Earth's Internal Structure and Plate Tectonics

Study of Earth's Interior

Indirect Methods for Studying the Deep Earth

• Seismic Method: This involves analyzing the echoes of sound waves produced by a small surface explosion. These waves bounce off different layers within the Earth.
• Gravimetric Method: This method detects small variations in the gravitational field caused by the distribution of rock mass deep within the Earth. Light rocks create a negative gravity anomaly, while dense rocks create a positive one.
• Measurements of Isotopes: This allows scientists to determine the exact temperature of the water in which an organism lived.
• Radiometric Dating: This is used to determine the age of a rock sample.
• Study of Meteorites: Meteorites provide valuable insights into the composition of the early solar system and the Earth.

Seismic Method

The seismic method is an indirect way to detect the boundary between materials of different compositions or states, as they divert seismic waves. These areas are called seismic discontinuities.

Seismic Waves

When an earthquake occurs, two types of seismic waves propagate through the Earth's interior:

• P-waves: These travel at great speed and are the first to be detected by seismographs. They are longitudinal waves, like those transmitted along a spring that is compressed and stretched. P-waves are transmitted through solids and liquids. Their speed increases with the stiffness of the material they pass through.
• S-waves: These are slower than P-waves and are received later in seismographs. They are transverse waves, like those formed by shaking a rope. S-waves propagate perpendicular to the motion of particles. They are transmitted through solids but not liquids.

These alterations in the path of seismic waves produce shadow zones, places where either no waves are received or only P-waves or S-waves are detected.

Earth's Crust and Mantle

Crust

There are two types of crust:

• Oceanic Crust: Primarily composed of basalt. It is covered by a layer of sediment that can be thousands of kilometers thick near continents but may be very thin or absent towards the middle of the ocean.
• Continental Crust: Mainly composed of granite. It also contains sedimentary, metamorphic, and volcanic rocks, which in some places can be thousands of feet thick.

Granite has a density between 2600 and 2700 kg/m³, while basalt has a density between 2700 and 3200 kg/m³. The significant difference in density between the granitic crust and the underlying mantle prevents them from mixing. This difference in density and thickness between these two types of crust produces two important effects:

1. The basaltic crust is thinner and forms ocean basins, while the granite stands out more on the land surface and forms the continents.
2. The rocks in the granitic crust are very old.

Mantle

The mantle has a more homogeneous composition than the crust. Its main component is peridotite, a group of rocks whose primary minerals are olivine and pyroxene. Despite its high homogeneity, its density increases with depth. At a depth of about 670 km, the rock becomes denser. This abrupt change in density is the Repetti discontinuity, which separates the upper mantle from the lower mantle.

Layer D''

At the Gutenberg discontinuity, where the liquid iron core meets the rocky mantle, the temperature is about 3000°C. In this area, there is a layer between 100 and 400 km thick that forms the transition between the mantle and core: the D'' layer.

The mantle as a whole is agitated by a very slow convective motion called convection currents.

Lithosphere

The most superficial part of the upper mantle, together with the crust, forms a rigid layer called the lithosphere. This layer is fragmented into blocks called lithospheric plates. These can be:

• Oceanic: Formed by basaltic oceanic crust and a few miles of the upper mantle. They have a thickness of between 30 and 50 km.
• Continental: Formed by granitic continental crust and a portion of peridotite mantle. They can reach thicknesses of 70 to 150 km, with some areas reaching up to 300 km.

Earth's Core

It is estimated that the core consists of at least 80% iron and more than 10% nickel. The remaining mass, less than 10%, is likely composed of carbon, oxygen, and sulfur.

Core and Terrestrial Magnetism

The outer core is liquid and is over 3000°C. The base is about 1000°C warmer than the top. This large temperature difference, coupled with its fluidity, produces violent convection currents. These currents are responsible for Earth's magnetic field.

