Earth’s Structure, Composition, and Geological Processes

Posted on Dec 11, 2024 in Geology

Direct and Indirect Methods of Studying the Earth

Direct methods of study are those that provide testable data on what is being investigated. On the other hand, indirect methods are applied to obtain information when material objects cannot be manipulated directly.

Indirect Methods

Seismic Method

The seismic method involves the study of earthquakes and seismic waves, which are vibrations of the Earth’s crust recorded as waves on a seismograph. The waves that can be recorded are:

  • P-waves: Primary or longitudinal waves are the first to reach seismographs and are therefore the fastest. They vibrate in the same direction as the wave propagation. These waves slow down when traversing fluids.
  • S-waves: Secondary or transverse waves are detected second on seismographs and are slower than P-waves. They vibrate perpendicular to the wave propagation and do not propagate in fluids.
  • L-waves: Long or surface waves are detected last and are the slowest, causing damage to the surface.

Only P and S-waves provide information about the Earth’s interior. By studying the velocity and behavior of P and S-waves through the Earth’s interior, areas with abrupt changes in speed were identified. These are called seismic discontinuities and are considered to separate Earth’s layers with different properties or compositions. The main discontinuities are:

  • Mohorovicic
  • Low-velocity zone
  • Gutenberg
  • Lehmann

Magnetic Method

The magnetic method studies and measures the magnetic field to detect possible anomalies or magnetic changes. The Earth’s magnetic field is similar to an electric dipole, with points called the North Pole and South Pole. Currently, the magnetic North Pole corresponds to the geographic South Pole and vice versa. This reverses periodically, a phenomenon called secular variation, which changes the orientation of magnetic minerals and is important for dating the Earth.

Gravimetric Method

The gravimetric method studies small variations in gravity due to changes in relief.

Other Sources of Information

  • Study of Densities: The Earth’s average density is 5.5 g/cm3, while the crust’s density is 2.8 g/cm3. This indicates a very dense core.
  • Study of Meteorites: Some meteorites have preserved traces of the origin of the solar system, and their composition should be similar to Earth’s. There are three types of meteorites:
    • Siderites: Composed of Fe and Ni (representing the core)
    • Siderolites: A mixture of siderite and lithometeorites (representing the mantle)
    • Lithometeorites: Composed of silicates (representing the crust)
  • Geothermal Studies: Based on heat production on the planet, there are two distinct areas: warmer areas at the mid-ocean ridges and colder areas in subduction zones.
  • Study of Material Ejected by Volcanoes: This allows us to determine the chemical composition of the deep crust and mantle.

Geologic Time

Time Divisions

The first divisions were based on:

  • Unconformities: Separating two sets of differently folded material.
  • Extinctions of Species: Some fossils are not found in rocks after a certain point in history.

Three main material groups were identified:

  • Primary Materials: Characterized by being highly folded and metamorphosed, containing fossils of ancient organisms. This time was called the Paleozoic Era.
  • Secondary Materials: Sedimentary rocks that were not as folded, with organisms very similar to those present today. This time was called the Mesozoic Era.
  • Tertiary Materials: Little or no folding, containing fossils of organisms very similar to those of today. This was called the Cenozoic Era.

The 4.5 billion-year history of the Earth is divided into four eons, each eon into eras, and eras into periods.

Fossils

The presence of fossils in strata is used as a criterion to correlate materials from different locations. To be considered an index fossil, a fossil should:

  • Be found in very remote areas (wide geographical distribution).
  • Have a short existence in geologic time.

Geologic Time Scale

Dating Rocks: Absolute and Relative

Absolute dating determines the specific age of a rock in millions of years.

Carbon-14 Dating

C-14 atoms are unstable and decay radioactively. When tissue dies and stops incorporating C-14 atoms, their number decreases by half every 5,730 years (the half-life of this element).

Relative Dating

Relative dating provides the order of geological materials or processes, sorting them chronologically. Several methods exist:

  • Superposition: In undisturbed sedimentary rocks, the layers on top are younger than those below.
  • Cross-cutting Relationships: Geologic processes that cut across strata are younger than the strata they affect.
  • Fossil Succession: When two sedimentary rocks contain fossils characteristic of the same geological period, the two rocks are of the same age.

