Formation and Structure of the Earth

1. Formation and Structure of the Earth

2. Standards • Recognize that radiometric data indicate that Earth is at least 4 billion years old and that Earth has changed during that period. • Describe the internal structure of Earth (e.g., core, mantle, crust) and the structure of Earth’s plates. • Describe how waves are used for practical purposes (e.g., seismic data). • Know that Earth’s systems are driven by internal (i.e., radioactive decay and gravitational energy) and external (i.e., the sun) sources of energy.

3. Formation of the Universe • Our universe began approximately 13.7 Ga (billion years ago) with a cosmic “explosion” called the BigBang. • Before the Big Bang, all matter and energy were compacted into a single, inconceivably dense point called a singularity.

4. Formation of the Universe • We don’t know what happened during the Big Bang or for the first fraction of a second after it. • But, during the billions of years that followed, the universe has expanded and gas, dust, stars and galaxies have formed.

5. The Big Bang Photo: http://oz.plymouth.edu/~sci_ed/Turski/Courses/Earth_Science/Intro.html

6. Formation of the SolarSystem • Earth formed along with the rest of the solar system (at the same time). • Our solar system formed from the collapse of an interstellar cloud (condensation theory).

7. Condensation Theory • Our model for the formation of the solar system. • A large cloud of interstellar (between stars) gas began to collapse under the influence of its own gravity. • As it contracted, it became denser and hotter, with most of its matter forming the sun at its center.

8. Condensation Theory 3. The rest of the matter formed into a flatteneddisk around the sun, hotter in the inner region where more of the matter accumulated, than in the less dense outer regions. 4. Gravitational attraction caused dust and cooling gas in the disk to collide and accrete (clump together) as small chunks called planetismals.

9. Condensation Theory 5.The planetismals continued to collide and accrete to form the planets and their moons. 6. Some planetismals did not clump together, and now form asteroids and meteorites.

10. Condensation Theory of Solar System Formation Photo: http://oz.plymouth.edu/~ sci_ed/Turski/Courses/Earth_ Science/Intro.html

11. Condensation Theory • This all happened 4.6 Ga (billion years ago). • But, how do we know when this occurred?

12. Dating the Solar System • Meteorites (the original planetismals) sometimes fall to Earth. • We can get an age for the meteorites by radioactive dating (more on this later). • The meteorites, which are left over pieces from the formation of the solar system, have an average age of 4.6 Ga. Photo: http://www.nizwa.net/env/meteorites/meteorites.html

13. Formation of the Earth • Early earth was rocky and uniform in composition and density throughout. • Then it began to heat up and melt. Photo: http://www.escepticospr.com/ images/early_earth.jpg

14. 3 Sources of Heat • Gravitationalcollapse – as earth contracted, it heated up. • Radioactivedecay – thermal energy released by the decay of radioactive elements. • Surface bombardment by meteorites – the early solar system contained many more meteorites than today.

15. Differentiation Occurs • As Earth heated it became partially molten. • 30 – 65% of the Earth formed an outer magmaocean hundreds of kilometers thick. • The interior of the Earth became soft. • The heaviest elements sank to the middle and the lightest elements floated on top. • This caused the formation of distinct layers in the Earth, called differentiation.

16. Compositional Layers of the Earth • Layers defined by what they are made of • The innermost layer of the Earth is the core (inner & outer) • The next layer is the mantle • The top layer is the crust

17. Core • Made of iron and nickel • Inner core: • Solid • Even though its hot, there is too much pressure for it to be liquid • 1255 km thick • Outer core: • Liquid • Rotates around the inner core (this gives us our magnetic field) • 2220 km thick

18. Mantle • Region surrounding the core • Makes up the bulk of the Earth • Made of peridotite (rock containing iron & magnesium) • Is a solid that flows • 2900 km thick

19. Crust • Thin outer layer of the Earth • Solid • Two types: 1. Continental crust • made of granite • 20 – 70 km thick 2. Oceanic crust • made of basalt • thinner than continental crust • 7 – 10 km thick

20. Mechanical Layers of Earth • Layers defined by how they behave • Lithosphere • Asthenosphere

21. Lithosphere • Layer that includes the crust and uppermostmantle. • Lithospheric Plates - The lithosphere is broken up into plates that move over the partially molten mantle (plate tectonics)

22. Asthenosphere • Part of mantle directly below the lithosphere, on which the plates move.

23. How do We Know the Earth’s Structure? • Drilling has only been done to a few kilometers depth in the crust. • How then do we know that the Earth is layered?

