Rocks That Make Up the Earth's Crust: Types and Structure Explained

The Earth's crust is the planet's thin, solid outer shell of rock, ranging from roughly 5 to 70 kilometres thick and made up overwhelmingly of silicate rocks. The rocks that make up the Earth's crust sit at different levels relative to the surface — in some places they emerge directly into the open air, while in others they lie hidden beneath a layer of soil.

What the Earth's crust is: definition and general characteristics

The Earth's crust is the outermost solid layer of the planet, resting on the mantle and forming the upper part of the lithosphere. It is the thinnest of Earth's main layers — the crust, the mantle, and the core (itself split into an outer core and an inner core) — yet it is the only layer humans live on and drill into directly. Compared with the mantle below it, the crust is rich in lighter elements and far less dense, which is precisely why it "floats" on the denser material beneath.

Geologists recognise two fundamentally different kinds of crust: continental crust, which forms the landmasses, and oceanic crust, which underlies the ocean basins. The boundary between the crust and the mantle is marked by a seismic surface called the Mohorovičić discontinuity, named after the Croatian geophysicist Andrija Mohorovičić who identified it in 1909 from a sudden change in the speed of earthquake waves.

The structure of the upper layers of the Earth's crust

The structure of the upper layers of the Earth's crust was first revealed in detail by studying how waves travel through the crust from blasting work — explosions carried out to free deposits of useful minerals, such as coal, from the rocks covering them. The propagation of a blast wave deep into the Earth's crust is measured with seismographs.

Using this method, the Earth's crust was studied down to a depth of 38 kilometres. It turned out to consist of several layers of differing density. The upper layer, covered by sedimentary rocks, is formed by granites about 10 kilometres thick. Beneath them lie, in succession, three layers of basalt with a combined thickness of over 28 kilometres.

Methods of studying the Earth's crust with seismographs

Seismographs let scientists study the Earth's crust indirectly by recording how earthquake waves change speed and direction as they pass through rocks of different density. Because seismic waves travel faster through denser, more rigid material, an abrupt jump in velocity signals a boundary between layers — this is exactly how the Mohorovičić discontinuity was detected. The seismologist R.D. Oldham used the same principle of wave analysis to demonstrate that the Earth has a distinct core, while later researchers such as Don L. Anderson refined our picture of the deep interior.

Seismic surveying is complemented by other indirect techniques. Gravity and magnetic field studies map variations in the density and magnetisation of buried rock, measurements of heat flow reveal how warmth escapes from Earth's interior, and laboratory experiments that subject rocks to high pressure and temperature reproduce deep conditions so the recorded wave speeds can be matched to real mineral assemblages.

The layers of the Earth's crust: granitic and basaltic

The continental crust is classically divided into an upper granitic layer and a lower basaltic layer, separated by a transition sometimes called the Conrad discontinuity. The granitic (or "sial") layer is rich in silicon and aluminium and gives continents their characteristic low density; the basaltic (or "sima") layer below is richer in silicon and magnesium and is denser. Oceanic crust, by contrast, lacks a true granitic layer and is composed almost entirely of basalt.

Variation in the thickness of the Earth's crust

The thickness of the Earth's crust varies dramatically depending on whether it is continental or oceanic. Continental crust averages about 30–40 kilometres and can exceed 70 kilometres beneath great mountain ranges such as the Himalayas and the Tibetan Plateau, where crust is piled up by collision. Oceanic crust is far thinner — typically only 5 to 10 kilometres. These variations are not random: the crust behaves according to isostasy, the principle that lighter crustal blocks float on the denser mantle and sink deeper where they are thicker, much as a thick iceberg projects both higher above and deeper below the waterline. Thick mountain roots beneath the Himalayan mountain range are a textbook example of isostatic compensation.

The chemical composition of the Earth's crust

The Earth's crust is dominated chemically by just two elements — oxygen and silicon — which together make up roughly three-quarters of its mass. The crust is therefore overwhelmingly silicate in character, built from minerals in which silicon and oxygen are bonded with metals. This is very different from the deep interior: the outer core and inner core are composed mainly of iron and nickel, while the mantle is rich in magnesium-iron silicates such as those found in peridotite.

The abundance of chemical elements in the Earth's crust

The eight most abundant chemical elements account for around 98% of the crust's mass, and their order of abundance is well established by analyses compiled by bodies such as the U.S. Geological Survey, part of the U.S. Department of the Interior.

