EESC 101 Lecture 2: Earth Systems and Internal Structure

Exam 1

Where did the heat that formed the early Earth come from?

1. heat from impacts on the forming Earth’s surface

   • much more prevalent early in Earth’s history, but there are still many (thousands) of small impacts each year

2. heat from differentiation of the Earth’s layers

   • mostly spent but density differences still drive Earth’s processes

3. radioactive materials

   • again, much stronger early on, but still creates the bulk of internal heat today

Radioactivity: Short Half Lives

Atoms with short half lives are not great for age dating rocks as they are too young

• H = 12yrs

• C = 20min to 5730yrs

  • Carbon dating is great for archeological purposes

• N = 10min

• Cs = 30yrs

• I = 13hrs to 15.7mil yrs

Radioactivity: Long Half Lives

Atoms with long half lives are great for age dating in geology

• U = 4.468billion yrs

• Th = 14.05billion yrs

• K = 1.248billion yrs

Layers of the Earth: Magnetosphere

• deflects solar winds

• caused by Earth’s outer core

• not unique to Earth, but causes may be different on other planets/moons

  • metallic hydrogen

Layers of the Earth: Atmosphere

• (78% N, 21% O)

• 1 atm (unit of pressure) is the amount of pressure exerted by the atmosphere at sea level

• all clouds form in the troposphere (lowest level)

  • all weather is in the troposphere

• planes cruse in low stratosphere

• the atmosphere’s boundary with space is gradual (700-10,000km)

• differences in temperature gradient due to balance of forces

• troposphere gets hot from Earth (radiation)

• stratosphere absorbs heat from solar radiation (UV rays)

Northern Lights

• aurora borealis/aurora australis caused by solar flares

• Interactions between atmosphere and magnetosphere

• charged particles deflected to poles by magnetic fields

• particles interact with molecules in the atmosphere and emit light

  • Oxygen: yellow/green

  • Nitrogen: red/violet

• occur in narrow bands of latitude

  • typically within 10-20 deg of the (magnetic) north and south poles

Layers of the Earth: Hydrosphere/Cryosphere

• covers 70% of the Earth’s surface

• part of what makes the Earth so unique in the solar system

• most freshwater is captured in glaciers

• last glacial maximum (~20,000 years ago)

• lots of ice=lower sea level

  • migration of land animals (ex. mammoth, horse, camel, people)

Layers of the Earth: Biosphere

• combination of all living organisms that occupy the Earth

• extends through the lower atmosphere…

  • kitting spiders found at 16,000 ft above sea level

• … to depths of ocean…

  • 6.6 miles below surface

• … and into the outermost layer of the Earth

  • ~1 mile underground

• plants and humans are the most obvious signs of the biosphere

Layers of the Earth: Geosphere

• includes the uneven land surface of the Earth and its inner layers

• when examining the Earth’s inner structure

  1. made up of meteorites

  2. study of xenoliths (alien-rocks)

  3. theoretical/experimental work simulations

  4. seismic waves

• deepest mine : ~2 miles

• deepest core : ~7.5 miles

• geosphere will include layers of the Earth

  • Crust (oceanic, continental)

  • Upper mantle

  • Lower Mantle

  • Core (inner/outer)

Crust

• upper part of Earth’s solid body

• oceanic crust:

  • 7-10km thick

  • very dense

  • Fe/Mg rich (mafic)

  • low viscosity (when melted)

  • darker in colour

  • density is ~3 g/cm³

• continental crust:

  • silica rich (felsic)

  • high viscosity (when melted)

  • lighter in colour

  • density is ~2.7 g/cm³

Mantle

• makes up the bulk of the Earth by volume

• stretches from the base of the crust to 2,900 km

• composed mostly of the rock peridotite

  • silicate minerals (SiO2) with lots of iron and minerals

• split into three parts

  1. upper (crust-660km)

  2. lower (660-2,900km)

  3. transition zone (400-660km)

• lower mantle is ductile

  • on long timescales it behaves like a fluid

  • flows at a rate of 15 cm/yr

    • three times faster than fingernails

Core

• composed primarily of iron, nickel, and other heavy elements

• core stats: 4,700 deg C and 3,600,000 atm

Alternate Layers

• crust/mantle/core layers defined by compositional differences

• another common say to divide Earth’s layers is by density

• crust/upper mantle = Geosphere

• lower mantle = asthenosphere