Geology 100 Introduction to Geology Telecourse
INTRODUCTORY LECTURE, Part 1
Opening Comments Why Geology? Important Concepts in Physical Geology Theory of Plate Tectonics Plates are in Motion Types of Convergent Boundaries Geologic Time

Opening Comments

In this lecture I pulled out some of the major concepts from the text that you should understand. Read it before you start your text assignments. Don't worry if at first reading the material seems complex; as you proceed during the semester with your text and videolesson assignments, you will understand more and more. Refer back to this lecture several times during the semester to assist you in summarizing some of the most important course material. This lecture and the final review are excellent study aids for your final exam.

Why Geology?

• Geology makes the environment more interesting
• Geology uses scientific method to explain the changes that take place in and on the earth

Who Needs Geology?

• Geology helps us to avoid geologic hazards such as:
  • Earthquakes
  • Volcanoes
  • Landslides
  • Floods
• Geology helps supply the things we need:
  • Energy
  • Metals
  • Other: gems, building materials and renewable resources
• Geology protects the environment
• Geology helps us understand our surroundings
  • Appreciating scenery while traveling

Geology is based on Scientific Method

• Question raised or problem presented
• Data gathered
• Hypotheses proposed
• Predictions made
• Predictions tested
• Only a hypothesis that withstands testing becomes a theory

Important Concepts in Physical Geology

Overview of Important Concepts: Heat Engines

• The earth has an internal and external heat engine
• Both convert heat energy into mechanical energy (movement)
• Both work through the process of convection

Pinwheel Heat Engine Diagram: This diagram shows the conversion of heat energy into mechanical energy. That is, heat energy can be used to cause movement. The heat under the teapot expands the water creating steam that escapes from the pot. If you place a pinwheel over the escaping steam, the wheel will spin.

Lava Lamp Heat Engine Diagram: Another way to envision the Earth's heat engines is a lava lamp. Once again heat causes motion. The warm red material rises away from the heat source. It then starts to cool and becomes heavier. This causes it to sink back towards the heat source and the cycle repeats itself.

Earth's Two Heat Engines

• Internal Heat Engine--Internal Processes
  Hot rock from earth's interior flows upward building the Earth's mountains
• External Heat Engine--External Processes
  Weather processes erode the surface of the Earth
• These processes balance each other through equilibrium

Internal Heat Engine

• Powered by the heat of the earth's interior causing material within the mantle to rise towards the earth's surface or crust.
• The movement of the crust and earthquakes are a result of this convection.

Tectonic Forces Diagram: This diagram shows the internal heat engine of the Earth. Material in the mantle is heated by the radioactive core and floats to the surface next to the crust. It then cools and moves back down towards the core. This continual movement (called convection currents) causes the crust to move in plate tectonics.

External Heat Engine

• Driven by solar power & gravity
• Weather patterns, hydrologic cycle & ocean currents
• Erosion--due to water, ice, wind, gravity
• Rock formed at high temperature becomes unstable at surface
  • Form new material stable under conditions at earth's surface
  • equilibrium
• Sediments are formed they can solidify (lithify) into sedimentary rock

Theory of Plate Tectonics

• Accepted because it accounts for many seemingly unrelated geological phenomena.
• Plate tectonics regards the surface of the earth or lithosphere, as being broken into plates that are in motion.
• Plate boundaries are where two plates are pulling away from each other, sliding past each other, or moving toward each other.

The World's Plates Diagram: This diagram shows some of the major plates of the Earth. It demonstrates how the plates move away from each other at divergent boundaries, such as the Mid-Atlantic Ridge. How they move toward each other at convergent boundaries, like where the Nazca Plate is colliding with the South American Plate to create the Andes Mountains. And how the plates slip past each other creating the San Andreas fault at the juncture of the North American and Pacific Plates.

Plates are in Motion

Divergent Boundaries

• Mid-oceanic ridges
• Magma enters fissures
• Lithosphere moves away from boundary
• New ocean crust is created

Divergent Boundary Diagram: This diagram shows the divergent boundary at the mid-oceanic ridge. Heated magma rises by convection beneath the crust. The pressure causes the crust to crack and the magma fills the crack/fissure. More magma pushes the crust away from the fissure. New crust is formed at the divergent plate boundary as the magma lithifies into rock and pushes the older crust away from the fissure.