• Oxygen — about 46%
• Silicon — about 28%
• Aluminium — about 8%
• Iron — about 5%
• Calcium — about 4%
• Sodium, potassium and magnesium — a few percent each

Because oxygen and silicon are so dominant, the rock-forming silicate minerals — chiefly feldspar and quartz — are the most common minerals in the crust. Feldspar alone is estimated to constitute more than half of the crust by volume, which is why it appears so prominently in granite.

Types of Earth's crust: continental and oceanic

There are two principal types of Earth's crust — continental crust and oceanic crust — and they differ in age, thickness, density, and composition. Understanding this distinction is the key to plate tectonics, because the two crust types behave very differently when tectonic plates collide.

Composition and properties of continental crust

Continental crust is thick, light, and ancient, composed largely of granitic rock rich in silicon and aluminium. Its low average density — around 2.7 grams per cubic centimetre — lets it stand high and resist being pulled down into the mantle, which is why continents persist for billions of years. The oldest parts of continents preserve a record of the planet's earliest history, making continental crust the principal archive of Earth's geological past.

Composition and properties of oceanic crust

Oceanic crust is thin, dense, and young, consisting mainly of basalt and, deeper down, gabbro overlying mantle peridotite. Its higher density — around 3.0 grams per cubic centimetre — means it sits lower than continents and is readily recycled into the mantle. New oceanic crust is created continuously along the mid-ocean ridge system, where magma rises and solidifies as plates pull apart. Slices of ancient oceanic crust and upper mantle occasionally become thrust onto land; these preserved fragments, called ophiolites, give geologists a rare direct look at material that normally lies beneath the sea.

Differences between continental and oceanic crust

The clearest way to compare the two crust types is side by side.

Density and distribution of material in the Earth's crust

Density differences govern how material is arranged in the Earth's crust and across the whole planet. Lighter granitic continental blocks ride higher than denser basaltic ocean floor, and both float on the still-denser mantle in accordance with isostasy. On a planetary scale, density increases steadily inward — from the light crust through the mantle to the iron-nickel core — a layering that Isaac Newton's work on gravitation first made possible to constrain through measurements of the Earth's overall mass and density.

Formation and evolution of the Earth's crust

The Earth's crust formed when the molten surface of the early planet cooled and solidified. Soon after Earth accreted, its outer layers were a magma ocean; as heat radiated away, the lightest silicate minerals crystallised and rose to form a primitive crust, while denser iron and nickel sank to build the core. Continental crust then grew gradually over billions of years through repeated melting, recycling, and the collision of plates — a process still ongoing today.

Plate tectonics drives the continuing evolution of the crust. Continents have repeatedly assembled into supercontinents and broken apart again: geologists trace this cycle through Rodinia, Gondwana, and Pangaea, with the present arrangement of continents merely the latest configuration. Each cycle of assembly and rifting reworks the crust, building mountains through orogeny and opening new ocean basins.

Cratons, shields, and platforms

Cratons are the stable, ancient cores of continents that have remained largely undeformed for hundreds of millions to billions of years. Where the old crystalline rock of a craton is exposed at the surface, it is called a shield — the Canadian Shield and the Baltic Shield (also known as the Fennoscandian Shield) are classic examples. Where that same ancient basement is buried under a relatively thin cover of younger sedimentary rock, it is called a platform. Together, shields and platforms form the durable foundations on which continents are built.

The oldest rocks of the Earth's crust and their age

The oldest known intact crustal rock is the Acasta Gneiss of northern Canada, dated to roughly 4 billion years. Even older are individual zircon crystals from the Narryer Gneiss terrane in Western Australia, which have been dated to about 4.4 billion years and are the oldest known fragments of Earth's crust. Such finds confirm that continental crust began forming very early in Earth's history and that some of it has survived essentially intact ever since.

Continental margins and their types

Continental margins are the transition zones where thick continental crust meets thin oceanic crust, and they come in two main types. A passive margin sits within a single tectonic plate, far from plate boundaries, and accumulates thick sedimentary wedges — much of the Atlantic coastline is of this kind. An active margin coincides with a plate boundary, where one plate descends beneath another in a subduction zone, generating earthquakes, volcanoes, and deep ocean trenches — the western edge of South America along the Andes is a leading example.

The main groups of rocks

However great the diversity of rocks, they are divided into three main groups:

1. igneous,
2. sedimentary,
3. metamorphic.

