What have we learnt?

In this chapter we have tried to find the engine driving the motion within the Earth and at its surface. We turned to the Earth’s thermal state to uncover how the internal heat produced by natural radioactive decay can be lost at the Earth’s surface without the interior temperature rising above the melting temperature of mantle rock. Slow convective flow in the mantle solves the Earth’s heat problem and the resulting mantle circulation explains the observed motion of oceanic and continental plates and, thus, plate tectonics. We have also seen that the mantle convection model coincides with the measured distribution of the heat flux through the surface of the Earth and that the thermal boundary at the CMB, predicted by the model, is confirmed by data from seismology and mineral physics.

So, did we find all the answers? No, as in most natural sciences, in geodynamics answering one question brings up another. For example, these questions remain unanswered (for now):

Has large-scale plate tectonics always been active on Earth? Or did plate tectonics only start when the Earth had cooled sufficiently? If so, was a smaller scale chaotic type of mantle circulation responsible for the heat loss of the young Earth?

Why do the other terrestrial planets Mercury, Venus and Mars and the Moon no longer show any signs of plate tectonics? This is one of the main questions of comparative planetology, a branch of science that has expanded over the last few years due to the enormous amount of data available from planetary research missions. Comparative planetology has recently been extended to planets outside our solar system i(see http://www.exoplanet.eu and http://kepler.nasa.gov).

Glossary

Accumulation of sediment: the build up of loose deposits.

Anomaly: a deviation from the average or reference value (for example for gravity).

Asthenosphere: the viscous part of the mantle that lies directly underneath the lithosphere, found

between a depth of approximately 80 and 300 km . The tectonic plates move over the asthenosphere.

Azimuth: the horizontal angle measured clockwise from the north of the fault orientation at the surface. Basalt: dark extrusive rock formed during volcanic eruptions. Basalt is rock that has cooled fast, so few

or no visible crystals are formed. During cooling, basalt usually fractures into polygonal columns. In The Netherlands, these columns are used to reinforce dikes.

Bathymetry: depth measurements of the ocean floor, or a map showing ocean depths.

Caldera: a circular depression of the landscape formed by an explosive volcanic eruption. A caldera is

formed when the roof of a volcano’s large, shallow magma reservoir collapses due to the extrusion of large amounts of magma over a short period of time.

Chondrites: the largest group of meteorites. They contain chondrules, small spherical particles, after

which they are named. The chemical composition of the Earth resembles that of the chondrites.

Conduction: the transport of heat through a substance; the passing on of vibrations by atoms in a

lattice.

Convection: The transfer of heat by flow of material caused by hotter, less dense material rising and

colder, denser material sinking under the influence of gravity.

Convergent plate boundary: the margin between two tectonic plates where the plates move toward

each other, one of which subducts underneath the other.

Core: the central part of the Earth, starting at a depth of 3450 km. The core is divided into a liquid outer

core and a solid inner core.

Crust: the outer layer of the Earth. The crust is relatively thin: oceanic crust is about 7 km thick and

continental crust 30 to 50 km thick.

Crustal root: the thickened crust underneath a mountain chain that allows the mountains to maintain

isostatic equilibrium.

Crystal: a homogeneous solid with well-developed surfaces which express the regular internal

arrangement of the crystal’s atoms. The more time and space available for the cooling of a melt, the larger a crystal will grow.

Deformation: the change in the shape, volume, position or orientation of a rock.

Divergent plate boundary: the margin between two plates that move away from each other.

Dome: a swell resulting from the slow extrusion of viscouslava from a volcano the build-up of viscous lava in the crater of a volcano. Sometimes this build-up hampers the extrusion of magma, causing it to slow down.

Epicentre: the location on the Earth’s surface directly above the hypocentre of an earthquake. The

effect of an earthquake is felt most strongly at its epicentre.

Erosion: the removal of rock through physical processes such as water flowing, ice or wind. Focus of an earthquake (= hypocentre): the point of origin of the earthquake within the Earth .

Force: this describes the push or pull exerted on an object. The unit of force is Newton [N].

Fracture zone: a region of cracks in the ocean floor continuing from a transform fault between two mid-

ocean ridge segments.

Granite: coarse, crystalline igneous rock. Granite cooled slowly beneath the Earth’s surface and had

enough time to grow large crystals.

Great circle: a special circle on the surface of a sphere. It is the largest circle on a sphere and dividing

it into two equal halves. A great circle therefore has the same centre as its sphere.

Half-life: the time it takes for a certain amount of radioactive isotope to decay to half its original value.

Every radioactive isotope has a unique half-life.

Hazard management: the measures taken to control or reduce the effects of natural hazards - such as

risk assessments, taking precautions and writing disaster scenarios.

Hotspot: a stationary plume in the mantle made up of hot mantle rock rising from large depths of the

Earth. Volcanism occurs in the region above the plume, where material starts to melt at shallower levels. Hawaii, for example, was formed in this way.

HPT experiments: High Pressure and Temperature experiments to model processes taking place in the

interior of the Earth.