Earth’s Internal Structure

Geochemical Composition

The Earth is divided into layers based on seismic discontinuities:

  • Continental Crust: The area corresponding to the Earth’s surface.
  • Oceanic Crust: Thinner than the continental crust, corresponding to the submerged area.
  • Mohorovicic Discontinuity: Occurs at a depth of 3 to 70 km.
  • Upper Mantle
  • Low-Velocity Zone: At a depth of 670 km.
  • Lower Mantle
  • Gutenberg Discontinuity: At 2,900 km depth.
  • Outer Core: Liquid.
  • Lehmann Discontinuity: At 5,150 km depth.
  • Inner Core: Solid.

Dynamic Composition

The Earth is divided into layers based on their dynamic behavior:

  • Lithosphere: A rigid layer formed by the crust and the uppermost part of the upper mantle, reaching a depth of 250 km.
  • Asthenosphere: Coincides with the upper mantle in some areas; it is a semi-viscous material due to the pressure and temperature conditions that allow the partial melting of rocks. Its existence is debated.
  • Mesosphere: Corresponds to the lower mantle.
  • D” Layer: A thin layer formed by molten mantle material.
  • Endosphere: Equivalent to the outer and inner core.

Structure and Composition of the Crust

Seismic wave velocities in the crust differ between inland and oceanic areas, leading to the distinction between oceanic and continental crust.

Oceanic Crust

Its age never exceeds 180 million years due to subduction at convergent plate boundaries. It has a layered structure:

  • Vertical Structure:
    • Surface layer of sediments: Thicker near continental margins and absent on the axes of mid-ocean ridges.
    • Layer of basalt and basaltic dikes.
    • Bottom layer of plutonic rocks (gabbros, pyroxenites).
  • Horizontal Structure: The difference between continental and oceanic crust is marked by emerged and submerged areas.
    1. Continental Shelf: Part of the continent covered by ocean water; shallow with a low gradient. It is of great economic value.
    2. Continental Slope: Connects the continental shelf to the ocean floor; has a steep inclination, often with deep valleys and canyons.
    3. Abyssal Trench: Narrow and deep depressions of the seafloor, close to continental margins or island arcs. They are seismically active.
    4. Abyssal Plain: Consists of oceanic crust; its surface is almost flat.
    5. Seafloor Elevations: Can be of various types, such as plateaus.
    6. Oceanic Ridges: Interconnected ridges with a central rift, traversed by transform faults. They are areas of intense seismic, volcanic, and tectonic activity.

Continental Crust

It is thicker than the oceanic crust but has a lower density because it is older. It is mainly composed of granite. Traditionally, three levels were recognized:

  • Level 1: Upper layer of sedimentary and volcanic rocks with granitic inclusions.
  • Level 2: Intermediate layer with plutonic and metamorphic rocks.
  • Level 3: Bottom layer of basic nature.

Horizontally, three types of structures are observed:

  • Shields: Tectonically stable areas, geographically smooth, without a sedimentary layer, and covered with endogenous rocks. They tend to be in the middle of continents.
  • Interior Platforms: Transition zones between shields and orogens. They have a layer of sedimentary rocks.
  • Orogens: Structures such as mountain ranges, thickened, and mostly close to convergent plate boundaries.

The interface between continents and oceans is called the transitional crust.

Mantle Structure and Composition

The mantle extends from the Mohorovicic discontinuity to the Gutenberg discontinuity. The low-velocity zone separates the upper mantle from the lower mantle. This discontinuity may be due to high pressures, which could also explain the lack of earthquakes in this zone. The upper mantle and the crust form the lithosphere. Below the low-velocity zone, the upper mantle is partially fluid and is called the asthenosphere, although its existence is currently debated. Below the upper mantle is the lower mantle, which is limited by the D” layer. The composition of the upper mantle is dominated by peridotite, with minerals like olivine and pyroxene.

Structure and Composition of Earth’s Core

The core is the innermost layer of the Earth, extending from the Gutenberg discontinuity to the Earth’s center. The Lehmann discontinuity separates the liquid outer core from the solid inner core. The calculated density for the core and the existence of a bipolar magnetic field suggest a metallic composition, primarily iron alloyed with nickel.

