October 27, 2003

Earth as a Planet (Chapter 7)

  1. Global Characteristics
  2. Size
  3. Composition (iron-nickel, iron-sulfur, silicon and oxygen – metal and silicate rocks) - very different from the composition of the Sun
  4. Orbit around the Sun
  5. Temperature range (-50 to 50 deg Celsius, average about 10-15 deg Celsius)
  6. Mass
  7. Density
  1. Earth’s Interior – several distinct layers with different compositions
  2. Crust (the “skin” of the Earth), 0.3 % of the total mass of Earth, density – 3 g/cubic cm
  3. crust under the ocean (55%) – 6 km thick, formed of volcanic rocks (basalt – silicon, oxygen, iron, aluminum, magnesium)
  4. continental crust – (45%) – 20 to 70 km thick, formed of silicates (granite)
  5. Mantle – largest part of the solid Earth – down to a depth of 2900 km. Density, temperature and pressure (more or less solid); Volcanic activities – samples of the mantle
  6. Outer core (liquid) – dense metallic core (iron, nickel, sulfur), diameter 7000 km
  7. Inner core (solid) – diameter 2400 km
  8. Differentiation – separation of the Earth into layers of different densities
  1. How can we study the interior of the Earth?
  2. Studying the magnetic field of the Earth

- Material in the core is moving - generates magnetic field

- Magnetoshpere – the Earth magnetic field extends about 60 000 km into surrounding space

- Asymmetric

  • Studying the seismic waves – speed and wavelength bring information about the layers
  1. Plate Tectonics – theory that explains how slow motions within the mantle of the Earth move large segments of the crust, resulting in a drifting of continents and formation of large scale geological features.
  • The science that study the crest - geology
  • Plate tectonics - basic concept to geology
  • Developed at mid 20th century
  • “Plate” – large rigid slab of solid rocks; “tectonics” – from the Greek root “to build”; “plate tectonics” – refers to how the Earth’s surface is built of plates
  • Earth’s crust and upper mantle are divided into about 10 major plates, moving relative to one another, located at the top of the hotter, more mobile material of the mantle
  • Convection – physical phenomenon that provides the power to move the plates
  • Explanation of convection
  1. Basic kinds of interaction between plates
  • Plates can pull apart (recede)
  • Rift zones (Mid Atlantic ridge, central African ridge). Most rifts zones – below the ocean; molten rocks (basaltic lava) fill the space between receding plates
  • 60000 km of active rifts have been identified
  • average rate of separation – 4 cm/yr – means 2 sq km new area added to the Earth each year – new oceanic crust in 100 000 000 years
  • oceanic crust – the youngest feature of our planet
  • Plates can burrow under one another
  • The zone where one plate is forced under another – subduction zone (subducted plate is forced down and partly melted)
  • the feature formed is called (ocean) trench (deep Japan trench along the coast of Asia)
  • subduction zones often give rise of thermal vents – earthquakes and volcanoes
  • Plates can slide alongside
  • Forming cracks or faults
  • Not a smooth motion – severe earthquakes
  • Average Rate – several cm/yr
  • San Andreas Fault – boundary between the Pacific plate (the coast of California) and the North American plate
  • Plates can jam together (colliding)
  • Push against each other under great pressure – recent plates collisions create mountains (Alps and Himalayas – young features)

6. Volcanoes - locations where magma rises to the surface

  • Along the mid-ocean ridges, along the undersea mountains (formed at plate boundaries)
  • Above the mantle hot spots, far from the boundaries of the plates
  • Volcanic mountains (lava flows slow from craters), volcanic plains (lava flows rapidly from a long cracks)

7. The Earth’s atmosphere

  • The ocean of air – pulled down by the earth gravitational field
  • Pressure that the Earth atmosphere exerts at sea level is 1 bar
  • 1 bar - each sq cm of the earth surface experience a force 10.3 newtons
  • total mass – 5 x 1018 kg (small fraction of the total mass of Earth)

8. Structure of the atmosphere

  • troposphere (0 – 10 km) – air convection all the time – warm air heated by the surface rises and is replaced by descending currents of cooler air - creates clouds and wind , temperature at 10 km - -50 deg Celsius
  • stratosphere – 10 – 50 km, cold and free of clouds
  • ozone layer (O3) - absorbs UV light
  • mesosphere (50 -100 km)
  • ionosphere

9. Atmospheric composition at the Earth’s surface

  • 72 % nitrogen
  • 21 % oxygen
  • 1 % argon
  • traces of water vapor, carbon dioxide
  • dust particles and water droplets