Fundamentals of Physical Geology: Structure, Systems, and Rock Cycle

Introduction to Geology Fundamentals

The Development of Geological Thought

The understanding of Earth's history evolved through competing theories:

• Mid-1600s – James Ussher: Catastrophism
  • Proposed that Earth’s landscapes were shaped primarily by sudden, short-lived catastrophes.
• 1795 – James Hutton: Uniformitarianism
  • States that the physical, chemical, and biological laws that operate today have operated throughout the geologic past.
  • Famous maxim: “The present is the key to the past.”

Energy Sources of the Earth System

The Earth system is powered by two main sources:

External Processes (Solar Energy)

The Sun drives external processes occurring in the:

• Atmosphere
• Hydrosphere
• At Earth’s surface

Internal Processes (Earth's Interior Heat)

The Earth system is also powered by the Earth’s interior. Heat remaining from the formation and heat continuously generated by radioactive decay power the internal processes that produce:

• Volcanoes
• Earthquakes
• Mountains

Earth’s Internal Structure

Layers Defined by Composition

Earth is divided into three major layers based on chemical composition:

• Crust: Earth’s thin, rocky outer skin, divided into continental and oceanic crust.
  • Oceanic crust: Approximately 7 kilometers thick, composed of basalt.
  • Continental crust: 35–70 kilometers thick, composed primarily of granodiorite.
• Mantle: Approximately 2,900 kilometers thick, composed of peridotite.
• Core: Composed of an iron-nickel alloy.

Zones Defined by Physical Properties

Earth’s interior is also divided into different zones based on physical properties (strength and rigidity):

• Lithosphere: The rigid outer layer of Earth that consists of the crust and the upper mantle.
• Asthenosphere: The soft, weak layer below the lithosphere.
• Transition Zone: A zone marked by a sharp increase in density below the asthenosphere.
• Lower Mantle (Mesosphere): A zone of strong, very hot rocks subjected to gradual flow, located below the transition zone.
• Outer Core: The liquid outer layer of the core.
• Inner Core: The solid inner layer of the core.

Rocks and the Rock Cycle

Rocks are divided into three major groups:

• Igneous
• Sedimentary
• Metamorphic

The Three Rock Types

• Igneous Rocks: Formed by the cooling and solidification of molten rock (magma or lava).
• Sedimentary Rocks:
  • Sediments are derived from the weathering of preexisting rocks.
  • Sediments will lithify (turn into rock) to form sedimentary rocks.
  • These rocks typically accumulate in layers at Earth’s surface.
• Metamorphic Rocks:
  • Formed by altering preexisting igneous, sedimentary, or other metamorphic rocks.
  • The primary driving forces for metamorphism are heat and pressure.

Earth's Surface Features and Topography

Earth’s surface is divided into major topographic units: ocean basins and continents. Their elevation difference is a result of differences in their relative density and thickness.

Features of the Ocean Floor

Key features of the ocean floor include:

• Continental margins
• Deep-ocean basins
• Oceanic ridges

Oceanic ridges are the most prominent feature on the ocean floor. They are composed of igneous rock that has been fractured and uplifted.

Features of the Continents

Key features of continents include:

• Mountain belts
• Cratons
• Shields
• Stable platforms

Mountain belts are the most prominent features of continents.