Looking inside the Earth
The internal structure of the Earth consists of three main parts, the crust, mantle and core. The division between the crust and the mantle is called the Moho.
The crust makes up only 0.5 % of the Earth's total mass and can be subdivided into two main parts, continental and oceanic. Both differ in thickness, density and composition. Although oceanic crust covers approximately 61 % of the Earth's surface, it only comprises some 30 % of the crustal mass, as the continental crust is much thicker.
The continental crust ranges from the surface of the Earth down to 30 km - 50 km. The exposed parts of the continental crust are less dense than oceanic crust (this is because oceanic crust contains minerals rich in heavier elements such as iron and magnesium). However, continental crust appears to be stratified (layered) and becomes denser with depth. It is largely composed of volcanic, sedimentary and granitic rocks, although the older areas are dominated by metamorphic rocks.
The continental crust can be divided into:
1. Stable continental regions (e.g. Precambrian cratons, platforms of undeformed sediments on crystalline basements)
Thickness: 35 km - 45 km
Density: 2.69 gm/cm3 - 2.74 gm/cm3 for the upper crust and 3.0 gm/cm3 - 3.25 gm/cm3 for the lower crust.
- Upper Crust (sial): felsic igneous and metamorphic rocks. Average composition close to that of either a mafic granodiorite or quartz diorite.
- Lower Crust (sima): anhydrous (no water) - quartz andesite or andesite; hydrous - amphibolite or diorite
2. Tectonically active regions (e.g. the Andes, Himalayas)
Thickness: 55 km - 70 km
Composition: varies from region to region but may consist of any of the following:
- mafic rocks
- ultramafic, mafic and felsic-intermediate rocks
The oceanic crust ranges from the surface of the Earth down to 10 km - 12 km. The oceanic crust can be divided into:
1. Ocean basins:
water depth exceeds 4 km, layered and very uniform and consists of three distinctive layers:
- Layer 1: thickness of < 1 km; deep water sediments in various stages of lithification (turning into rock), commonly foraminiferal ooze, chert and mudstone.
- Layer 2: thickness of 1.6 km - 2 km; basalts and dolerites with pillow lavas, dykes and sills (i.e. volcanic extrusive and intrusive rocks).
- Layer 3: thickness of 3.0 km - 5.7 km; the main crustal layer; dolerite, gabbro and amphibolite (i.e. intrusive mafic rocks).
- Layer 3B: thickness is variable but up to 3 km; not always present; metagabbros and pyroxenites (i.e. intrusive mafic and ultramafic rocks).
2. Mid-ocean ridges
In mid-ocean ridges (such as the Mid-Atlantic Ridge), Layer 1 is absent; Layer 2 crops out on the surface and is thicker than normal; Layer 3 is thinner than normal and passes transitionally into the upper mantle.
3. Island arcs
The structure beneath island arcs (such as the Indonesian Arc) is very complex and the composition of the crust in these regions is very heterogenous. The most common rocktypes in surface exposures are volcanic andesites (from explosive volcanoes) and deep-sea sediments.
Differences between Continental and Oceanic Crust/Lithosphere Thicknesses
The boundary between the crust and mantle is known as the Mohorovicic Discontinuity (Moho). It marks a significant change in chemical composition and is the boundary between the crust and mantle. It is a sharp boundary at which both P- and S-wave speeds rise from crustal to higher values. This change occurs over a distance of 0.1 km to 0.5 km in oceanic and stable continental regions. In tectonically active regions, the change is far more gradual and generally ill-defined - in many cases, there may not even be a Moho. At present there are two main hypotheses for what the Moho represents:
- a phase transition: The Moho may mark an isochemical phase transformation (i.e. change in mineralogy but not chemistry) from a gabbroic lower crust to an eclogitic upper mantle. However, this phase transformation appears to be a gradual process in oceanic and stable continental regions.
- a chemical discontinuity: The most widely accepted theory for the Moho is that it represents a chemical change from intermediate and mafic crustal rocks, to an ultramafic mantle.
The mantle is thought to be primarily composed of ultrabasic rocks (rocks rich in magnesium and iron, and poor in silica; mostly peridotites). As seismic velocity change through the mantle, we can subdivide it into an upper mantle, transition zone, and lower mantle. There is also a zone in the upper mantle that we call the low velocity zone (located at depths of 70 km to 250 km) where S-wave speeds decrease rapidly to a minimum and then gradually increase again. We believe that most magmas (molten rock) are generated in this zone.
