Title: 17.2 Seafloor SpreadingPage #: 88

Date: 3/18/2012

Students will be able to summarize evidence that led to the discovery of seafloor spreading.

Students will be able to explain the significance of magnetic patterns on the seafloor.

Students will be able to explain the process of seafloor spreading.

Basalt: A dark grey to black fine grained igneous rock.

Main Idea: Pg. 473

Oceanic crust forms at mid-ocean ridges and becomes part of the seafloor.

MappingThe ocean Floor:Pg. 473

Early 1900s: People believe that oceans are carved out by erosion.• Believe that ocean floor is flat and

old.

1940s: WWII submarine warfare results in the development of SONAR technology.• Also results in MAGNOMETER

technology.

Mapping the Ocean Floor

pg. 473

Sonar: Echolocation. Sub sends sound out, it bounces off an object and reveals its location.

Magnometer: Device that can detect changes in magnetic fields (used to find enemy subs.)

Sonar Magnetometer

Mapping the Ocean Floor

pg. 473

Late 1940s-1950s: WWII ends. Scientists use new technology to study depth and topography of the ocean floor (sonar.) • Scientists study magnetic fields of

ocean floor rocks (magnometer.)

Mapping the Ocean Floor

pg. 474

1947: Bruce Heezen and Marie Tharp of Columbia University began to systematically map Earth’s seafloor features.

Heezen went out on research cruises and gathered data from 1947 – 1965.

Tharp: stayed on shore to construct maps. (Women were not allowed on research cruises.)

Heezen and Tharp

World Ocean Floor; Published 1974

Mapping the Ocean Floor

pg. 474

Surprise Findings! 1. Ridges: huge underwater

mountain ranges, 80,000 km long and up to 3 km high. Largest mountain range on Earth. Earthquakes and volcanoes are

common along ridges.

Mid-AtlanticRidge

Mapping the Ocean Floor

pg. 474

Surprise Findings! 2. Deep-Sea Trenches: Narrow

elongated depressions in sea floor. Can be 100s of km long and many

km deep. Example: Marianas Trench; Mt.

Everest (9 km above sea-level) can fit in Marianas Trench with 9 Empire State Buildings on it.

Mapping the Ocean Floor

pg. 474

• Ridges and trenches lead to more questions:

1. What formed mountains?2. What is source of valcanism

on mountain range?3. What forces depressed Earth’s

crust to create huge trenches?

Ocean Rocks and Sediments

pg. 475

2 Discoveries from samples of Deep Sea rocks and sediments:

1. Age of rocks increases as you get farther away from a mid-ocean ridge – oldest rocks at trenches.

2. Thickness of sediments increases as you get farther away from mid-ocean ridge (older crust has more time to collect sediments.)

Ocean Rocks and Sediments

pg. 475

Oceanic Rocks only 180 million years old.

Continental Rocks can be up to 3.8 billion years old.

Layer of sediment on oceanic crust is only a few hundred meters thick.

Layer of sediment on continental crust is 20 kilometers thick.

Thickness of sediments increases as you get farther from a mid-ocean ridge.

Ocean Rocks and Sediments

pg. 475

Oceanic rocks and sediments are symmetrical in age and depth of sediments (mirror images) on either side of a mid-ocean ridge.

Magnetismpg. 476

Earth has a magnetic field.• Causes compasses to point

north.• A result of flow of molten iron in

liquid outer core of the earth.

Magnetic Reversal:• When Earth’s magnetic field

changes direction.• Happens when there is a change

in flow of the earth’s core.

Magnetic Polarity Time Scale

pg. 476

Paleomagnetism: Study of history of Earth’s magnetic field.

• Lava contains magnetite – when it solidifies crystals point toward the magnetic pole because of Earth’s magnetic field.

Magnetic TimePeriods

Magnetic Symmetry

pg. 476

Magnetic Symmetry: Oceanic crust is mostly basaltic.

• Contains large amounts of iron bearing minerals.

Magnetometers revealed a pattern in the ocean floor.

Magnetic Pattern in Ocean Floor

pg. 476

Regions with normal and regions with reverse polarity form a series of stripes across the floor parallel to mid-ocean ridges.

• Age and width of the stripes matched from one side of the ridge to the other.

Seafloor Striping

Magnetic Pattern in Ocean Floor

pg. 477

Isochron: A line on a map that shows points with the same age.

• Scientists were able to match patterns of reversals on land to the reversals on the sea floor to determine the age of the ocean floor.

Seafloor Spreading

pg. 479

Seafloor Spreading: Theory that explains how new crust is formed at ocean ridges and is destroyed at deep-sea trenches.

Steps:1. Less dense magma rises at ridges and

some solidifies.2. Solid magma makes new seafloor.3. Most magma gets turned horizontally away

from ridge under the crust.

Seafloor Spreading

pg. 479

Continental Drift questions answered:

• Continents don’t move through oceanic crust.

• Continents are carried as passengers that ride along as ocean crust moves.