Earth History: Formation and Early Years

Earth's First 2 Billion Years

• Early Earth was hot due to the decay of radioactive elements within the planet and associated with Earth's formation processes.
• The heat comes from the thermal energy contained within the objects that formed Earth, as well as from collisions that caused compression and heating of materials.
• As Earth grew, its gravitational force became stronger, causing material compression and heating.

Chicxulub Asteroid Impact

• The asteroid impact occurred 65 million years ago at the edge of the Yucatan peninsula in Mexico, at the end of the Cretaceous Period.
• The asteroid was around 10-15 km in diameter and produced superheated plasma of over 10,000 degrees.

Heating and Differentiation of Earth

• As Earth heated up, silicate minerals and metals melted, leading to differences in density and the unmixing of Earth's layers.
• This process, called differentiation, resulted in the separation of silicate minerals and metals into a rocky outer layer and a metallic core.
• The movement of these materials also caused the Earth to heat up even more.

Formation of Earth's Layered Structure

• Chemical differentiation led to the formation of Earth's layers, including:
  • Iron-rich cores
  • Primitive crust
  • Mantle

Development of the Atmosphere

• Large quantities of gases escaped from Earth's interior, leading to the development of an atmosphere.

Evolution of Continents and Ocean Basins

• The primitive crust was lost to erosion and other geologic processes, leading to the evolution of continents and ocean basins.

Geologic Time Scale

• The geologic time scale is essential for interpreting Earth's history.
• There are two types of dates:
  • Numerical dates: specify the actual number of years that have passed since an event occurred.
  • Relative dates: cannot tell us how long ago something took place.

Principles of Relative Dating

• Principle of Superposition: in an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below.
• Principle of Original Horizontality: layers of sediment are generally deposited in a horizontal position.
• Principle of Lateral Continuity: sedimentary beds originate as continuous layers that extend in all directions until they eventually grade into different types of sediments or thin out.
• Principle of Cross-Cutting Relationship: geologic features that cut across rocks must form after the rocks they cut through.

Unconformities

• Unconformity: a long period where deposition ceased, erosion removed previously formed rocks, and then deposition resumed.
• There are three basic types of unconformities:
  • Angular unconformity: tilted/folded sedimentary rocks overlain by younger, flat-lying strata.
  • Disconformity: a gap in the rock record that represents a period of erosion rather than deposition.
  • Nonconformity: younger sedimentary strata overlie older metamorphic/or intrusive igneous rocks.

Geological Contacts

• Contacts and formations can be used to determine the geological history of an area.

Volcano Formation

• Volcanoes typically form in three settings: constructive plate boundaries, destructive plate boundaries, and hot spots.
• Volcanoes do not typically occur at transform boundaries due to limited magma availability.

Constructive Plate Boundary Volcanoes

• Form where tectonic plates are moving away from each other.
• The Earth's crust is pulled apart, creating a new pathway for rising hot magma to flow onto the surface.

Destructive Plate Boundary Volcanoes

• Form at subduction zones where tectonic plates are moving towards each other and one plate descends beneath the other.
• Example: ocean-continent subduction.

Hot Spots Volcanism

• Form above a mantle plume, or hot spot, where magma rises through the mantle and melts the crust above to form a volcano.
• Can occur in areas distant from plate boundaries, referred to as hot spot or intraplate volcanism.

Hot Spot Track

• A chain of volcanoes forms as a tectonic plate moves over a plume of hot mantle material rising from deep within the Earth.
• Examples: Hawaii-Emperor Hotspot Track and Volcanic Activity along Columbia Plateau – Yellowstone Hotspot Track.

Hawaiian Volcanoes

• Formed as a result of the Pacific plate moving over a hot spot.
• Characteristics: Hawaiian Island Attributes.

Continental Hot Spots

• Formed as a result of a tectonic plate riding over a deep-mantle hotspot.
• Example: Volcanic Activity along Columbia Plateau – Yellowstone Hotspot Track.
• Characteristics:
  • Initial surfacing of plume head forms basalts.
  • Magma mixes with melted continental crust, forming rhyolite lavas.