EarthQuakes & Volcanoes (e.g. Shake and Bake)

1. EarthQuakes & Volcanoes(e.g. Shake and Bake) • The most visible manifestations of the earth’s internal heat are earthquakes and volcanoes. • There is an interesting relationship between the two that led researchers in the 1960’s to overturn our view of the Earth

2. Igneous Rocks • Formed when minerals and other rocks melt • Are classified according to their chemistry and texture • Bowen’s Reaction Series Explains all there is to know

3. Texture of Igneous Rocks • Texture describes how the minerals fit together in a rock and their size • Fine-grained are usually volcanic rocks • Coarse grained are commonly plutonic rocks.

4. Definitions • Plutonic = Intrusive (magma stays underground) • Volcanic = Extrusive (magma flows to the surface) • Lava = Magma that has reached the surface.

5. Geothermal Gradient • Why do rocks melt? • Because the earth is hot inside! • The change in temperature with increasing depth is called the Geothermal gradient.

7. What Happens When Rocks Melt? • They expand which causes…. • The density to decrease (because you have the same amount of rock in a larger volume) which causes….. • The melted rock to rise toward the surface!

8. Volcanic eruptions • We have discussed why volcanic material (magma) rises to the surface. • There are several different types of volcanic eruptions that are related to the composition of the magma. • Volcanoes are found all over the world, but tend to occur along linear belts.

9. Locations of Quakes/Volcanoes Earthquakes/Volcanoes Occur along Linear Belts. The most famous of these regions is called the “Pacific Ring of Fire”

10. Locations of Volcanoes

11. Types of Volcanoes/Locations • Rhyolitic or Andesitic Volcanoes: Concentrated near the boundaries between oceans and continents. • Basaltic Volcanoes: Concentrated in the middle of the Oceans (Mid-Atlantic Ridge).

12. Types of VolcanoesStrato or Composite Cones • Strato Volcanoes: Made up of alternating layers of rhyolitic to andesitic flows and ashfall deposits. • Top picture is Mt. St. Helens in Washington and the bottom is Mt. Fuji in Japan.

13. Types of VolcanoesShield Volcanoes Shield Volcanoes tend to be very large, but not as steep or tall (with respect to the land surface) as Composite volcanoes. Shield volcanoes tend to be dominated by basaltic volcanism. Eruption of Kiluaea, Hawaii

14. Types of VolcanoesCinder Cones • Cinder cones are the simplest type of volcano. They are built from particles and blobs of congealed lava ejected from a single vent. • They tend to be much smaller than the other types of volcanoes. Mt. Capulin, New Mexico

15. Frequency of Eruptions • Volcanic activity on Earth is nearly constant. • This activity, more than anything else, indicates the dynamic hot nature of the planet.

16. Predicting Volcanic Eruptions • Predicting volcanic eruptions has become increasingly more accurate. • Two Good Books • Surviving Galeras by Stanley Williams • No Apparent Danger by Victoria Bruce

17. Bowen’s Reaction Series (Chemistry of Igneous Rocks

18. Additional Notes on Bowen’s • As SiO2increasesdensitydecreases. • As SiO2increasesmelting temperaturedecreases. • As SiO2increasescrystallization temperaturedecreases. • As SiO2increasesviscosityincreases. • As SiO2increasesexplosive potentialincreases.

19. More Interesting Trivia • Why are continents, continents? • Why are oceans, oceans? • Answer: • Continental Crust: Thick and Light (2.7) • Oceanic Crust: Thin and Heavy (2.9)

20. EarthQuakes • Earthquakes, like volcanoes, reflect the dynamic nature of the planet earth. • Earthquakes, like volcanoes, tend to occur along linear belts. • There are different types of earthquakes in different regions.

21. Details of Earthquakes • What causes an earthquake? • Where did the earthquake occur? • How big was the earthquake? • What type of fault generated the earthquake? • How frequently do earthquakes occur? • Can Earthquakes be predicted?

22. What Causes an Earthquake? • The simple answer is the buildup of stress in solid rock. • Elastic Rebound Theory states that stress will build to a critical level and then the rock will break suddenly causing an earthquake. • Earthquakes occur on faults which are breaks in the earth’s crust along which movement occurs.

24. Where did the Earthquake Occur? • We need the following information to accurately locate an earthquake. • Time that P and S waves arrive at 3 earthquake monitoring stations. • P-waves are also known as ‘primary’ or ‘push’ waves because they are the first to arrive at a seismic station. P-waves can travel through gas, solid or liquid. • S-waves are known as ‘shear’ waves or ‘secondary’ waves because they are normally the second type of wave to arrive at a seismic station.

27. Locating an Earthquake Difference in P-S gives an estimate of distance, but not direction. In essence, we end up with a ‘circle of possible locations’ for each station.

31. Focus-point of rupture on fault Epicenter: Point on the earth’s Surface directly above the focus

32. How Big? • The size of an earthquake is called its magnitude. • Originally developed by Charles Richter, there are now many different magnitude scales. • Most of these depend on Distance to the quake, amplitude of a particular wave and period of that wave. • Station corrections are added to account for the crust in the region of the seismometer.

33. Magnitudes Sample Formula would look like: Mag= log (A/T) + log D + 3.2 Where A=amplitude (mm) T= period (seconds) D=Distance And the last number is a station correction.

34. Magnitude Relationships • Because the scale is logarithmic, each single digit increase in magnitude represents a 10x increase in the amplitude of the wave. • Perhaps more importantly, each single digit increase in magnitude represents a 32x increase in the amount of energy released by a quake.

35. What type of fault? • The type of fault depends on the type of stress generating the earthquake. • Normal fault= Extensional vertical stress (pull-apart) • Reverse fault= Compressional vertical stress (push-together) • Strike-slip fault=sliding horizontal stress

36. Normal fault Hanging Wall: Miners hang their lamps on it. Foot wall is the side of the fault that miners could walk on.

37. Reverse fault

38. Strike-Slip Fault Left-lateral motion: Observers standing on opposite sides of the fault would see each other moving to their left. Right-lateral motion: Observers standing on opposite sides of the fault would see each other moving to their right.

39. Focal Mechanism Maps • Often called ‘beach balls’ they give geologists a way to determine fault type remotely. • Based on the first motion of the P-wave (either up or down. • If the first motion of the P-wave is ‘up’, then this signifies upward movement of the ground surface beneath the seismograph. • If the first motion of the P-wave is ‘down’, then this signifies downward movement of the ground surface beneath the seismograph

40. In this case, which direction is the movement on the fault?

42. Strike-Slip Fault Plane Solution---In this case a right lateral fault.

43. Normal Fault Focal Mechanism Reverse Fault Focal Mechanism

44. Relationship between Faults/Volcanoes • Normal faulting is associated with basaltic volcanism. • Reverse faulting is associated with andesitic and rhyolitic volcanism. • Strike-slip faulting is amagmatic.

45. How Often? • Earthquakes occur on a daily basis. • In your homeworks, you will learn more about the frequency-energy relationships of earthquakes. • Can Earthquakes be predicted?

46. Earth Structure From Earthquakes • Recall: • P-Waves are fastest and can travel through all media • S-Waves cannot travel through liquid or gas.

47. Snell’s Law • As a wave (represented by its ray path) moves through layers of different density it will bend. Case 2: Layer 2 is ‘slower’ than Layer 1 Case 1: Layer 2 is ‘faster’ than Layer 1

48. The Real Earth

49. S-Wave Shadow Zone

50. Can Earthquakes Be Predicted?