Sea-Floor Spreading
Oceanic crust forms at ocean ridges and becomes part of the seafloor. Review Vocabulary basalt: a dark-gray to black fine-grained igneous rock
I. Mapping the Ocean Floor Until the mid-1900 s, many scientists thought that the ocean floors were essentially flat and that oceanic crust was unchanging and was much older than continental crust. Advances in technology during the 1940 s and 1950 s showed that all of these widely accepted ideas were incorrect.
A. Magnetometer - device that can detect small changes in magnetic fields
B. Sonar 1. Bounces sound waves off underwater objects 2. Echoes determine distance to object 3. Maps ocean floor topography
II. Ocean Floor Topography A. Mid-Ocean Ridges
What name was given to the large underwater mountain ranges that were discovered during the 1940s and 1950s? A. ocean ridges B. ocean trenches C. sea mounts D. sea stacks 0% 0% 0% 0% A. B. C. D.
B. Deep-sea trenches Pg. 474 Fig. 17.8
The deepest trench, the Mariana Trench, is more than 11 km deep. Mount Everest, the world s tallest mountain, stands at 9 km above sea level, and could fit inside the Mariana Trench with six Empire State buildings stacked on top.
III. Ocean Rocks & Sediments A. Glomar Challenger (drilling ship) 1. Youngest rock sample near ridge 2. Oldest rocks near continents
B. Evidence 1. Molten material a. Alvin (submarine) b. Rock samples
C. Sediments 1. Thickness of ocean-floor sediments increases with distance from an ocean ridge
How did scientists discover that rocks farther away from the mid-ocean ridge were older than those near it? A. by observing eruptions of molten material on the sea floor B. by mapping rocks on the sea floor using sonar C. by determining the age of rock samples obtained by drilling on the sea floor D. by measuring how fast seafloor spreading occurs 0% 0% 0% 0% A. B. C. D.
What technology did scientists use in the mid-1900s to map the mid-ocean ridge? A. satellites B. deep-sea diving C. submarines D. sonar 0% 0% 0% 0% A. B. C. D.
IV. Magnetism A. Earth s magnetic field 1. Flow of molten iron in the outer core 2. Magnetic reversal a. Flow in the outer core changes b. Earth s magnetic field changes direction
B. Magnetic polarity time scale 1. Paleomagnetism - the study of the history of Earth s magnetic field 2. Lava solidifies a. iron-bearing minerals crystallize b. align with Earth s magnetic field
Pg. 476 Fig. 17.11 Periods of normal polarity alternate with periods of reversed polarity. Long-term changes in Earth s magnetic field, called epochs, are named as shown here. Short-term changes are called events.
FIG 17. 12 Pg. 477 Regions of normal and reverse polarity form a series of stripes across the ocean floor parallel to the ocean ridges. The ages and widths of the stripes match from one side of the ridges to the other.
3. Isochron - imaginary line on a map a. points that have the same age
Which layer of Earth generates Earth s magnetic field? a. the crust b. the mantle c. the outer core d. the inner core 0% 0% 0% 0% 1 2 3 4
A rock cools from lava and acquires a normal magnetic orientation. What happens to this rock s magnetism when Earth s magnetic field next reverses? A. It changes direction. B. It stays the same. C. It gets erased. D. It has a different strength. A. 0% 0% 0% 0% B. C. D.
V. Sea-floor spreading A. Harry Hess studied mid-ocean ridges 1. Maybe Wegener was right
B. New material added to ocean floor 1. Spreads apart at ridges 2. Moves ocean floor & continents
C. Molten material rises 1. Pushes older rock outward 2. New material cools
In sea-floor spreading, molten material rises from the mantle and erupts A. along the edges of all the continents. B. along mid-ocean ridges. C. in deep-ocean trenches. D. at the north and south poles. 0% 0% 0% 0% A. B. C. D.
What happens to an oceanic plate after it moves to a trench? A. It is welded to a continent. B. It sinks into the mantle. C. It is uplifted to form mountains. D. It breaks into small pieces. 0% 0% 0% 0% A. B. C. D.
VI. Subduction at Trenches A. Ocean floor sinks back into mantle 1. Gravity pulls old, dense rock down 2. Occurs at deep-ocean trenches
The process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle is known as A. convection. B. continental drift. C. subduction. D. conduction. A. 0% 0% 0% 0% B. C. D.
Where in an ocean basin is the oldest oceanic crust usually located? A. near an ocean ridge B. near an ocean trench C. along the middle of the basin D. along any edge of the basin A. 0% 0% 0% 0% B. C. D.
Visualizing Seafloor Spreading Data from topographic, sedimentary, and paleomagnetic research led scientists to propose seafloor spreading.
B. Changes size & shape of oceans 1. Pacific Ocean is shrinking a. More subduction than new crust forming 2. Atlantic is expanding
seafloor401spreading
Oceanic crust forms at ocean ridges and becomes part of the seafloor. Studies of the seafloor provided evidence that the ocean floor is not flat and unchanging. Oceanic crust is geologically young.
New oceanic crust forms as magma rises at ridges and solidifies. As new oceanic crust forms, the older crust moves away from the ridges.