Unit 1: History of Earth

Formation of the Earth

The Earth, along with the rest of our solar system, formed approximately 4.6 billion years ago. The most widely accepted theory is the Nebular Hypothesis.

1. Solar Nebula: A vast, spinning cloud of gas and dust (mostly hydrogen, helium, and heavier elements from older supernovas) existed in space.
2. Gravitational Collapse: Gravity caused this cloud to contract and spin faster, flattening it into a protoplanetary disk. The vast majority of the mass collapsed to the center, igniting to form our Sun.
3. Accretion: In the spinning disk, dust particles began to stick together (accretion). These clumps grew into larger bodies called planetesimals.
4. Proto-Earth: Through continued collisions and gravitational attraction, planetesimals clumped together to form a "proto-Earth." This process generated immense heat, causing the entire planet to be in a molten (liquid) state.
5. Differentiation: While molten, heavier elements (like iron and nickel) sank to the center due to gravity, forming the Core. Lighter elements (like silicon and oxygen) floated to the surface, forming the Mantle and Crust. This separation process is called planetary differentiation.

Formation and Composition of Earth's Layers

As a result of differentiation, the Earth is a layered planet. These layers are defined by their chemical composition and physical properties.

Core

• Location: The center of the Earth.
• Outer Core:
  • State: Liquid
  • Composition: Primarily liquid iron (Fe) and nickel (Ni).
  • Function: The convection (flow) of this liquid metal generates the Earth's magnetic field, which protects us from harmful solar wind.
• Inner Core:
  • State: Solid (despite being hotter than the outer core, it is solid due to immense pressure).
  • Composition: Primarily solid iron (Fe) and nickel (Ni).

Mantle

• Location: The thick layer (about 2900 km) between the core and the crust.
• Composition: Makes up ~84% of Earth's volume. Composed of hot, dense, iron- and magnesium-rich silicate rocks (e.g., olivine, pyroxene).
• Sub-layers:
  • Lithosphere (Rigid): The crust AND the uppermost rigid part of the mantle. This is what makes up the "tectonic plates."
  • Asthenosphere (Plastic): A semi-molten, "plastic-like" layer just below the lithosphere. It flows very slowly, allowing the rigid tectonic plates above it to move.

Crust

• Location: The outermost, thin, rocky "skin" of the Earth.
• Continental Crust:
  • Thickness: Thicker (30-70 km).
  • Composition: Less dense, "granitic" (rich in silicon and aluminum).
  • Example: The landmasses we live on.
• Oceanic Crust:
  • Thickness: Thinner (5-10 km).
  • Composition: Denser, "basaltic" (rich in silicon, iron, and magnesium).
  • Example: The crust beneath the oceans.

Atmosphere

The atmosphere formed in stages:

1. Primordial Atmosphere: Composed of Hydrogen (H) and Helium (He) from the solar nebula. This was quickly lost to space.
2. Secondary Atmosphere: Formed by volcanic outgassing. Volcanoes released gases like water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), and nitrogen (N₂). There was virtually no oxygen.
3. Living Atmosphere (Present):
   • Cyanobacteria (blue-green algae) evolved and began photosynthesis, consuming CO₂ and releasing Oxygen (O₂).
   • This "Great Oxidation Event" (starting ~2.4 billion years ago) filled the atmosphere with oxygen, paving the way for complex, air-breathing life.
   • Present Composition: ~78% Nitrogen (N₂), 21% Oxygen (O₂), 0.9% Argon (Ar), and trace gases.

Hydrosphere

Definition: The hydrosphere includes all the water on Earth, in all its forms (liquid, solid, gas). This includes oceans, lakes, rivers, glaciers, groundwater, and water vapor.

Formation:

• Volcanic Outgassing: The primary source. The huge amounts of water vapor (H₂O) released by volcanoes during the secondary atmosphere stage condensed as the Earth cooled, forming clouds and falling as rain for millions of years to fill the ocean basins.
• Comets and Asteroids: A significant portion of Earth's water may also have been delivered by "icy" comets and asteroids that impacted the early Earth.

Introduction to Geological Time Scale

The Geological Time Scale (GTS) is a "calendar" that organizes Earth's 4.6-billion-year history into hierarchical divisions based on major geological and biological events (like mass extinctions).

Hierarchy of Geologic Time

The hierarchy, from largest to smallest, is:

Eon > Era > Period > Epoch

Major Divisions and Events

Earth's history is split into two major Eons:

1. Precambrian Eon: (Covers ~88% of Earth's history, from 4.6 billion to 541 million years ago). This includes the Hadean, Archean, and Proterozoic Eons.
   • Key Events: Formation of Earth, formation of oceans, first single-celled life (Archean), Great Oxidation Event (Proterozoic), first multi-celled organisms.
2. Phanerozoic Eon: ("Visible Life," from 541 million years ago to present). This is the Eon we are most familiar with. It is divided into three Eras:

Era	Nickname	Timeframe (Approx.)	Major Changes & Life Forms
Paleozoic	"Old Life"	541 - 252 Mya	"Cambrian Explosion" (rapid diversification of life). Dominated by marine invertebrates (trilobites), first fish, first land plants, amphibians, and insects. Ended with the Permian-Triassic Extinction ("The Great Dying").
Mesozoic	"Middle Life" (Age of Reptiles)	252 - 66 Mya	Dominated by dinosaurs. Appearance of first mammals, first birds, and flowering plants. Ended with the Cretaceous-Paleogene Extinction (asteroid impact) that killed the dinosaurs.
Cenozoic	"New Life" (Age of Mammals)	66 Mya - Present	With dinosaurs gone, mammals diversified and became the dominant large land animals. Appearance of grasses, primates, and eventually, humans.

Holocene and the Emergence of Humans

The Holocene Epoch

Definition: The Holocene is the current geological epoch. It began approximately 11,700 years ago, at the end of the last major ice age (the Pleistocene).

• Major Change: The Holocene is characterized by a relatively stable and warm climate, which allowed for the development of human civilization.
• Geological Process: Melting of continental ice sheets led to a significant rise in sea levels, shaping the coastlines we see today.

Emergence of Humans

Human evolution is part of the Cenozoic Era, mostly within the Neogene and Quaternary Periods.

• The genus Homo (our genus) first appeared in the Pleistocene Epoch (the "Ice Age").
• Anatomically modern humans, Homo sapiens, evolved in Africa around 300,000 years ago.
• The Holocene Epoch coincides with the entirety of recorded human history, the invention of agriculture, and the rise of cities and modern civilization.

Real-World Application (The "Anthropocene"): Many scientists propose that the Holocene has ended and we have entered a new, unofficial epoch called the Anthropocene. This is defined by the period where human activities (e.g., industrialization, CO2 emissions, plastic pollution, nuclear testing) have become the dominant influence on Earth's climate and environment.