2: Earth's origin and structure

2: Earth's Origin and Structure

Earth formed approximately 4.54 billion years ago through the process of accretion within the solar nebula. This vast cloud of gas and dust, left over from the formation of the Sun, began to coalesce due to gravity. Dust particles collided and stuck together, forming progressively larger bodies called planetesimals, which eventually merged to create the proto-Earth. A colossal impact with a Mars-sized body named Theia is theorized to have led to the formation of our Moon and caused the early Earth to melt entirely.

This early molten state allowed for planetary differentiation, a critical event where denser materials, primarily iron and nickel, sank to form the core, while lighter silicate minerals rose to form the mantle and crust. This process established Earth's foundational layered structure, which can be defined in two ways: by composition and by mechanical behavior.

Compositional Layers:

• Crust: The thin, solid, outermost layer. There are two types: continental crust (less dense, granitic) and oceanic crust (denser, basaltic).
• Mantle: The thickest layer, composed of solid but ductile ultramafic silicate rocks. It is divided into the upper and lower mantle.
• Core: Composed primarily of iron and nickel. The outer core is liquid and generates Earth's magnetic field through convection, while the inner core is solid due to immense pressure.

Mechanical Layers (Rheological):

• Lithosphere: The rigid outer shell, comprising the crust and the uppermost, brittle part of the mantle. It is broken into tectonic plates.
• Asthenosphere: A ductile, partially molten zone in the upper mantle over which the lithospheric plates move.
• Mesosphere: The stronger, more rigid lower mantle.
• Outer Core: Liquid.
• Inner Core: Solid.

This differentiated, layered structure is fundamental to all geologic processes, from volcanism and plate tectonics to the generation of Earth's protective magnetic field.