Earth’s Formation, Interior, and Plate Tectonics
Earth’s Formation and Early Characteristics
Formation of Earth and Solar System (4.6 Billion Years Ago)
The Earth and solar system originated from a nebula, a swirling cloud of gas and dust. As the nebula spun faster, hydrogen concentrated at the center, forming the Sun.
The accretion theory proposes that remaining nebula materials formed planetesimals, varied rocks floating in space. Collisions between planetesimals created a molten mass of rocks, the primitive Earth.
As the molten Earth rotated, the outer layer cooled, forming a layered structure with denser rocks at the core and less dense rocks towards the surface.
Key Characteristics of Earth
- Shape: Earth is not perfectly spherical, but rather an oblate spheroid, wider at the equator and flattened at the poles. The polar radius is 6,371 km and the equatorial radius is 6,378 km. This information allows for calculations of Earth’s volume, average radius, mass, and circumference.
- Movement: Earth rotates on an inclined axis of approximately 23 degrees. This rotation is responsible for day and night. Earth also revolves around the Sun, and this translational motion, combined with the axial tilt, causes the seasons.
- Structure: Earth is a layered planet with a surface largely covered by water and an outer layer of protective gases. It is the third planet from the Sun, approximately 150 million km away.
Studying Earth’s Interior
Direct Methods
Direct methods involve analyzing samples. Drilling has revealed that surface rocks have an average density of 2.7g/cm³, suggesting denser materials within Earth’s interior.
Volcanoes provide direct insights into the composition of interior rocks brought to the surface. Meteorites, remnants from Earth’s early formation, also offer clues about the planet’s composition.
Indirect Methods
Indirect methods, such as gravimetric and magnetic studies, do not involve direct sampling. Gravimetric studies reveal gravitational anomalies, indicating Earth’s heterogeneous composition.
Terrestrial magnetism, measured by compasses, has shifted throughout history, suggesting anomalies and a solid iron core immersed in a molten metal layer.
Seismology
Seismology, the study of earthquakes, uses seismic waves to probe Earth’s interior. These waves originate from the hypocenter (the earthquake’s origin) and travel through Earth, their speed and direction changing based on the materials they traverse.
Seismographs record seismic waves, which are then analyzed by seismologists. Discontinuities, where seismic waves change speed and direction, reveal boundaries within Earth:
- Mohorovičić Discontinuity (Moho): Boundary between the crust and mantle at approximately 30 km depth, marked by an increase in wave speed.
- Repetti Discontinuity: Located at approximately 670 km depth, also marked by an increase in wave speed.
- Gutenberg Discontinuity: Located at approximately 2,900 km depth, where S-waves disappear and P-waves slow down.
- Lehmann Discontinuity: Located at approximately 5,100 km depth, where P-waves accelerate again.
There are two main types of seismic waves:
- P-waves (Primary waves): Longitudinal waves that travel quickly through both solids and liquids.
- S-waves (Secondary waves): Transverse waves that travel slower and only through solids.
Models of Earth’s Interior
Geochemical Model
- Crust (0-30 km): Abundant in minerals like granite, silicates (quartz, feldspar, mica), aluminum, and oxygen.
- Upper Mantle (30-670 km): Denser silicate rocks.
- Lower Mantle (670-2,900 km): Peridotite, silicon, magnesium, and less oxygen.
- Outer Core (2,900-5,100 km): Liquid iron and nickel.
- Inner Core (5,100-6,373 km): Solid iron and nickel.
Geodynamic Model
- Lithosphere (0-150 km): Includes the crust and upper mantle. Cold, brittle, and broken into plates.
- Asthenosphere (150-700 km): Soft, plastic layer where temperature increases sharply. Contains radioactive elements.
- Mesosphere (700-2,900 km): Solid, rigid layer with high pressure and temperature.
- D” Layer: Boundary between the mesosphere and outer core where rocks begin to melt.
- Outer Core (2,900-5,100 km): Liquid iron and nickel, losing heat through convection.
- Inner Core (5,100-6,373 km): Solid iron and nickel due to immense pressure.
This model explains Earth’s slow cooling through radiation and convection, driving plate tectonics and geological events like earthquakes and volcanoes.
Earth’s Crust
Continental Crust
Very old, with rocks up to 3.9 billion years old. Average density of 2.7g/cm³. Composed of sedimentary rocks overlying granitic rocks, with metamorphic rocks at the deepest levels. Horizontally, it consists of cratons (old, stable rocks) and orogens (younger, unstable mountain ranges). The continental shelf, submerged up to 200m, marks the crust’s edge.
Oceanic Crust
Thin and young (less than 200 million years old) with a density greater than 3g/cm³. Primarily composed of basaltic rocks. The continental slope marks a sharp drop to the ocean floor, featuring abyssal plains, mid-ocean ridges, and ocean trenches.
Theory of Plate Tectonics
The lithosphere is broken into tectonic plates that move due to convection currents in the asthenosphere. Plates can be macroplacas, microplates, or mixed plates (containing both continental and oceanic crust).
Plate Boundaries
- Divergent Boundaries: Create new lithosphere at mid-ocean ridges, causing seafloor spreading.
- Convergent Boundaries: Destroy lithosphere at ocean trenches, leading to subduction (one plate moving under another) or obduction (continental collision).
- Transform Boundaries: Plates slide past each other, creating transform faults.
Continental Drift and the Wilson Cycle
Alfred Wegener’s Continental Drift
Alfred Wegener proposed that continents were once joined as Pangaea and have since drifted apart. He presented paleontological (fossil distribution), geographical (continental fit), paleoclimatic (glacial remnants), and geological (matching rock formations) evidence.
Wilson Cycle
(Designs of Page 168 – 169)
– This cycle explains the theory of plate tectonics:
1.Se originates in the continental crust, which is very cold and brittle. Upon receipt of the interior heat loss, the crust bulges until it breaks and collapses forming intracontinental rift, filled with lakes.
2.The lakes reach a sufficient density to form a small sea that comes from the union of all lakes in the rift. In such areas, is forming a new divergent boundary.
3.Es true oceanic phase of stable margins. This area is full of sediment that is collected daily and a lot at the foot of the slope.
4.If this stage there is a subduction emergence of a new convergent boundary where the lithosphere is destroyed.
5.The oceans are being reduced significantly.
Finally 6.Por obduction occurs when the ocean disappears as both continental collision produces a new intercontinental range.