Structure and Interior of the Earth

Sources of Information About the Earth’s Interior

Because direct access to the Earth’s interior is extremely limited, knowledge of its internal structure comes from both direct and indirect sources.

Direct Sources

Mining and drilling

Mining and drilling allow material beneath the Earth’s surface to be examined directly. Their usefulness is limited because humans cannot excavate very deeply.

• The Mponeng and TauTona gold mines in South Africa reach depths of roughly 4 km.
• The Kola Superdeep Borehole, drilled in the former Soviet Union, reached a depth of about 12 km.

These depths are very small compared with the Earth’s radius.

Volcanic eruptions

At active plate boundaries and other weak zones in the crust, molten material from the mantle may reach the surface. The erupted material provides direct evidence about material below the crust. This source is also limited because volcanic activity is restricted in geographical extent and does not bring material from all depths.

Indirect Sources

Gravitational field

The force of gravity varies with the mass and density of material beneath a place. At the same distance from the Earth’s centre:

• A higher gravitational value suggests denser or more massive material below.
• A lower gravitational value suggests less-dense material below.

A difference between the observed and expected gravitational values is called a gravity anomaly. Gravity anomalies help scientists infer variations in the density and distribution of subsurface rocks.

Magnetic field

The strength of the local magnetic field is influenced by magnetic minerals, especially iron-bearing minerals. Strong magnetic readings may indicate a greater concentration of such minerals, while weak readings may indicate fewer magnetic minerals.

Temperature and pressure

Temperature and pressure generally increase with depth. The high internal temperature and the presence of partially molten or plastic layers provide clues about conditions within the Earth. Radioactive decay is one important source of the Earth’s internal heat.

Meteorites and other celestial bodies

The Earth and other bodies in the Solar System formed from related primordial material. Studying the mineral and chemical composition of meteorites therefore provides indirect clues to the composition of the Earth’s deep interior.

Seismic waves

Plate interactions and fault movement release energy. When this energy travels through or over the Earth as seismic waves and causes the ground to shake, the event is called an earthquake or seismic activity.

Seismic waves are classified into two broad groups:

1. Body waves, which travel through the Earth’s interior:
   • Primary waves (P-waves)
   • Secondary waves (S-waves)
2. Surface waves, which travel along the Earth’s surface:
   • Love waves
   • Rayleigh waves

Body waves are particularly useful for studying the Earth’s interior because their speed and path change with the density and physical state of the material through which they pass.

P-Waves and S-Waves

The speed of a seismic wave depends on the density, elasticity, and physical state of its medium. In general, P-waves travel faster through solids than through liquids and gases. The inability of S-waves to pass through liquids is especially important evidence about the Earth’s internal layers.

Focus, Epicentre, and Seismograph

• The focus or hypocentre is the point inside the Earth where an earthquake originates.
• The epicentre is the point on the Earth’s surface directly above the focus.
• A seismograph records seismic waves.

Seismic Evidence for the Earth’s Internal Structure

Seismograph records reveal several important patterns:

1. P-waves change speed and are refracted at boundaries between layers of different density and composition.
2. At the mantle-core boundary, P-waves slow sharply and bend as they enter the liquid outer core. Their speed increases again in the solid inner core.
3. P-waves are not detected in an angular belt of approximately 103° to 142° from the epicentre. This is the P-wave shadow zone.
4. S-waves do not pass through the outer core and are not detected beyond approximately 103° from the epicentre. This is the S-wave shadow zone.

These observations show that:

• The outer core is liquid, because S-waves cannot pass through it.
• The inner core is solid, as indicated by the behaviour and velocity of P-waves.
• The mantle contains materials of changing density and composition with increasing depth.
• The Earth can be divided into major internal layers: the crust, mantle, outer core, and inner core.

Layers of the Earth

Differences in mineral composition and density divide the Earth into three principal layers-the crust, mantle, and core-and six broad sublayers:

1. Upper crust
2. Lower crust
3. Upper mantle
4. Lower mantle
5. Outer core
6. Inner core

Each major transition between layers is marked by a discontinuity, where seismic-wave velocity or direction changes abruptly.