Structure and Composition of the Atmosphere

The atmosphere’s composition is relatively simple: 78% nitrogen, 21% oxygen, 0.93% argon, 0.035% carbon dioxide, and 0.035% water vapor, along with dust particles.

Two main atmospheric layers can be distinguished: the homosphere and the heterosphere.

The Homosphere

Extends up to 80 km altitude. Its composition is relatively constant. It consists of:

  • Troposphere: Up to 13 km in altitude. Most of the CO2 and water vapor are found here, making it the zone where weather phenomena occur.
  • Tropopause
  • Stratosphere: 50-60 km in altitude. The temperature remains low at the bottom but increases significantly in the ozone layer (due to the absorption of UV radiation).
  • Stratopause
  • Mesosphere: Up to 80 km. Characterized by low temperatures.

The mesopause separates the homosphere from the heterosphere.

The Heterosphere

Also called the ionosphere or thermosphere. It is characterized by high temperatures because gas molecules are ionized by high-energy solar radiation.

General Atmospheric Circulation

Solar radiation heats the Earth’s surface unevenly, with equatorial regions being warmer than polar regions. Warmer air is less dense and rises, while colder, denser polar air sinks and moves towards the equator, replacing the warm air. This creates a surface circulation of cold air from the poles to the equator, which, when heated, rises and flows towards the poles, where it cools, and the cycle begins again.

Due to the Earth’s rotation and the difference in speed at different latitudes, air masses are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is known as the Coriolis effect. As a result, there is a latitudinal distribution of high and low-pressure areas:

  • Subtropical Areas: High-pressure areas around 30 degrees latitude, generating trade winds.
  • Equatorial Zones: Low-pressure areas with westerly winds.
  • Subpolar Areas: High-pressure areas with polar easterly winds.

The Hydrosphere

The hydrosphere refers to all the water on the Earth’s surface.

Physical Properties

The key physical property of water is its density, approximately 1 g/cm3. Temperature significantly influences density, with ice being less dense than liquid water, and the maximum density occurring at 4°C.

Because the Earth’s average temperature is 15°C, it is close to the triple point of water, allowing it to exist in all three phases (solid, liquid, and gas).

Water is unevenly distributed on our planet. The vast majority is in the oceans (97%), with a smaller portion on the continents, mostly in glaciers and groundwater. Only a tiny fraction is available for human use in rivers, lakes, soils, living beings, and the atmosphere.

The Water Cycle

The movements and phase changes of water in the hydrosphere form a practically closed circuit called the water cycle or hydrologic cycle. The energy driving this cycle comes from the sun, which evaporates water from oceans and inland waters. Water vapor in the atmosphere condenses into clouds, which then cool, releasing liquid water or snow, depending on the degree of cooling. Rivers and glaciers form and eventually flow into the ocean. Some continental water can infiltrate the ground, forming groundwater, which also ultimately returns to the ocean.

Ocean Water

The oceans have acted as sinks for materials transported by rivers from the continents. Their salinity comes from ions contributed by rivers. Currently, mid-ocean ridges are also considered sources of soluble elements in the sea.

Ocean temperature varies with depth and latitude. The oceans consist of a thin layer of warm water (12-30°C) 200-500 m deep, situated above a large mass of cold water (-1 to 5°C). The boundary between warm and cold water is called the thermocline and is located between 200 and 1,000 m depth.

Water temperature varies with latitude, with warmer intertropical seas and colder seas elsewhere. Salinity is highest in the tropics due to less rainfall diluting the salts. The oceans act as huge thermal buffers and regulate the content of oxygen and CO2, playing a crucial role in modulating the biosphere and climate.

Circulation of Ocean Waters

  • Ocean Currents: Rivers within the ocean, primarily driven by differences in temperature and salinity, along with the Earth’s rotation and the effect of winds. This results in two types of currents: thermohaline (driven by salinity differences) and surface currents (driven by temperature differences).
  • Waves: Progressive oscillatory waves on the ocean surface, produced by wind or sudden movements of the seafloor.
  • Tides: Periodic movements of rising and falling sea levels caused by the differential gravitational attraction of the Moon and the Sun.