Lesson 4 Instructional Materials . 1 3.4 Seafloor Spreading MAINIDEA: Oceanic crust forms at ocean ridges and becomes part of the seafloor. Mapping the Ocean Floor Common misconceptions until the mid-1900s: Ocean floors are flat. Oceanic crust is unchanging and older than continental crust. Technological advances used to study the ocean floor: Sonar: uses sound waves to measure distance by measuring the time it takes for sound waves sent from the ship to bounce off the seafloor and return to the ship. Ocean-Floor Topography Using sonar and other new technology, scientists were able to measure water depth and map the topography (or physical features) of the ocean floor. What did they find?! Ocean ridges : vast, underwater mountain chains Up to 40,390 miles long and 1.86 miles tall. FUN FACT: Ocean ridges form the longest continuous mountain range on Earth (that’s almost 2 times around the earth’s circumference)! Deep-sea trenches : narrow, elongated depression in the seafloor Thousands of miles long and many miles deep. The deepest trench is called the Mariana Trench and is more than 6.8 miles deep. FUN FACT: Mount Everest plus SIX Empire State Buildings could fit inside the Mariana Trench. Vocabulary Terminology Definition Ocean ridge Vast, underwater mountain chains with enormous length and height. This is where new crust is formed. Earthquakes and volcanism are common here. Deep-sea trench A narrow, elongated depression in the seafloor. This is where ocean crust is destroyed. Ocean Rocks Scientists collected samples of ocean-floor rocks. IMPORTANT DISCOVERIES 1. Ages of rocks vary across the ocean floor. These variations are predictable. 2. Samples taken near ocean ridges were younger than samples taken from areas near deep-sea trenches. 3. Even the oldest parts of the seafloor are geologically young (180 million years).
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Lesson 4 Instructional Materials .
1
3.4 Seafloor Spreading MAINIDEA: Oceanic crust forms at ocean ridges and becomes part of the seafloor.
Mapping the Ocean Floor
Common misconceptions until the mid-1900s:
Ocean floors are flat.
Oceanic crust is unchanging and older than continental crust.
Technological advances used to study the ocean floor:
Sonar: uses sound waves to measure distance by measuring the time it takes for sound waves sent from the ship to bounce off the seafloor and return to the ship.
Ocean-Floor Topography
Using sonar and other new technology, scientists were able to measure water depth and map the topography (or physical features) of the ocean floor. What did they find?!
Ocean ridges: vast, underwater mountain chains Up to 40,390 miles long and 1.86 miles tall. FUN FACT: Ocean ridges form the longest continuous mountain range on Earth
(that’s almost 2 times around the earth’s circumference)!
Deep-sea trenches: narrow, elongated depression in the seafloor Thousands of miles long and many miles deep. The deepest trench is called the Mariana Trench and is more than 6.8 miles
deep. FUN FACT: Mount Everest plus SIX Empire State Buildings could fit inside the
Mariana Trench.
Vocabulary Terminology Definition
Ocean ridge
Vast, underwater mountain chains with enormous length and height. This is where new crust
is formed. Earthquakes and volcanism are common here.
Deep-sea trench A narrow, elongated depression
in the seafloor. This is where ocean crust is destroyed.
Ocean Rocks
Scientists collected samples of ocean-floor rocks.
IMPORTANT DISCOVERIES
1. Ages of rocks vary across the ocean floor. These variations are predictable.
2. Samples taken near ocean ridges were younger than samples taken from areas near deep-sea trenches.
3. Even the oldest parts of the seafloor are geologically young (180 million years).
Lesson 4 Instructional Materials .
2
Why do you think ocean-floor rocks are so young compared to continental rocks?
Ocean Sediments
Scientists collected deep-sea sediments on the ocean-floor.
IMPORTANT DISCOVERIES
1. Ocean-floor sediments are typically only a few 100 meters thick (while continental sediments can be as thick as 20,000 meters).
2. Thickness of sediments increases with distance from ocean ridges.
Age of rocks & thickness of sediments Seafloor Spreading
Using all of this new data, the seafloor spreading theory was proposed. The theory explains how new oceanic crust is formed at ocean ridges, slowly moved away from ocean ridges, and destroyed at deep-sea trenches.
What type(s) of plate boundaries are found in sea floor spreading?
Lesson 4 Instructional Materials .
3
Real World Connection: Glomar Challenger
1968: launched a 15 year long expedition to retrieve core samples of deep ocean rock and sediment. These core samples provided evidence for continental drift and seafloor renewal. Scientists also found that the ocean floor is youthful compared to Earth’s geologic history.
Real World Connection: Glomar Challenger
Super shark and seafloor spreading.
Vocabulary Terminology Definition
Theory of Seafloor Spreading
The idea that new oceanic crust is formed at ocean ridges, slowly moved
away from ocean ridges, and destroyed at deep-sea trenches.
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