Transform Plate Boundary

• Plates slip past each other
• San Andreas Fault is a famous transform boundary

Motion Along San Andreas Fault Diagram: This diagram shows a famous transform fault -- the San Andreas. It the best known geological feature on Earth, because it has been so well studied. The Pacific Plate is moving on a north-west course while the North American Plate is moving to the south-east, creating this fault which runs most of the length of California. Note the sea floor spreading causing the gap which we know as the Gulf of California and an additional Spreading Axis to the north.

Convergent Boundaries

• Subduction Zones are caused when ocean plates slide under each other or continental plates
• This subduction created magma at depth which moves upward, pushing up the land above it.
• This magma/lava solidifies into intrusive/extrusive igneous rock.
• Heat from the magma can change the rock around it. Rock that recrystallizes without melting becomes metamorphic rock.

Types of Convergent Boundaries

• Ocean-Ocean Convergence: Older, denser oceanic plate subducts if plate melts creates island arc. The Philippines Islands are an example.
   

  Ocean-Ocean Convergence Diagram: The diagram shows two pieces of ocean crust colliding. The older, denser crust/plate will sink or subduct under the younger lighter crust/plate. If the plate subducts far enough into the mantle it will melt. The resulting magma is buoyant and floats toward the surface, pushing up the crust above it. When the magma finds a weakness in the crust above it, it will flow out in the form of lava. This pushing up of crust and lava flow creates a Volcanic Island Arc.

  
  
• Ocean-Continent Convergence: Denser oceanic crust subducts. Oceanic crust is always denser than continental crust, so it is always the oceanic crust that subducts in ocean-continent convergence. If the plate subducts far enough it will melt and young continental mountains are created. The Andes Mountains in South America are an example.
   

  Convergent Plate Boundary Diagram: This diagram illustrates a convergent plate boundary where oceanic crust collides with continental crust. The oceanic crust is denser than continental crust so it always subducts under the continental crust. The oceanic crust has subducted far enough to be heated by the mantle and melt. The magma is buoyant and rises, pushing up the crust above it. This creates a young mountain belt along the coast of the continent which parallels the subduction zone offshore. If the magma reaches the surface, an active volcano is formed.

  
  
• Continent-Continent Convergence: Older, denser plate sinks a small ways under, but doesn't subduct. The continental crust is too buoyant to subduct. Land crumples into mountains.
   

  Convergent Boundaries Diagram: This diagram shows that usually when two continents come together there was once an ocean between them. The oceanic crust subducts. But when the oceanic crust runs out, the continents slam together. The older continental crust will slip a small ways under the younger continental crust, creating a suture zone, but subduction stops. As the two continents press against each other there is no where to go but up and mountains are formed in the middle of the new land mass. (It is like pushing on opposite ends of a table cloth and watching wrinkles form in the middle) The Himalayas in Northern India are an example.

Geologic Time

• James Hutton felt that earth processes of the past were the same as today
• Principle of uniformitarianism: "the present is the key to the past"
• Earth is around 4.5 billion years old
• Major subdivisions of geologic time
  • Cenozoic Era -- 65 million years (age of mammals)
  • Mesozoic Era -- 245 million years (age of dinosaurs)
  • Paleozoic Era -- 545 million years (age of fishes)
    Oldest abundant fossils
  • PRECAMBRIAN -- all time before Paleozoic, an eon of time!

Geological Time Scale Diagram: The diagram above shows the Geologic Time scale broken down into: Precambrian Time (an Eon, since it is comprised of three Eras) lasting from the formation of Earth approximately 4.5 billion years ago until the first abundant fossils appeared 545 million years ago. Paleozoic Era and its 7 periods -- 545-245 million years ago when the primary Earth inhabitants were fishes. Mesozoic Era and its 3 periods -- 245-66 million years ago when the primary Earth inhabitants were reptiles. Cenozoic Era and its two periods -- 66 million years ago until present when the primary Earth inhabitants are mammals.

Go to Introductory Lecture, Part 2

E-mail your instructor: John McNamara
Copyright © 2003 by Debbie Secord. All rights reserved.

| Top | Back | Home | Schedule |
| Distance Learning | Virtual Library | Coastline Home Page |