Igneous rocks

Igneous rocks, also called extrusive when erupted, formed through the solidification of magma, which either poured out onto the surface or halted at certain depths without reaching the surface of the Earth. Igneous rocks are not layered and contain no animal or plant remains. They are in turn divided into:

• effusive (volcanic) rocks,
• intrusive rocks.

Effusive (volcanic) rocks

Rocks that pour out onto the surface are called effusive (from the Latin verb effundere — to pour out) or volcanic. Effusive rocks cool quickly, turning either into a porous, slag-like mass — volcanic lavas are an example — or into dense rocks such as basalt and obsidian, the latter also known as volcanic glass.

Basalt

A widely distributed effusive rock, sometimes covering tens of thousands of square kilometres, is basalt. Basalt. Basalt is the dominant rock of oceanic crust and erupts abundantly at volcanic hotspots such as Hawaii, where vast lava flows build entire islands.

Obsidian

Another very common effusive volcanic rock is obsidian. Its colour is velvety-black and dark grey, more rarely other shades, with a strong glassy lustre.

In chemical composition (which is, however, extremely variable) and hardness, obsidian approaches the feldspars. Obsidian is easily distinguished from other rocks by its dark colour and deep conchoidal fracture. Obsidian stone

When split, it produces fragments with sharp, cutting edges. This valuable property of obsidian was used by prehistoric humans in making tools and hunting implements. Outcrops of obsidian are often found in the Transcaucasus, and especially in the vicinity of Lake Sevan.

Intrusive (deep-seated) rocks

Intrusive rocks, or deep-seated rocks (from the Latin verb intrudere — to push in), are also called plutonic.

The word plutonic comes from the name of Pluto — the god of the underworld who, according to the beliefs of the ancient Greeks, sent riches up to people. The expression plutonic rocks emphasises their deeper position in the bowels of the Earth. Cooling at depth in the Earth's crust takes place extremely slowly and under the enormous pressure of the overlying strata of rock.

Deep-seated (intrusive) rocks formed under such conditions differ from volcanic (effusive) rocks by their granular-crystalline structure. Granite is an example.

Granite

Granite is the most important intrusive rock of the continental crust and the chief reason continents are so light and buoyant. Because it crystallises slowly at depth, granite develops large, interlocking crystals of feldspar, quartz, and mica that give it its characteristic speckled appearance and great durability — qualities that have made it both a key building stone and the dominant rock of the upper continental crust.

Sedimentary rocks

By their distribution in nature, sedimentary rocks rank next after igneous rocks. They are:

• clastic sedimentary rocks,
• chemical sedimentary rocks,
• organic sedimentary rocks.

Clastic sedimentary rocks

The predominant role within the sedimentary group belongs to clastic sedimentary rocks, or mechanical sediments: crushed stone, pebbles, gravel and sand:

• crushed stone refers to sharp-angled fragments and splinters of rock;
• pebbles are rounded stones ranging from 10 millimetres to 10 centimetres;
• gravel consists of small stones from 2 to 10 millimetres;
• sand consists of grains smaller than 2 millimetres.

Mechanical sediments

The same rocks, but bound (cemented) by clayey, calcareous, ferruginous, and siliceous solutions, are set apart as a special group of cemented clastic rocks:

• cemented crushed stone is called breccia (in Italian, breccia means crushed stone);
• cemented gravel and pebbles form conglomerates (from the Latin, meaning gathered into a heap);
• sand under the same conditions turns into sandstone.

Chemical sedimentary rocks

Sands and clays occupy a notable place among sedimentary rocks. Chemical sedimentary rocks form from aqueous solutions containing dissolved substances and salts. Examples are Glauber's salt (bitter salt) and rock salt, gypsum and others; they form from supersaturated solutions as a body of water evaporates. Such deposits are commonly left behind in enclosed seas and lagoons — for instance, evaporite layers preserved around basins like the Ionian Sea record episodes of intense evaporation in the geological past.

Organic sedimentary rocks

Organic sedimentary rocks include the group of biolites, or organogenic rocks (that is, rocks formed with the participation of organisms). In this group, the predominant position is held by limestone, chalk, coal, peat. Oil and gas also belong here. Limestone stone. These rocks are economically vital, and organisations such as the World Economic Forum repeatedly highlight that the availability of such resources shapes global energy and material supply.

Sedimentary rocks are distinguished by their layering; they frequently contain fossils and imprints of the most ancient plants and animals.

The alteration of rocks

The rocks that make up the Earth's crust do not remain unchanged. At the surface, the leading role in altering rocks is played by climate, especially sharp swings in temperature, which in deserts can reach 50–60° over the course of a single day. The minerals composing the rocks then undergo unequal expansion and contraction.

