Download PLATE TECTONICS

UNIT 4 CONCEPT
PLATE
TECTONICS
WHAT IS IT?
Plate tectonics is a geological theory that explains movement in the Earth’s lithosphere, the
rocky surface layer on which we live that includes the crust and the upper portion of the mantle.
The lithosphere consists of plates that move separately on top of the mantle, the semi-molten
region that extends to the Earth’s core. Tectonic comes from the Greek word meaning “to build”
and it is plate tectonics that explains how mountains form and how continents are positioned
around the world.
WHY IS IT IMPORTANT?
The movement of the continents has a direct impact on the evolution of life.
• How continents are positioned in relation to the Sun determines whether their climate
is tropical, temperate, or arctic.
• The collision of drifting continents creates mountain ranges, which divide species and
affect weather patterns, water distribution, and other factors that drive evolution.
The movement of tectonic plates has a direct impact on contemporary life.
• Earthquakes happen when two passing plates scrape each other. In the San Francisco
earthquake of 1906, the sudden release of stress pushed one plate 20 feet northward
along a 270-mile stretch of the San Andreas Fault in just one minute.
• Most volcanoes form along plate boundaries. Some eruptions cause major casualties
and a recent eruption in Iceland disrupted airplane traffic in Europe for days.
HOW DO WE KNOW ABOUT IT?
• Alfred Wegener’s theory of continental drift, published in 1915, claimed the continents once
formed a single supercontinent that broke apart, setting the separate landmasses adrift.
Wegener observed that 1) the coastlines of some continents fit like a jigsaw puzzle, and
2) similar fossils are found on opposite sides of the Atlantic. But he could not explain why
continents would drift.
• In 1959, Harry Hess proposed seafloor spreading: molten rock seeps up from the mantle
through mid-ocean ridges, spreads out to create new ocean floor, and re-enters the mantle
through trenches. This explained why drift occurs, but needed proving itself.
• Scientists determined that Earth’s polarity flips its orientation, on average every 450,000
years. This means that sometimes a compass needle would point south instead of north
and led to the hypothesis that if new oceanic floor is being created, ocean-floor rocks will
record flips of the magnetic field. In the 1960s, research proved they do.
BIG HISTORY PROJECT / UNIT 1 CONCEPT CARD
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