24. Seismic Waves • Vibrations that travel through the Earth. • Produced by earthquakes and surface explosions.

25. Seismic Waves • An earthquake generates waves that spread out in all directions, like light from a light bulb. • Seismograph stations detect all of the waves that arrive at that location. • We can learn about the deep interior of Earth by: • Recognizing what kind of waves have arrived • Knowing exactly when they arrived • Calculating when and where the earthquake occurred

26. Seismic Waves • Seismic waves refract (bend) and reflect when they pass from one material into another material. • They refract and reflect at the boundaries in the Earth because the layers are made of different materials. • They can also change in acceleration when passing from one material to another.

27. Seismic Waves • Mohorovičić Discontinuity • Moho for short • Boundary between the crust and mantle • Where seismic waves accelerate as they pass from the lower density crust into the higher density mantle

28. Types of Seismic Waves • There are two main types of seismic waves: • Surface waves • Body waves

29. Surface Waves • Travel on the Earth’s surface. • Can’t be used to determine structure.

30. Body Waves • Travel through Earth, so can be used to determine structure. • Two types of body waves: • Primary or P waves • Secondary or S waves

31. Primary or P waves • First to arrive at seismograph station • Compressional waves – particle motion is in same direction as wave travel • Can travel through both solids and liquids

32. Secondary or S Waves • Arrive after P waves • Shear waves – particle motion is perpendicular to wave motion. • Can travel through solids but not liquids. • They do not travel through the outer core, therefore we know the outer core is liquid.

33. S-Wave Shadow Zone • Zone in which no S waves are recorded by seismic stations (because of the outer liquid core).

34. Continents • Continental growth: began soon after differentiation & has continued through geologic time. • The less dense materials on Earth’s surface accrete (clump together) to form the continents.

35. Continents • Continents are made up of large regions of stable, ancient crystalline rocks (igneous and metamorphic rocks), called shields. • The shields are surrounded or buried by sedimentary rocks called platforms. • Shields and platforms make up the stable (no longer undergoing deformation) parts of the continents, called cratons.

36. Earth’s Atmosphere Today • Breathable by humans • Mixture of gases: • 78% nitrogen • 21% oxygen • Trace amounts of argon, carbon dioxide & water vapor • Large amount of oxygen makes Earth’s atmosphere unique in solar system.

37. Origin of Earth’s Atmosphere • Atmosphere of early Earth made up of gasesmost common in solar system (hydrogen, helium, methane, ammonia & water vapor) • Escaped into space • Secondary atmosphere was outgassed (expelled) from planet’s interior by volcanoes • Surface temperature fell and water vapor condensed, forming oceans • Life appeared in oceans and began to produce oxygen • Oxygen in present-day atmosphere is direct consequence of evolution of life on Earth.

38. Earth Systems • The Earth system is composed of all the parts of our planet that work together. • These are the geosphere, atmosphere, hydrosphere, biosphere and cryosphere • It is an open system that exchanges energy and mass with its surroundings.

39. Earth Systems • Earth systems are driven by both external and internal sources of energy. • The external source of energy is our sun. • The sun’s energy drives climate and weather, and energizes the atmosphere, hydrosphere, cryosphere and biosphere.

40. Earth Systems • The internal sources of energy are gravitational (trapped heat from Earth’s formation) and heat from the decay of radioactive elements. • Internal energy drives plate tectonics and the geodynamo (interaction between inner and outer core) and energizes the lithosphere, asthenosphere, deep mantle, and inner & outer core.

41. Our Changing Earth • Earth is dynamic - it is always changing • It continues to change even as we speak • Change is natural and unstoppable