Plate Tectonics, Earthquakes, Weathering and Ecological Balance

• The Earth has been in constant change since it formed; the rocks of the crust do not stay in their original state.
• They are subjected from time to time to internal and external forces that make them take new positions and shapes.
• The forces released from inside the Earth cause earthquakes, the agitation of seas and oceans, the advance or retreat of their waters over the land, and the shifting and movement of the continents relative to one another.

The role of the lithosphere and the asthenosphere

• The lithosphere is made of the crust plus the rigid upper part of the mantle, and it is about 100 km thick.
• Below it lies the asthenosphere, about 350 km thick: a plastic rock material that behaves like a fluid under pressure and temperature.
• Convection-current eddies spread through the asthenosphere and help the continents float and move over it.

In 1912 the scientist Alfred Wegener assumed that the continents were once a single giant land mass called Pangaea ("the mother of continents"), which began to break apart, separate and drift until it settled in its present positions — more than 200 million years ago.

Wegener's evidence

• The fit of the continental margins: the east coast of South America matches the west coast of Africa.
• Paleomagnetism: tracing the direction of the magnetic needle preserved in ancient rocks.
• Paleoclimate: glacial deposits found in the south of several continents that are far apart today.
• Matching fossils and geological structure across the separated continents.

The question that remained

What force moves the continents? The answer came with the theory of plate tectonics.

Isaacs, Oliver and Sykes (1968) stated that the lithosphere is divided into major tectonic plates — continental, oceanic, or both together — which float on the plastic asthenosphere and move very slowly under the effect of the heat convection-current eddies.

Results of divergent movement

• Oceanic plate with oceanic plate → a mid-ocean ridge and new oceans, as in the Atlantic Ocean.
• Continental plate with continental plate → a continental rift, as in the rifting of the Red Sea, produced by the Arabian plate moving away from the African plate.

Results of convergent movement

• Continental with continental → huge fold mountain chains, as in the Himalayas.
• Continental with oceanic → the denser oceanic plate is subducted beneath the continental plate and melts in the mantle, forming volcanic mountain chains as in the Andes.
• Oceanic with oceanic → one plate sinks under the other, forming deep ocean trenches and volcanic island arcs.

Results of transform movement

Large deformations and transform faults accompanied by violent volcanoes and earthquakes. Examples: the San Andreas fault and the Gulf of Aqaba.

Relation to geological structures

These movements produce the folds and faults already studied: compression produces folds and reverse faults, while tension produces normal faults.

An earthquake is huge stored energy released suddenly from inside the Earth as elastic waves and vibrations that travel through the rocks and cause destruction in the crust.

Focus and epicentre

• The focus is the point inside the Earth from which the energy is released.
• The epicentre is the point on the surface lying vertically above the focus, where the effect is usually most severe.

Types of earthquakes

• Volcanic earthquakes: local tremors that accompany volcanic activity and do not extend far.
• Tectonic earthquakes: the most common and most destructive type; they occur in zones of rock fracturing and plate sliding along faults.
• Plutonic (deep-focus) earthquakes: their focus lies at great depth, sometimes more than 500 km below the surface.

Seismic waves are recorded at observing stations by the seismograph.

What the waves revealed about the Earth's interior

• The passage of P waves through the whole core with a change in their speed allowed scientists to identify a molten outer core and a solid inner core.
• The inability of S waves to cross the outer core proved that it is liquid (molten).
• Analysis of these waves divided the core into an outer core (about 2,100 km) and an inner core (radius about 1,386 km).

Earthquake intensity

The intensity of an earthquake and its destructive effect are measured on the Modified Mercalli scale.

Weathering is the breakdown and decomposition of rocks under the action of atmospheric agents at the surface. It is the first stage of the rock cycle: rocks crumble, then agents of transportation carry and deposit the debris to form sedimentary rocks. It is divided into mechanical and chemical weathering.

Mechanical weathering

• Breaking the rock into smaller pieces without changing its chemical or mineral composition.
• Freezing and thawing of water in rock cracks: blocks separate and talus (scree) slopes form.
• Thermal expansion caused by the difference in temperature between night and day in the desert.
• Unloading: by erosion, buried rocks rise upward, expand and peel off in sheets, as in granite.

Chemical weathering

• Decomposition and change of the mineral and chemical composition of rocks by water and air, producing new, weaker minerals suited to surface conditions.
• Oxidation: dissolved oxygen acts on iron-rich rocks such as basalt, so they rust.
• Carbonation: carbonic acid forms when CO₂ dissolves in rainwater; it dissolves limestone and forms chasms and caves.

Isostatic balance (Airy's theory): the Earth's crust is in a state of permanent balance even though its rocks differ in density and thickness. This appears in the oceanic crust (sima, higher density) and the continental crust (sial, lower density and greater thickness), which are in constant equilibrium.

Mountain roots

• Large mountain chains are made of light rock with an average density of about 2.7 g/cm³.
• They have deep roots in the denser mantle rocks reaching a depth of several times their height (about 4 times, so the total depth is about 5 times the visible height).
• A mountain resembles a floating iceberg: the greater part is submerged below.

How balance is restored during erosion

• Erosion breaks up the mountain peaks and carries the debris to settle in sedimentary basins.
• The load on the mountain decreases while the load on the basin increases, producing a difference in pressure.
• Plastic mantle material moves from beneath the high-pressure zone (the basin) toward beneath the low-pressure zone (the mountain), so the mountain rises and the balance is restored.
• This repeats over millions of years.

Why the balance matters for the environment

• Isostasy keeps the crust stable and limits changes in the levels of land and sea.
• Weathering, transportation and deposition constantly reshape the surface, while the balancing mechanisms preserve its overall stability.
• It joins the internal forces (tectonic movements and earthquakes) with the external forces (weathering and erosion) in one balanced cycle.