Crust

The crust is the Earth’s outermost and thinnest layer.

• Its average thickness is approximately 35 km, though it varies considerably: oceanic crust is generally 5-10 km thick, while continental crust averages roughly 30-50 km and may be thicker beneath major mountains.
• Continental crust is relatively thick and less dense. Older geographical terminology describes its silica- and aluminium-rich upper portion as SIAL.
• Oceanic crust is thinner and denser. Older terminology describes its silica- and magnesium-rich composition as SIMA.
• The most abundant elements in the crust include oxygen, silicon, aluminium, iron, calcium, sodium, potassium, and magnesium.
• The crust accounts for less than 1% of the Earth’s volume and mass.
• Crustal density generally increases with depth, from roughly 2.7-2.8 g/cm³ in continental rocks to about 3.0 g/cm³ in oceanic rocks.

The Conrad discontinuity is traditionally used for the boundary between the upper and lower continental crust. The Mohorovičić discontinuity (Moho) separates the crust from the mantle.

Mantle

The mantle lies below the crust and extends to a depth of approximately 2,900 km.

• It contains silicate minerals rich in magnesium and iron.
• Density increases from approximately 3.3 g/cm³ near the top to about 5.5 g/cm³ near the core.
• It accounts for about 84% of the Earth’s volume and roughly 67% of its mass.
• Mantle convection drives much of the tectonic and volcanic activity that reshapes the Earth’s surface.

The crust and rigid uppermost mantle together form the lithosphere, which is commonly taken to extend to about 100 km depth, although its thickness varies. Beneath it lies the weaker, plastic asthenosphere, broadly associated in these notes with depths of about 100-350 km.

The Repetti discontinuity is traditionally used for the transition between the upper and lower mantle. The Gutenberg discontinuity, at about 2,900 km depth, separates the mantle from the outer core.

Core

The core is the Earth’s innermost layer and extends from approximately 2,900 km depth to the centre at about 6,371 km.

• It consists mainly of iron and nickel and is therefore traditionally called the NiFe layer.
• Density rises from roughly 9.9 g/cm³ in the outer core to about 13 g/cm³ near the centre.
• The core accounts for about 15% of the Earth’s volume and approximately 32% of its mass.
• The outer core is liquid, primarily because its very high temperature prevents solidification despite immense pressure.
• The inner core is solid because pressure at the centre is high enough to keep the material solid.
• Convection of electrically conducting molten iron in the outer core generates the Earth’s magnetic field through the geodynamo.

The Lehmann discontinuity separates the liquid outer core from the solid inner core.

By bulk composition, the most abundant elements in the whole Earth are approximately ordered iron, oxygen, silicon, and magnesium.

Summary of layers and discontinuities

Minerals

A mineral is a naturally occurring, generally inorganic, homogeneous solid with a definite chemical composition, an ordered atomic structure, and characteristic physical properties such as colour, lustre, hardness, cleavage, and crystal form.

Based broadly on composition and physical properties, minerals may be grouped as follows.

Metallic minerals

Metallic minerals contain metals and are generally good conductors of heat and electricity.

• Ferrous minerals: contain iron; examples include iron ores.
• Non-ferrous minerals: do not contain iron as the principal metal; examples include ores of aluminium, copper, zinc, and lead.
• Precious metallic minerals: rare, valuable metals such as gold, silver, and platinum.

Non-metallic minerals

Non-metallic minerals generally lack metallic properties and are poor conductors of heat and electricity. Examples include phosphates, sulphur-bearing minerals, and nitrates. Many are important to plant and animal growth and to industry.

Minerals combine to form rocks.

Rocks and Petrology

A rock is a naturally occurring aggregate of one or more minerals or mineraloids held together by physical or chemical bonds. Quartz and feldspar are among the most common rock-forming minerals. The scientific study of rocks is called petrology.

Based on origin and mode of formation, rocks are classified into three major groups:

1. Igneous rocks
2. Sedimentary rocks
3. Metamorphic rocks

Igneous Rocks

Igneous rocks are the primary rocks of the crust. They form when magma or lava cools and solidifies. They are generally unstratified and do not contain fossils.