Inside the Thermal Machine of Earth

The thermal energy that the Earth holds in its interior is residual heat generated during its formation, mainly by three processes:

1. The intense meteorite bombardment during the accretion of the planet.
2. Gravitational differentiation by density, with the consequent formation of the nucleus. The fall of dense metallic materials inward and the ascent of rocky material forming the mantle and crust generated heat by friction.
3. The decay of radioactive elements, which heats the material bombarded by the generated subatomic particles.

Geothermal Gradient and Convection in the Mantle

The existence of a cold surface and a warm interior on Earth, i.e., the presence of a geothermal gradient, causes convection currents. These currents cause rocks from the deep mantle to reach the Earth's surface and produce volcanism.

When a substance melts and then crystallizes, it releases the latent heat of fusion into its environment. The crystallization of iron in the outer core produces large amounts of heat. Convection in the outer core transfers this heat to the base of the mantle.

At the Gutenberg discontinuity, the mantle and core do not mix easily because their densities and chemical compositions are very different. Mantle convection effectively evacuates heat to the surface.

Heat Input

The cooling of the Earth has been slowed by two processes that still provide heat to the system:

• The decay of radioactive elements: The spontaneous fission of unstable atoms of elements such as uranium, plutonium, etc., emits subatomic particles at high speeds. When these particles collide with other atoms, they increase the temperature.
• The crystallization of the metallic core: The molten iron in the outer core is crystallizing due to high pressure, and the solidified iron is decanted, gradually thickening the inner core. This crystallization process releases the latent heat of fusion of iron, which significantly retards core cooling.

Geothermal Gradient

This gradient is responsible for convection in the mantle, which in turn is related to:

• The recycling of basaltic crust.
• Volcanism.
• The movements of the continents.

Fixed Theories

These are explanations about the origin of reliefs that assume continents have always been in their current positions, suggesting not horizontal movements of the Earth's crust but only vertical movements of rising and sinking.

Investigation of the Ocean Floor: The Ridges

Spreading ridges are mountain ranges running through the ocean floor, with an altitude above the abyssal plains between 2000 and 3000 m. The ridges have several features:

• They are reliefs of volcanic origin.
• They are not covered with sediment.
• They exhibit a symmetrical paleomagnetic banding.
• The age of oceanic basalts increases with distance from the ridge.

Expansion of the Ocean Floor

In 1962, Harry Hess proposed the theory of seafloor spreading. According to this theory, ridges are fractures in the lithosphere through which mantle material escapes in the form of basaltic lava. As this lava solidifies, it forms a new oceanic crust that pushes outward on both sides, forcing the ocean to become wider and the continents to drift apart.

Subduction

Subduction is the sinking of an oceanic plate into the sublithospheric mantle. It begins when the end of an oceanic plate has thickened and cooled, becoming denser. Once subduction starts, it accelerates as the subducting lithosphere, subjected to increasing pressure, is compressed, and its density increases, accelerating the collapse.

Vertical Movement: Isostasy

The term"isostas" was introduced by geologist Dutton in 1892 to explain the vertical movements of subsidence and uplift of the crust. Dutton postulated that the crust floats on the mantle, which behaves like a very viscous liquid. The crust could sink when overloaded with weight. This is the isostatic equilibrium model, based on Archimedes' principle.

Horizontal Movements of Plates

• Divergent Movement: Two plates tend to separate. This occurs at constructive edges.
• Convergent Movement: Two plates tend to approach each other. This type of movement occurs at destructive edges.
• Shear Movement: This occurs at passive edges located in transform faults.

Activity at Plate Edges

This activity is manifested in:

• Volcanism: Both at plate boundaries and in areas near destructive edges and large fractures in the lithosphere.
• Seismicity: Earthquakes are caused by friction between plates, especially at edges with converging movements.
• Deformation of Rocks: In areas where there is convergence between plates.
• Metamorphism: Rocks affected by compression and increased temperature experience changes in their structure and composition.
• Magmatism: The melting of rocks to form magmas is linked to plate boundaries.
• Formation of Relief: In subduction zones, the overriding plate is under severe compression, which leads to shortening of its length and increased thickness.