Metamorphic rocks

Both igneous and sedimentary rocks, under the influence of enormous pressures — for example, during mountain building — high temperatures, and chemical solutions, undergo significant changes. Such altered rocks are called metamorphic rocks (from the Greek verb metamorphoo — I change, I transform). The high pressures and temperatures involved are routinely reproduced in laboratory experiments on rocks, which let geologists work out exactly how each mineral transforms.

Examples of metamorphic rocks are:

• marble,
• clay slate,
• gneiss,
• quartzite,
• graphite.

Marble and limestone

Marble is limestone that has recrystallised under the influence of pressure from overlying rocks, hot solutions, and the high temperatures of the Earth's interior. Limestone and marble have the same chemical composition, but their structure differs: one is dense and non-crystalline (limestone), the other is crystalline (marble), (more: Minerals and precious stones).

Clay slate and clay

Clays become compacted and acquire a slaty structure, that is, the ability to split into thin, dense plates when struck with a hammer. Hence the name of this altered rock — clay slate. Clay slate

Unlike clay, clay slate does not soften in water and possesses sufficient hardness. Some varieties of clay slate were used for roofing, which is why it is also called roofing slate.

Gneiss

Here is another interesting example of metamorphism. Granite has a remarkable twin in terms of its rock-forming minerals — gneiss. It likewise consists of feldspar, quartz, and mica, except that the flakes of the latter are arranged not in disorder, as in granite, but in parallel rows. Gneiss

This gives the rock a thinly layered, or foliated, structure. Gneisses are often found in the most ancient deposits. They may have formed through the metamorphism of granites, feldspathic sandstones, and other rocks. Quartzite forms through the metamorphism of sand, and graphite through that of coal.

Metamorphic rocks contain no fossils and play a secondary role in the structure of the Earth's crust.

Crystalline rocks

Beneath the layer of soil there usually lie clays, sand, and other rocks. However, wherever a borehole is sunk, granites or other crystalline rocks are inevitably encountered at one depth or another. For example, on the Kola Peninsula and in Karelia, outcrops of granites at the Earth's surface occupy vast expanses. Granite outcrops on the Kola Peninsula

In St Petersburg such rocks were found at a depth of 198 metres, while in Moscow much deeper — at 1,655 metres. Just how great is the thickness of this shell, and what rocks lie beneath it? Scientists obtain the answer to this question both directly and indirectly. The thickness of sedimentary rocks is also determined by the drilling of oil wells.

To carry out geophysical research on rocks at great depths, a borehole was sunk on the Kola Peninsula. Its drilling began in 1970 and was completed in 1990, with the well reaching a depth of 12,262 metres. The depth at which the layer of solid granites lay at the drilling site was 7,000 metres. To this day it remains the deepest borehole in the world, but at present, owing to a lack of funds for its upkeep, it is on the verge of ruin.

Granite

Granite consists of certain minerals known precisely as rock-forming minerals. These are feldspar, quartz, and mica. In each fragment they are distinguished by uniformity of structure and chemical composition. Feldspar, quartz, mica

These three minerals form a very widespread crystalline-granular rock, which is confirmed by its name: the word granite comes from the Latin granum — grain. Thus granite, translated into Russian, means a granular rock.

In granite (more: Ferrous and non-ferrous metals and their ores), grains with a smooth, shiny surface are clearly visible. This is feldspar. Feldspar is the most widespread mineral in nature. It is mainly responsible for giving granite its colour.

By colour, one distinguishes red granites of various shades:

• flesh-red,
• pinkish-red,
• yellowish-red,
• brownish-red;

also widespread are:

• grey granites,
• white,
• more rarely yellow,
• green.

The feldspar grains in these granites are of the same colour. In some varieties of granite they are large, in others small.

Hence the second classification of granites:

• fine,
• medium,
• coarse-grained granite.

Granite stone

Other grains — dull grey, with a greasy or glassy lustre and an uneven surface, well visible in granites — are formed by quartz, also one of the most widespread rock-forming minerals. Even more distinctly visible in granites are the shiny black or light plates of mica.

Quartz in granite increases in volume considerably more than feldspar. As a result of uneven expansion and contraction, the rock becomes covered with cracks on its surface.

Physical weathering

Deepening more and more, the cracks promote further destruction of the rock, especially when water gets into the cracks and freezes (more: The work of snow and ice). Gradually the rock turns into a heap of sharp-angled fragments.

