Introduction to Physical Geology

Geology is the science that pursues an understanding of planet Earth

• Geology involves both outdoor fieldwork and laboratory work

Physical geology examines materials composing Earth and seeks to understand the many processes that operate beneath and on the surface of our planet

Historical geology seeks an understanding of the origin of Earth and its development through time

Geology, People, and the Environment

Natural hazards are part of living on Earth

• Geologists study volcanoes, floods, tsunamis, earthquakes, and landslides.
• These are natural processes but become hazards when they occur where people live.
• World population growth increases the risk of death and damage from significant geologic hazards.

Global climate change

• Climate is naturally variable
• Human activities influence changes globally

Resources are another important part of geology

• Include water, soil, metallic and nonmetallic minerals, and energy

• The nature of Earth has been a focus of study for centuries
  • Mid 1600s – James Ussher
    • Catastrophism
    • Earth’s landscapes shaped primarily by catastrophes
  • 1795 – James Hutton
    • Uniformitarianism
    • The physical, chemical, and biologic laws that operate today have operated throughout the geologic past
    • The present is the key to the past

• Geology Today
  • Uniformitarianism still valid
    • Some geologic processes are not directly observable but have well-established evidence to suggest they occur.
  • Earth processes vary in intensity but still take a very long time to create or destroy major landscape features.
    • The magnitude of geologic time involves millions and billions of years
    • Earth is 4.6 billion years old

The Nature of Scientific Inquiry

• Science is a process of producing knowledge
  • Based on making observations and developing explanations
  • Assumes the natural world behaves consistently and predictably
• The goal of science is to discover patterns in nature and use that knowledge to make predictions.
• Data are essential to science and the development of scientific theories.

• How or why things happen are explained using:
  • Hypothesis – a tentative (or untested) explanation
    • A hypothesis must fit observations and be testable
  • Theory – a well-tested and widely accepted view that the scientific community agrees best explains certain observable facts
  • Scientific Method – the process by which researchers raise questions, gather data, and formulate and test scientific hypotheses

There is no fixed path that scientists follow that leads to scientific knowledge

Earth as a System

• Earth is a dynamic body with many separate, but interacting, parts:
  • Hydrosphere – global ocean and fresh water
  • Atmosphere – gaseous envelope
  • Geosphere – the solid Earth
  • Biosphere – all plant and animal life

• A system is a group of interacting parts that form a complex whole.
• Earth system science:
  • Aims to study Earth as a system composed of numerous interacting parts
  • Employs an interdisciplinary approach to solve global environmental problems
• Processes that characterize the Earth system vary on spatial and temporal scales and are powered by energy from the Sun and heat from the Earth’s interior.

• Origin of our Solar System
  • The universe began with the Big Bang.
  • The components of the solar system formed at essentially the same time out of the same material.
  • The nebular theory proposes that the bodies of our solar system evolved from an enormous rotating cloud called the solar nebula.

Origin and Early Evolution of the Earth

Nebular Theory

• The solar nebula consisted of hydrogen, helium, and microscopic dust grains.
• Something (perhaps a supernova explosion) causes the nebula to collapse under its own gravitation.
• During collapse the nebula evolves from a huge rotating cloud to a smaller, faster-spinning disc.
• Through collisions and other interactions, gases and particles begin to orbit in one plane, with most of the matter condensing into the center where the protosun (pre-Sun) forms.
• Inner planets began to form from metallic and rocky substances.
• Larger outer planets began forming from fragments of ices (H₂O, CO₂, and others).

• As material accumulated forming early Earth, temperature was high enough for iron and nickel to melt.
• Formation of Earth’s layered structure
  • Metals sank to the center
• Chemical differentiation
  • Molten rock rose to produce a primitive crust
  • Established the three basic divisions of Earth’s interior: core, mantle, and crust
• A primitive atmosphere evolved from volcanic gases

Earth’s Internal Structure

• Earth is divided into three major layers by composition:
  • Crust – Earth’s thin, rocky outer skin, divided into the continental and oceanic crust
    • Oceanic crust is approximately 7 kilometers thick and composed of basalt.
    • Continental crust is 35–70 kilometers thick and composed primarily of granodiorite
  • Mantle – approximately 2900 kilometers thick and composed of peridotite
  • Core – composed of an iron-nickel alloy

• Earth’s interior is divided into different zones based on physical properties:
  • Lithosphere – (rock sphere) the rigid outer layer of Earth that consists of the crust and the upper mantle
  • Asthenosphere – (weak sphere) the soft, weak layer below the lithosphere
  • Transition zone – a zone marked by a sharp increase in density below the asthenosphere
  • Lower Mantle – a zone of strong, very hot rocks subjected to gradual flow below the transition zone
  • Outer core – liquid outer layer of the core
  • Inner core – solid inner layer of the core

• Rocks usually consists of smaller crystals called minerals:
  • The minerals that compose a rock strongly influence its nature and appearance.
  • A rocks texture (size, shape, and/or arrangement of its constituent minerals) also has an effect on appearance.

• Rocks are divided into three major groups:
  • Igneous, sedimentary, and metamorphic
• The rock cycle
  • Allows us to visualize the interrelationships among different parts of the Earth system.
  • Helps us understand the origin of igneous, sedimentary, and metamorphic rocks and see that each type is linked to the others by Earth processes

The Rocks and the Rock Cycle

• Igneous rocks
  • Cooling and solidification of molten rock
• Sedimentary rocks
  • Sediments are derived from weathering of preexisting rocks
  • Sediments undergo lithification which means “conversion into rock”. Sediment is usually compacted and/or cemented together
  • Accumulate in layers at Earth’s surface
• Metamorphic rocks
  • Formed by “changing” preexisting igneous, sedimentary, or other metamorphic rocks
  • Driving forces are heat and pressure

The Face of Earth

• Earth’s surface is divided into ocean basins and continents.
  • Their elevation difference is a result of differences in their relative density and thickness.
• Ocean basins
  • Average depth is 3.8 kilometer below sea level
  • Composed of approximately 7 kilometer thick basaltic rocks
• Continents
  • Relatively flat plateaus average 0.8 kilometer above sea level
  • Composed of granitic rocks, average 35 kilometer thick

• Features of the ocean floor include continental margins, deep-ocean basins, and oceanic ridges.
  • Continental margins are the portion of the seafloor adjacent to major landmasses.
    • The continental shelf is a gently sloping region of continental crust extending from the shore.
    • The continental slope is a relatively steep dropoff that extends from the continental shelf to the deep ocean floor.
    • The continental rise consists of a thick wedge of sediment that moved downward from the continental shelf and slope to accumulate on the sea-floor.
  • Deep ocean basins are the portions of the seafloor between the continental margins and the oceanic ridges.
    • The abyssal plain is a flat feature of the deep ocean basin.
    • Deep-ocean trenches are deep and relatively narrow depressions that make up only a small portion of the ocean floor.
    • Seamounts are small volcanic structures that dot the ocean floor.
  • Oceanic ridges are the most prominent feature on the ocean floor and are composed of igneous rock that has been fractured and uplifted.

• Features of continents include mountain belts, cratons, shields, and stable platforms.
  • Mountain belts are the most prominent features of continents.
  • Stable interiors of continents, called cratons
  • Shields are expansive, flat regions of deformed crystalline rocks within cratons.
  • Stable platforms are the flat portions of cratons covered with a thin veneer of sedimentary rocks.