Introduction to Environmental Processes Lecture 3 THE SOLID EARTH
Outline • Internal structure and composition • Earthquakes • Geothermal heat • Evolution during geological time
The Solid Earth • The internal structure of the Earth is known from: • the study of the travel times and paths of earthquake (seismic) waves, • the magnetic field, • also from the gross density and moment of inertia of the planet.
The Solid Earth • The Earth is thought to have a layered structure due to chemical and mineral segregation. • There are three principal layers: the crust, the mantle and the core. • The crust is different beneath the oceans and the continents:
The Solid Earth • OCEANIC CRUST is about 10 km thick and is composed of basaltic rocks (iron- and magnesium-rich slicates) erupted from mid-ocean ridge volcanos. • Beneath this crust is a depleted zone about 70km thick composed of peridotite, from which the basalt was formed. • These two layers together make up a LITHOSPHERIC PLATE.
The Solid Earth • CONTINENTAL CRUST is about 30 km thick and is largely composed of granitic rocks, plus various sediments and metamorphic rocks. • It was formed by secondary melting of the ocean crust when the latter was subducted back into the Earth’s interior. • Further rocks have been formed by erosion and metamorphism during continental collision.
The Solid Earth • The MANTLE is about 2000 km thick and is composed of iron and magnesium silicate minerals. • The increasing pressure and temperature cause phase changes in these minerals that are revealed as internal zonation in the mantle, distinguishable from seismic evidence. • In particular, the upper mantle is recognised as the source area for deep plumes of heated rock that erupt along lines of crustal weakness.
The Solid Earth • The CORE is a semi-liquid sphere of iron and nickel (both free and as their sulphides). • Electrical currents, produced at the core-mantle boundary and within the core, are thought to generate the magnetic field.
The Solid Earth • Earthquakes occur as a result of stick-slip motion at plate tectonic boundaries. • Several types of wave are generated: • P-Waves • S-Waves • Surface waves • These waves travel at different velocities and thus arrive at a distant point at differing times
The Solid Earth • P and S waves travel through the earth and thus provide information about its interior. • Their velocity depends on density, elasticity and temperature. Thus it varies with depth and so with the length of the travel path. • P-waves are faster than S-waves and so will arrive first at any point. The difference between the arrival times indicates the distance to the earthquake source.
The Solid Earth • Using the time difference, the distance from a receiving station to the source can be plotted. • Observations of the same earthquake at three different receivers thus enable the position of the source (strictly its epicentre) to be determined.
The Solid Earth • If the arrivals from a given event are recorded at receivers around the globe, it is possible to deduce the travel path of the wave through the earth. • This has revealed the major layers into which the earth is divided and also provides information on their physical properties. • This information helps to determine their composition.
The Solid Earth • The Earth has a source of internal energy as evidenced by an outward flow of heat beneath every part of the globe, together with local heat outflows due to volcanic processes.
The Solid Earth • The Earth’s internal energy arises from radio-active decay, mainly the uranium and potassium series. • These are found in granite. This is a crustal rock and so most heat production is in the crust. • Hence the interior of the Earth is effectively heated from the outside inwards.
The Solid Earth • The interior temperature rises until it is balanced by the outward heat flow along the geothermal gradient. • The equilibrium temperature at the centre is around 6000ºC.
The Solid Earth • The Earth’s internal heat thus is not the residual heat from its formation. It is likely that the planet formed by cold accretion of planetoids and then became hotter over time. • Although some early heat would have been generated by the accretionary impacts, and by the later gravitational segregation of the iron core, this is not sufficient to account for the internal heat flow that persists at the present day.
The Solid Earth • The Earth has evolved during geological time by the processes of bombardment, segregation, outgassing and plate tectonics. • The early Earth suffered a bombardment phase (4,500 – 4,000 Myr ago) during which the debris of the solar system was swept out of its orbit. At about the same time, increasing internal heat caused the iron core to segregate from the mantle.
The Solid Earth • Internal heat caused local melting of the mantle and the segregation of a basaltic crust. Lateral movement and subduction of this crust allowed the formation of a granitic crust that accreted into continental blocks.
The Solid Earth • Rocks as old as 3,500 Myr show evidence of this process. • Continental accretion has probably continued throughout geological time by the addition of secondary andesitic or granitic rocks formed at compressive plate margins. • These added units can sometimes be recognised as so-called exotic terranes.
The Solid Earth • The early volcanism was accompanied by the outgassing of volatiles, mainly nitrogen, methane, water and carbon dioxide, to form the initial atmosphere and oceans. • Thermal dissociation of water vapour, and the photosynthetic reduction of carbon dioxide, gradually produced an oxygen atmosphere during the period from about 3,000 Myrs to 1,000 Myrs ago.
Summary • Internal structure and composition • Earthquakes • Geothermal heat • Evolution during geological time
Introduction to Environmental Processes Lecture 4 PLATE TECTONICS
Plate TectonicsSummary of Lecture 4 • Basic concepts • Plate motions and boundaries • Evidence for plate tectonics • Plate history of the Atlantic Ocean
Plate Tectonics • The lithosphere is the name given to the outermost 100 km to 150 km of the earth. • The lithosphere acts as a series of rigid plates that move relative to one another across the asthenosphere, which forms a plastic layer beneath them.