Earth in 2-D, 3-D & 4-D

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1 Earth in 2-D, 3-D & 4-D We will consider the scientific tools and techniques used to map surface features, reconstruct the layered structure of Earth, and interpret Earth history, including the origin of the ocean

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3 Prior to the early 20th century, soundings were the only means to determine water depth weighted lines lowered from ships time consuming, relatively few, doubtful accuracy

4 Voyage of the H.M.S. Challenger First dedicated oceanographic (scientific) exploration of the world ocean

5 Echo Sounders sound source & receiver (hydrophone) on hull of ship high frequency sound waves travel through the water, reflect off the seafloor, and are recorded by the hydrophone provide continuous depth profiles along a ship s track but only 2-dimensional

6 Seismic Reflection Profiles sound source and hydrophone towed by ship 2-D, continuous profile lower frequency energy source (stronger sound source with fewer sound waves per second) deeper penetration into sedimentary layers and ocean crust

7 Echo Sounder Seismic Reflection Profiler (sound source and hydrophone)

8 1/27/2014

9 Echo sounder record of a seamount

10 Seismic Profiler record

11 Seismic reflection survey

12 Other tools to map the seafloor: Side Scan Sonar like echo sounder, but it images a 60 km swath of seafloor overlapping swaths = complete coverage (3-D)

13 Seabeam swath producing 3-D bathymetric map of seafloor (a bathymetric map is similar to a topographic map used to depict relief on land)

14 Sidescan Sonar of Ship Wreck

15 This is a bathymetric map based on a seabeam survey for Deep Sea Drilling Project sites off northwest Africa (contour interval = 50 m); note the very steep Mazagan Escarpment dropping off to the deep-sea isobaths

16 Multibeam echo sounding survey This image shows a depiction of the beam of sound waves mapping the ocean floor. Multibeam surveying provides incredibly detailed imagery of the seabed. On board survey ships, instruments emit multiple beams of sound waves, which are reflected off the ocean floor. As the sound waves bounce back with different strengths and timing, computers analyze these differences to determine the depth and shape of the seafloor, and whether the bottom is rock, sand or mud.

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18 Schematic diagram showing the various types of seafloor-mapping systems used by the Western Coastal and Marine Geology team. Drawing by Bruce Rogers, modified from image on a Web page posted by the USGS Woods Hole Sea-Floor Mapping Group.

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21 Satellites Precise altimeters (using microwaves) can map the relief of the ocean surface with centimeter-scale resolution Bathymetric highs and lows on the seafloor, and differences in rock density, cause measureable gravitational distortion of the ocean surface

22 Right now, T/P's measurement precision for sea surface height is 4.3 cm (1.7 inches). Because the satellite flies at about 1330 km (830 miles) above the Earth's surface, that's comparable to knowing the sea surface height to much less than the thickness of a dime while flying in a jet at 35,000 feet altitude. Note: seafloor features distort the ocean surface, these very subtle distortions can be measured from space!

23 Map of the central and North Atlantic from satellite

24 Map of the world ocean from space. What do you see?

25 application: Caribbean tsunami and earthquake hazards studies (USGS) Perspective view of the seafloor of the Atlantic Ocean and the Caribbean Sea. The Lesser Antilles are on the lower left side of the view and Florida is on the upper right. The purple seafloor at the center of the view is the Puerto Rico trench, the deepest part of the Atlantic Ocean and the Caribbean Sea.

26 Radiometric Dating The primary method used to determine absolute ages of geologic and some biologic materials.

27 Recall the basic structure of the atom: a nucleus with protons and neutrons surrounded by shells or orbitals of electrons.

28 Protons: + charge Electrons: - charge Neutrons: no charge # protons = # electrons # neutrons are different in different isotopes of an element.

29 Unstable isotopes of certain elements (called parents) radioactively decay to the stable isotopes of other elements (called daughters). This happens in the nucleus by several mechanisms. The bottom line is that the number of protons and neutrons in the parent isotope changes as it decays to the daughter. This decay occurs at a precisely determined rate called a half-life.

30 For example, the parent isotope 238 U decays to the daughter isotope 206 Pb with a half-life (t 1/2 ) = 4.5 x 10 9 years.

