Section #: 17.2 Seafloor Spreading

Practice Test
  1.The thickness of ocean sediments that are close to a mid-ocean ridge is __________ the thickness of ocean sediments that are far from mid-ocean ridges.  
  a.   the same as  
  b.   always 100 times  
  c.   less than  
  d.   more than  
  2.Harry Hess' theory of __________ explained how ocean crust is generated and destroyed.  
  a.   continental drift  
  b.   seafloor spreading  
  c.   plate tectonics  
  d.   continental movement  
  3.What is the polarity of a magnetic field that has the same polarity as the present magnetic field?  
  a.   a magnetic reversal  
  b.   an isochron  
  c.   a normal polarity  
  d.   a reverse polarity  
  4.A __________ is a device that can detect changes in the strength of the magnetic field.  
  a.   satellite  
  b.   magnetometer  
  c.   sonar  
  d.   GPS  
  5.What is the study of Earth's magnetic record?  
  a.   paleogeography  
  b.   magnetometer  
  c.   paleontology  
  d.   paleomagnetism  
  6.What two topographic features of the ocean floor were discovered only with the use of sonar?  
  a.   volcanic islands and mid-ocean ridges  
  b.   mid-ocean ridges and deep-sea trenches  
  c.   shallow-sea trenches and volcanic islands  
  d.   mid-ocean ridges and earthquakes  
  7.What is an isochron?  
  a.   a line on a seafloor map that connects points of equal elevation  
  b.   a line on a seafloor map that connects points of equal age  
  c.   a point on a seafloor map that has a magnetic reversal  
  d.   a part of a mid-ocean ridge  
  8.An expedition finds a sample in the rock on the seafloor thought to be 4 million years old. What was the polarity of the Earth at that time, according to the figure?

  a.   normal  
  b.   reversed  
  c.   nonexistent  
  d.   half-reversed  
  9.An expedition brings up a piece of rock from the Gauss normal epoch. According to the figure, what is the probability that it came from a time when the magnetic field is the same as today?

  a.   highly unlikely  
  b.   unlikely  
  c.   likely  
  d.   very likely  
  10.What causes the seafloor to spread?  
  a.   rising molten magma  
  b.   drifting continental crust  
  c.   sinking magma  
  d.   earthquakes  
  11.Rocks close to mid-ocean ridges are __________ rocks far from mid-ocean ridges.  
  a.   older than  
  b.   the same age as  
  c.   younger than  
  d.   a different composition than  
  12.According to the data shown in the figure, what was Earth's magnetic polarity like 5 million years ago?

  a.   the same as today  
  b.   the reverse of today  
  c.   there was no magnetic field  
  d.   pointing in an east-west direction  

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