vicinity of the continents, which are gradually deposited on the magmatic crust once consolidated.

The oceanic crust is chemically and mineralogically distinct from the adjoining mantle. Level III gabbros and level II basalts are distinguished by their structure, derived from their mode of formation, plutonic in the first case and volcanic in the second, but not by their composition, which qualifies them as basic or mafic rocks; it is then a phase difference. The peridotites of the mantle, on the other side of the Moho, are, on the contrary, ultrabasic (ultramafic). The most abundant minerals in this layer are pyroxenes and feldspars and the elements are silicon, oxygen, iron and magnesium.

Most of the oceanic crust lies below the sea, several thousand meters deep, but there are exceptions: Iceland and Djibouti are interpreted as parts of the network of mid-ocean ridges rising above sea level. In addition, there are formations in the orogens, called ophiolites or ophiolitic complexes, which are fragments of oceanic crust, especially submarine volcanic edifices, which the dynamics of the plates have lifted over the continent.

The thickness of the magmatic levels of the oceanic crust is 6-12km, with a typical value of 7km. The oceanic lithosphere, of which the oceanic crust is the uppermost layer, is constantly recycled, being generated at the mid-ocean ridges and descending into the mantle next to the trenches through the phenomenon of subduction. The oldest rocks are thus only 180 million years old. Its extension (% of the Earth's surface) is 55%, much less than that of the ocean, because a significant part of the seas have continental-type crust on their bottom. Its relative density is high (2.9 g/cm³), as corresponds to basic plutonic rocks.

Continental crust

The continental crust is less homogeneous in nature, as it is made up of rocks of diverse origins and is horizontally heterogeneous. It is necessary to distinguish in it geologically active regions, where tectonic and magmatic processes abound, which we call orogens; and old, consolidated regions, which we call cratons. In the tectonically consolidated regions that we call cratons, most of their thickness, since the Mohorovičić discontinuity, is made of granites, acid magmatic rocks, although a physical phase boundary called the Conrad discontinuity (in some regions a sudden change between felsic rocks and mafic rocks appears). Old metamorphic rocks usually appear on the granites, formed by regional metamorphism in the orogens, which together with the previous ones form the continental base. Except for the shields, the plinth is covered by a covert, made up of highly varied sedimentary rocks. As a whole, the continental crust contains more silicon and lighter cations and is therefore less dense than the oceanic crust and certainly the mantle. It is also thicker than the oceanic crust. Unlike this, it does not return to the mantle, it is not recycled, although it does spread, which occurs due to orogenesis processes, so that its contribution to the total earth's crust grows.

The most abundant minerals in this area are quartz, feldspars and micas, and the most abundant chemical elements are oxygen (46.6%), silicon (27.7%), aluminum (8, 1%), iron (5.0%), calcium (3.6%), sodium (2.8%), potassium (2.6%) and magnesium (2.1%).

Training and explanation

Earth formed approximately 4.605 million years ago from a disk of dust and gas orbiting the newly formed Sun. It formed through accretion, where planetesimals and other smaller rocky bodies collided and got stuck, gradually growing into a planet. This process generated an enormous amount of heat, causing the early Earth to melt completely. As planetary accretion slowed, the Earth began to cool, forming its first crust, called the primary or primordial crust. This crust was likely destroyed repeatedly by large impacts, then reformed from the magma ocean left behind by the impact. None of the Earth's primary layers have survived to this day; it was all destroyed by erosion, impacts, and plate tectonics in the last few billion years.

Since then, Earth has been forming a secondary and tertiary crust. Secondary crust forms at mid-ocean spreading centers, where partial melting of the underlying mantle produces basaltic magmas and new forms of oceanic crust. This "crest push" it is one of the driving forces of plate tectonics, and it is constantly creating new oceanic crust. That means the old crust must be destroyed somewhere, so opposite a spreading center there's usually a subduction zone: a trench where an oceanic plate is being pushed back into the mantle. This constant process of creating new oceanic crust and destroying old oceanic crust means that the oldest oceanic crust on Earth is only about 200 million years old.

By contrast, most of the continental crust is much older. The oldest continental crustal rocks on Earth have ages in the range of about 3.7 to 4.28 billion years and have been found in the Narryer Gneiss Terrane in Western Australia, in the Acasta Gneiss in the Northwest Territories on the Shield Canadian and in other cratonic regions such as those on the Baltic Shield. Some Zircon as old as 4.3 billion years has been found in the Narryer Gneiss Terrane.

The average age of Earth's present continental crust has been estimated to be about 2 billion years. Most crustal rocks formed before 2.5 billion years ago are found in cratons. Such ancient continental crust and the underlying mantle asthenosphere are less dense than anywhere else on Earth and are therefore not easily destroyed by subduction. The formation of new continental crust is linked to periods of intense orogeny; these periods coincide with the formation of supercontinents such as Rodinia, Pangea, and Gondwana. The crust is formed in part by the aggregation of island arcs, including granitic and metamorphic fold belts, and is partly preserved by depletion of the underlying mantle to form a buoyant lithospheric mantle.