31 This means that with the passage of every 4.5 x 10 9 years, the number of remaining 238 U is reduced by 50%: t 1/2 = U = Pb = 0 t 1/2 = U = Pb = 50 t 1/2 = U = Pb = 75

32 The parent isotope 14 C decays to the daughter isotope 14 N with a t 1/2 = 5730 years.

33 How do we know what the internal structure of Earth is like? See pages in Investigating the Ocean

34 Seismic Waves & Earthquakes: Earth Structure Revealed Earthquakes release a tremendous amount of energy ( seismic energy ) seismic waves radiate away from their point of origin (= focus) 3 different forms of seismic waves: 1. Rayleigh waves travel along the surface of the Earth 2. P-waves travel fast through the Earth 3. S-waves are slower and cannot travel through liquids p. 94

35 Refraction of seismic waves P-waves and S-waves bend (refract) when they pass from a material of one density into a material with a different density By measuring the arrival times of P- and S- waves around the globe from many earthquakes, it is clear that our Earth is layered in concentric spheres of different composition and density p. 94

36 I shown a laser pointer at the floor where it produced a spot. Then I placed a clear plastic block in the path of the beam. Part of the beam reflected off the block and produced a spot on the ceiling. Part of the beam passed through the clear plastic, but its path was bent or refracted causing the spot on the floor to move to a different position. This is the way seismic rays pass through the Earth. When they encounter a new layer part of the seismic ray reflects back toward the surface, and part of it passes through the layer but is refracted.

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38 Earthquake-generated P-wave refraction and reflection within the Earth.

39 Seismic Energy: P & S Waves This text figure is shown in color in the follow two slides.

40 S-waves are blocked by the liquid outer core yielding the S-wave shadow zone. So we know the outer core is liquid and from the size of the shadow we know the size of the outer core.

41 P-waves are focused by the core yielding the P-wave shadow zone. This gives us more information about Earth s internal layering.

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45 The breakup of the supercontinent Pangaea, the drifting of the continents, and seafloor spreading over the last 225 million years.

46 Now we are going to compare a simple bar magnet to the magnetic field of the Earth. We can use this information to demonstrate continental drift and seafloor spreading!

47 Iron filings outlining the dipolar field of a bar magnet.

48 Earth s dipolar field with its magnetic lines of force is similar to a bar magnet.

49 Note that the magnetic lines of force intersect the surface of the Earth at different angles. At the equator the lines of force parallel the ground, but the higher the latitude the steeper the lines of force intersect the ground. At the poles the lines of force are vertical. So you can determine the latitude of an area by the dip of the lines of force in that area.

50 Paleomagnetism: the study of ancient magnetic fields As a magma cools and solidifies into an igneous rock minerals crystallize from the magma. Among these crystallizing minerals are usually small quantities of minerals that are magnetic and/or with magnetic susceptibilities, such as magnetite. As these minerals cool below their Curie temperatures, they record the surrounding magnetic field of the Earth that exists at the time of cooling. Sediments being deposited at the bottom of a lake or ocean may also record the magnetic field, so the magnetic field may also be recorded in sedimentary rocks.

51 This preserved magnetism is also called remnant magnetism. The latitude at which an ancient rock formed can be determined from the inclination of the remnant magnetic field.

52 Now these cooling lavas will record the ambient magnetic fields for their latitudes as they cool down below their Curie temperatures.

53 Remnant magnetism recorded in igneous rocks that have cooled below their Curie point. Notice how the different rocks have different fields preserved in them. Even if subsequent continental drift moves these rocks to different latitudes, they will preserve their original fields.

54 Real example: 200 million year old lava flows just a few miles from campus have remnant fields that tell us that 200 million years ago Massachusetts was close to the equator! Continental drift really happens!

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56 By measuring the magnetic fields of rocks of different ages, it has been discovered that the Earth s magnetic field has reversed polarity episodically many times. The magnetic reversals recorded in ocean floor basalts have proven valuable in demonstrating sea floor spreading.

57 Polarity reversal in the Earth s magnetic field.

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59 History of magnetic reversals. So the Earth s magnetic polarity flips in an irregular way over geologic time.

60 Development of magnetic stripes on the seafloor on either side of the spreading ridge. Lava comes up in the axial valley as the two halves of the ocean floor spread apart. The lava cools and records either a normal or reversed polarity. As spreading continues, the rocks move away from the axial valley of the ridge as new lava fills in the axial valley, cools down, and records the new polarity. The magnetic stripes on the seafloor are symmetrical about the ridge.

61 Another view of the magnetic stripes on the seafloor.

62 Age of the ocean crust. The blue is the oldest ocean floor (about 200 million years old) and the red is the youngest floor (forming right up to the present day). Seafloor spreading!