Types based on place of solidification

Intrusive or plutonic rocks

Intrusive rocks form when magma cools slowly at considerable depth below the surface. Slow cooling allows large crystals to develop, giving the rock a coarse-grained texture.

Examples include granite, diorite, and gabbro.

Extrusive or volcanic rocks

Extrusive rocks form when lava cools rapidly at or near the Earth’s surface. Rapid cooling produces fine-grained or glassy textures.

Examples include basalt, andesite, and rhyolite.

Hypabyssal or intermediate rocks

These rocks solidify at shallow depths between deep-seated plutonic rocks and surface volcanic rocks. Their texture varies with the rate of cooling. Typical examples include dolerite and porphyry.

Types based on composition

Sedimentary Rocks

Sedimentary rocks form from sediments derived from pre-existing rocks, organisms, or chemical precipitation. They are called secondary rocks because their material commonly comes from older rocks.

The conversion of loose sediment into solid rock is called lithification. Its principal stages are:

1. Compaction: overlying material presses sediment grains together.
2. Cementation: dissolved minerals precipitate between grains and bind them.

Deposition often occurs in successive layers, producing stratification. Sedimentary rocks are therefore also called stratified or layered rocks. Fossils and fossil fuels occur mainly in sedimentary rocks.

Examples of sediment deposits include:

• Till: deposited by glaciers.
• Fluvial deposits: deposited by rivers and surface runoff.
• Loess: fine sediment deposited by wind.
• Organic or fossil-bearing deposits: contain remains of plants and animals.
• Placer deposits: concentrations of heavy and valuable minerals, such as gold, produced by flowing water.

Mechanically formed or clastic rocks

These form through the physical weathering, transportation, deposition, and lithification of rock fragments by rivers, wind, glaciers, and other agents.

• Argillaceous rocks are rich in clay. They have high porosity but generally low permeability and a relatively soft texture. Shale is an example.
• Arenaceous rocks are rich in sand-sized particles. Many are porous and permeable and can serve as reservoirs for groundwater, petroleum, and natural gas. Sandstone is an example.

Organically formed rocks

Organic remains accumulate, undergo biological and chemical change, and are lithified.

• Carbonaceous rocks are rich in carbon. Coal grades include peat, lignite, bituminous coal, and anthracite, with carbon content generally increasing along that sequence.
• Calcareous rocks are rich in calcium carbonate and commonly form from shells and skeletons of organisms. Examples include limestone and chalk; dolostone is a related carbonate rock.

Chemically formed rocks

These form when dissolved minerals precipitate, commonly because of evaporation or chemical reactions. Examples include rock salt, gypsum, chert, and some limestones.

Limestone may form through organic, chemical, or clastic processes.

Metamorphic Rocks

Metamorphic rocks form when pre-existing rocks undergo recrystallisation and mineral reorganisation under elevated temperature, pressure, or chemically active fluids without completely melting.

Minerals may become arranged in:

• Lineation: linear alignment of minerals.
• Foliation: planar layers or sheets.
• Banding: alternating layers of different mineral composition or thickness.

Based on the dominant agent, metamorphism may be described as:

• Thermal or contact metamorphism: heat is dominant.
• Dynamic or regional metamorphism: pressure and deformation are dominant, usually together with heat.

Common transformations include:

The Rock Cycle

The rock cycle is the continuous process through which rocks form, change from one type to another, melt, and form again.

1. Molten material cools and solidifies to form igneous rock.
2. Igneous, sedimentary, or metamorphic rocks may be weathered and eroded into sediments.
3. Sediments are deposited, compacted, and cemented to form sedimentary rock.
4. Igneous and sedimentary rocks-and older metamorphic rocks-may be transformed by heat and pressure into metamorphic rock.
5. Any rock type may be carried deep into the Earth by subduction and melt into magma.
6. Cooling magma again forms igneous rock, completing the cycle.

Igneous rock forms only through the solidification of molten material, while sedimentary and metamorphic rocks can arise from more than one pre-existing rock type.