This kind of destruction of rocks under the influence of the processes described above is called physical weathering (more: Weathering of rocks).

Chemical weathering

Water, especially when it contains carbon dioxide, also exerts a dissolving effect on the rocks over which it flows. This kind of weathering, associated with a change in the composition of the rock, is called chemical weathering.

Organic weathering

Vegetation that settles on rocks exerts both physical and chemical effects on rocks. At the ends of the rootlets are root hairs. They secrete oxalic acid, which corrodes the rock and deepens the cracks. The effect of bacteria is even stronger. The destruction of rocks under the influence of organisms is called precisely organic weathering.

The rocks making up the Earth's crust are subject not only to metamorphic changes but, over time, to the effects of external factors. As a result of weathering, granites, for example, yield sand and clay.

Under the appropriate conditions, these turn into sandstones and clay slates, and at considerable depths in zones of higher pressure and temperature, clay slates, feldspathic sandstones, and granites change into gneisses.

Gneisses formed from sedimentary rocks are distinguished by a darker colour and sharper foliation. Igneous rocks, by contrast, yield light-coloured gneisses. And if gneisses are assimilated by magma, then when it solidifies, granites become the predominant rock.

Processes that change the Earth's crust

The Earth's crust is constantly reshaped by forces from deep within the planet as well as by surface weathering. The deepest driver is the slow churning of the mantle, which sets the rigid plates of the crust in motion and ultimately builds mountains, opens oceans, and triggers earthquakes.

Convection currents in the mantle

Convection currents in the mantle are the engine of plate tectonics. Heat escaping from Earth's hot interior makes the mantle's soft, partly molten layer — the asthenosphere — flow extremely slowly, like a thick fluid, rising where it is hot and sinking where it has cooled. These circulating currents drag the overlying rigid lithosphere, splitting it into tectonic plates. Where currents rise, new crust forms at a mid-ocean ridge and plates are pushed apart by ridge push; where currents descend, crust is dragged down at a subduction zone. The Pacific plate, the North American plate, the Juan de Fuca plate, and many others all move at the pace of growing fingernails because of this deep circulation.

Where one plate dives beneath another in a subduction zone, oceanic crust is consumed and destroyed, and the descending rock heats up enough to generate magma that feeds chains of volcanoes — the Aleutian Islands are a classic volcanic arc built above such a zone. Where continents collide instead, neither plate sinks easily, and the crust crumples upward in orogeny to raise great mountain belts: the Himalayas, the Andes with the high Altiplano, and the Tibetan Plateau all owe their height to colliding continental crust.

Earthquakes and crustal hazards

Earthquakes occur when stress built up along plate boundaries is released suddenly as rock breaks and slips. Because tectonic plates grind past, over, and away from one another, friction locks their edges until the strain becomes too great; the abrupt rupture sends out earthquake waves that seismographs record around the world. Regions sitting astride active boundaries — California, where the North American plate and the Pacific plate slide past each other, is a well-known case — face the greatest hazard. The same seismic studies that warn of these dangers also serve science: by analysing how earthquake waves travel, researchers have mapped the crust, the mantle, and the core, turning a destructive hazard into one of geology's most powerful investigative tools.

Frequently asked questions about the Earth's crust and rocks

Earth is not the only body with a crust, and comparing planetary crusts puts our own into perspective. The Moon has a thick, ancient crust rich in feldspar that formed when its early magma ocean cooled, overlying a mantle and a small core; its internal structure has been mapped by seismometers left by astronauts. Mercury, Venus, and Mars all possess solid silicate crusts, while Jupiter's moon Io is wrapped in a crust constantly resurfaced by intense volcanism. What sets Earth apart is active plate tectonics, which keeps recycling its crust — a process that, as researchers including geologists at institutions such as the National University of Singapore and science communicators like Muhammad Nawaz have stressed, appears rare among the worlds we have explored so far. Documentary and educational coverage from bodies such as the National Geographic Society has helped bring these comparisons to a wide audience.

In their economic activity, people use rocks and minerals as mineral resources.

Fuel resources:

• coal,
• peat,
• oil shale,
• oil,
• gas.

Ore resources:

• ferrous metal ores (iron ores, manganese ores);
• non-ferrous metal ores (aluminium, copper, tin);
• precious metals (gold, platinum, silver).

Non-metallic resources:

• building materials (granite, sand, clay, limestone);
• chemical raw materials (table and potassium salt, phosphates, sulphur).