With advances in technology, especially during World War II, more clues to the earth's geologic history were unearthed. Scientists were better able to map and study the ocean floor and discovered the presence of an underwater mountain range chain called the mid-ocean ridge system and deep-sea trenches. They also discovered that the ocean floor is not as old as expected and that the magnetic properties of the ocean sediment alternate in a matching pattern on either side of a mid-ocean ridge. These discoveries led geologist Harry Hess to develop his seafloor spreading theory. According to this theory, the mid-ocean ridges are spots where the earth's crust is weak, allowing magma from the earth's mantle to come up to the surface and create new seafloor. Deep-sea trenches are areas where old seafloor is being driven back into the mantle. Further research found that earthquakes and volcanic activity are concentrated near ridges and trenches. The clues all led to the modern day theory of plate tectonics.
To understand plate tectonics, we must first look at the make-up of the earth. The earth is divided into three main layers:
The semi-solid mantle moves in a circular motion, pulling the overlying crustal plates with it. The mantle's circular motion comes from two sources. First, heat from within the earth warms the mantle material, causing it to rise toward the crust. Near the crust, the mantle material cools and sinks down toward the core. The second force impacts the mantle at deep sea trenches. The heavy crustal plate is pulled by gravity down into the mantle dragging mantle material with it (this is called "slab pull"). Due to this convection motion in the mantle, the large plates covering the earth's surface are constantly moving. The boundaries between these moving plates are very active areas. The plate boundaries can be one of four types:
Data Activity
Author
Lisa Ayers Lawrence, Virginia Sea Grant, Virginia Institute of Marine Science
Grade Level
8-12
Lesson Time
45 min.
Objectives
Vocabulary
Continental drift, Plate tectonics, ore, Mantle, Crust, Plate boundaries, Divergent, Convergent, Transform, Boundary zones
Materials Required
world map, Map of Major Earthquakes 1900-2017, USGS map
Natl. Science Standards
IK-1 IK-2 PH5-2 PH5-3 ES5-1 ES5-2 ES9-1 ES9-3 PS5-3 PS9-3 PS9-5 N5-3 N9-3
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