Hypocentre: the location within the Earth’s crust where earthquake vibrations originate. Insolation: solar radiation that reaches the surface of the Earth.

Isotopes: atoms of the same element that have a different atomic mass. Isotopes have the same

number of protons in their nucleus, but a different number of neutrons.

Isostasy: the concept that where the crust is in gravitational equilibrium with the mantle. Because the

Earth’s crust has a lower density than the mantle, it ‘floats’ on top of the mantle (like wood floating on water).

Lava: liquid rock that extrudes from a volcano, flows over the Earth’s surface and eventually solidifies. Limestone: sedimentary rock mainly consisting of calcite (CaCO3).

Lithosphere: the crust and the upper part of the mantle that form the tectonic plate. Beneath the ocean

the lithosphere is about 70 km thick, whereas a continental lithosphere can be well over 125 km thick.

Longitudinal wave (primary wave, P-wave): a seismic wave that oscillates in the same direction as

its propagation direction. The P-wave is the first wave to arrive at a seismic station.

Magma: liquid rock below the Earth’s surface. When magma extrudes slowly, not explosively, and flows

over the Earth’s surface, it is called lava.

Magma chamber: part of the crust where magma is contained in a closed reservoir. This chamber is the

source of volcanism.

Mantle: the 2900-kilometre thick layer between the Earth’s crust and core. The solid mantle is divided

into an upper mantle and a lower mantle.

Marble: metamorphic limestone consisting of carbonate crystals (mostly calcite, CaCO3). Marble comes

in several different colours and is often used for tiles and statues.

Metamorphic rock: the rock formed due to transition of the mineral content of the original rock (for

example, sedimentary or igneous rock) as a result of high pressures and/or temperatures.

Mid-Atlantic Ridge and Mid-Oceanic Ridge (MOR, spreading ridge): the plate boundary

underneath an ocean where two plates diverge and new oceanic crust is formed.

Milankovitch: The Serbian mathematician who described the variation in the Earth’s orbit around the

Sun with the help of mathematical equations. The Earth’s orbit varies in three ways: its eccentricity varies with a period of 100,000 years, its obliquity has a period of 40,000 years and its precession a period of 20,000 years. These kinds of orbital variations change the insolation of the Earth, which in turn affects the Earth’s climate. Thus Milankovitch linked the Earth’s orbital variations to the ice ages.

Orogenesis: mountain building.

Phase change: the transition of a material from the liquid phase to a solid or gas phase or vice versa.

Solid material can also exist in different phases. For example, carbon can be found in a diamond form and a graphite form.

Plates: the Earth’s surface is divided into several tectonic plates. These plates are made up of the

lithosphere, which consists of oceanic and/or continental crust, and the underlying upper part of the mantle.

Plate tectonics: the theory that states that the Earth’s surface is divided into several large, rigid plates

that move slowly with respect to each other. Along the plate boundaries, geological phenomena such as volcanism, earthquakes and mountain building take place.

Polar jet stream: the strong, concentrated winds in the troposphere that drive weather systems. Pressure: force per unit area exerted on an object. Pressure is stress equal in all directions.

Sedimentary rock: rock formed through the transport and deposition (sedimentation) of loose material. Seismograph (seismometer): a device that records seismic waves.

Shield volcano: a shield-shaped volcano made of mostly basaltic lava flows. Shield volcanoes are

characterized by slow extrusions that result in gently sloping flanks and are commonly found at MORs and hotspots.

Shear modulus: the material property of elasticity with respect to shear deformation. Slip: the size and direction of the relative motion along a fault plane.

Stress: the force exerted per unit area, in Pascal (1 Pa = 1 N/m2_).

Igneous rock (extrusive or intrusive rock): the rock formed through the crystallization of magma. Strain partitioning: plate motion is not accommodated perpendicular to the plate contact, but the

relative motion is partitioned into two components, one perpendicular and one parallel to the contact.

Stratovolcano: a conical volcano built up of alternating layers of lava and tephra (ashes, pyroclastic

flow deposits and more). The volcano forms through explosive eruptions and is most commonly found along subduction zones.

Subduction: the process by which the ocean floor dives underneath a continent or island arc.

Tephra (pyroclastic deposit): the collective term for deposits of fragmented volcanic material that are

produced in an explosive volcanic eruption, such as ash, volcanic bombs and blocks and mud flows.

Tectonics: motion and deformation of (parts of) the Earth’s crust or lithosphere.

Thermal boundary layer: the lower mantle layer above the core-mantle boundary that is characterized

by a large contrast in temperature.

Tomographic map: map or image based on the spatial variation of seismic wave speeds in the mantle. Transform fault: a fault formed when two tectonic plates slide past each other.

Transverse wave (secondary wave, S-wave): a wave that oscillates in the direction perpendicular to

its propagation direction. Transverse waves arrive at seismic stations after the longitudinal body waves.

Tsunami: extremely long waves of water generated by vertical movement of the ocean floor.

Volatile: the constituent part of magma that can easily escape when it is in its gas phase (when the

magma rises or cools). For example, water (H2O), CO2, SO2, HCl and H2S.