Lecture 1 - The Earth

Geology

• Study of the earth

  • Both interior and exterior surface

  • Rocks (and other materials)

  • Processes that formed the minerals (in rocks)

  • Water flowing over and underneath the surface

  • Changes that has happened and will happen in items mentioned above

• “ge” (Greek) - meaning earth, “logia” (Greek) - meaning study of

Engineering Geology

• Subfield of Geology

• Serves the art and science of engineering through description of the structures and attributes of the rocks connected with engineering works

• Also aims to identify and evaluate natural hazards that may affect the success of an engineering project

Why do we need to study the Earth?

• It is our “home” and understanding it is necessary to take care of it

• We rely on Earth for resources (soil, water, minerals, fuels, etc.)

• Studying rocks and fossils will help us understand the evolution of our living things along with the environment

• Understanding Earth means we can minimize the risk of hazards (earthquakes, landslides, volcanoes, etc.)

• Understand the changes that have happened

• Understand how humans have altered the environment and the course of the Earth

Earth’s Age

• James Hutton has this to say about the age of the Earth: “The result, therefore, of our present enquiry is, that we find no vestige of a beginning — no prospect of an end”

• The age of the Earth is VAST

• Around 4.57 billion yrs old

• Geologic Time is the time covered by Earth’s Geologic History (starting from ~4.6 billion years ago)

• We can visualize how vast geologic time is by “condensing” it to a unit time — some have done it by compressing it into an hour, some into a year. Afterwards, key events (like mass extinctions or arrival of humans) are shown when they would have happened in relation the chosen unit time.

Theories of Geological Evolution

• Uniformitarianism

  • The present reflects what happened in the past

  • Based on James Hutton’s work; Supported and spread by Charles Lyell

  • Proposes that uniform slow processes shaped the Earth over a very long period of time (soil transfer like erosion and deposition, wave actions, etc.)

  • The actions that shaped the Earth in the past are the same ones that can be observed in the present day.

• Catastrophism

  • The Earth was shaped by short-lived, forceful events (catastrophes)

  • Developed by Georges Curvier

  • Proposes that the Earth has been shaped by catastrophes (short, powerful events)

  • This was due to Cuvier’s observation of gaps in fossil record which he believed to be the mass extinction events

Earth Structure

• Earth can be divided either chemically (contents/composition of each layer) or mechanically (state of each layer)

  • Chemical:

    • Crust

    • Mantle

    • Core

  • Mechanical:

    • Lithosphere

    • Asthenosphere

    • Mesosphere

    • Outer Core

    • Inner Core

• Core (chemical) - composed mainly of iron

  • Outer Core (mechanical) - liquid

  • Inner Core (mechanical) - solid, even though hotter than the outer core, it remains solid due to the pressure it experiences

• Mantle (chemical) - composed of iron and magnesium silicate minerals

  • Mesosphere (mechanical) - a big part of the mantle is solid rock (starting outside the outer core)

  • Asthenosphere (mechanical) - material after the mesosphere flow gradually (plastic) until the partially molten layer

• Crust (chemical)

  • Mainly granite (continents) and basalt (beneath the oceans)

  • Rigid in nature

  • 95% igneous and metamorphic rock with the remaining being sedimentary rock

  • Lithosphere (mechanical) - rigid layer from the outermost mantle to crust, divided into ~20 tectonic plates

• The Earth, by volume, is:

  • Core: 16.1%

  • Mantle: 82.5%

  • Crust: 1.4%

• Geothermal Gradient - the way temperature increases as one goes deeper into the Earth

  • Temperature increase with depth: 0°C to ~7000°C form surface to the center of the core

  • The rate of increase is 30°C for every km while in the crust. For other layers/sublayers, the rate of increase is different.

Convection Cells

• As heat tends to transfer from an area of higher temperature to an area of lower temperature, the heat travels from the insides and towards the surface of the Earth

• This heat transfer creates convection cells in the mantle

• In places where convection cells flow:

  • Upward:

    • New Lithosphere forms (ocean ridges)

    • Plates diverge (move apart)

  • Downward:

    • One plate subducts into the mantle beneath the other

    • Plates converge