Upper mantle (measured from the base of the crust down to 400 km).
10 % of the Earth's total mass.
Density of 3.25 gm/cm3 to 3.40 gm/cm3
Composition: Peridotite (e.g. olivine + pyroxene) along with plagioclase (< 30 km depth), spinel (30 km - 70 km depth) and garnet (> 70 km depth). In tectonically active regions, eclogite (amphibole + garnet) is a major component.
Although it is thought that almost all basalts are derived from the upper mantle, experiments have shown that their compositions cannot be formed by the partial melting of peridotite alone. The upper mantle therefore is probably composed of two main zones: an upper peridotite zone and an underlying primitive mantle or pyrolite zone composed of pyroxene, olivine, and/or garnet and/or plagioclase, where basaltic magmas are generated.
The pyrolite model was first proposed by two Australian geoscientists, Green and Ringwood (1963), in order to explain the nature of the composition of the upper mantle. They recognised that most mantle xenoliths had previously undergone some partial melting in order to produce basaltic magmas. They suggested that a model composition of the undepleted upper mantle could be tested by experimental methods to determine the types of basaltic magmas that were produced. They made this by mixing three parts dunite (olivine-rich) with one part basalt (pyroxene and feldspar rich) and called this mixture pyrolite (pyroxene-olivine). Experimental melting studies on this mixture at differing values of pressure, temperature and volatile content has yielded the range of most known basalt compositions suggesting that it is a valid model for the composition of the upper mantle.
Transition zone (400 km - 1000 km below the Earth's surface).
17 % of the Earth's total mass.
The top of the transition zone is marked by the phase transformation of normal olivine to a proto-spinel structure polymorph of olivine which is a higher pressure phase and is 9 % denser than normal olivine and by the phase transformation of normal pyroxene to a garnet-structure polymorph of pyroxene. Within the transition zone itself, there are a number of irregular seismic velocity changes including a major one at 680 km which is marked by the breakdown of olivine into its constituent oxide components of periclase (MgO) and stishovite (SiO2). Also, garnet is broken-down to its component oxides from 680 km - 1000 km
Lower mantle (1000 km - 2900 km below the Earth's surface). 41 % of the Earth's total mass.
The lower mantle is a region of relatively low seismic velocity gradients. It most likely consists of mixed oxides of pyrolite composition but with an increased iron content.
The core is marked as that point within the Earth where S-waves cannot penetrate. It is believed to be composed primarily of a nickel-iron alloy (along with abundant platinum-group elements), consisting of a liquid outer zone, and a solid inner zone. It is also marked by an abrupt increase in pressure.
- Outer core (2900 km - 5000 km below the Earth's surface). 30 % of the Earth's total mass.
- Inner core (5000 km - 6370 km below the Earth's surface). 2 % of the Earth's total mass.
Chemical composition of the Earth
The overall composition of the Earth is dominated by the elements iron (Fe), oxygen (O), silicon (Si), magnesium (Mg), nickel (Ni) and sulfur (S). This is because most of the mass of the Earth occurs within the mantle which is composed largely of the ferromagnesium silicate minerals olivine and pyroxenes. The core of the Earth is largely composed of iron and nickel. The crust of the Earth mainly comprises the minerals plagioclase, quartz and hornblende and is dominated by the elements oxygen, silicon, aluminium, iron, calcium, sodium, and potassium.
The overall composition of the Earth is very similar to that of meteorites, and because of this, it is thought that the Earth originally formed from Planetesimals composed largely of metallic iron and silicates.
P- and S-waves
Seismology is the study of the origin and propagation of elastic waves through planetary bodies. It was originally regarded as the study of earthquakes. The subdivisions in the Earth's interior are marked by abrupt changes in seismic velocities at specific depths. The two types of seismic wave relevant to studying the Earth's interior are:
- P-waves: Primary or Compressional Waves are the fastest type of wave and have velocities of 5 km/s at the top of the crust, 8 km/s at the top of the mantle, and 14 km/s at the bottom of the mantle. P-waves can travel through solids and liquids, so they can travel through the Earth's core (the outer core is liquid).
- S-waves: Shear or Transverse waves that travel at slower speeds than P-waves. They can only travel through solids, so they do not travel through the Earth's core.