Download Magnetic strips in ocean-floor rocks

After Wegner has published his book
on the origin of continents and
oceans in 1915. Scientists started to
have interest in studying Wegner’s
hypothesis. Later, the new data came
from the least-known part of earthThe Ocean floor.
Exploring the ocean floor
• In early 1900s, new technology helped the
scientists to make it easy to discover the sea
floor. It is SONAR ( sound navigation and
ranging). T is a system that uses sound waves
to calculate the distance to an object.
• Sound waves off under-water objects and
then records the echoes of these sound
waves. The time it takes for the echo to arrive
indicates the distance to the object.
• The deeper the water, the longer it takes the
sound waves to return back to he ship.
The process of sea-floor spreading
• Harry Hess believed that the mid-ocean ridges
and deep-ocean trenches might help to explain
how ocean floor was formed.
• His hypothesis was ‘ in the process of sea-floor
spreading, new ocean floor forms along Earth’s
mid-ocean ridges, slowly moves outward across
ocean basins, and finally sinks back into the
mantle beneath deep-ocean trenches.’
New oceanic lithosphere is formed, and the ocean
floor gets wider. Example: The Atlantic ocean would
have been like the Red Sea.
• Eruption along Mid-Ocean Ridges
Scientists found evidence that is formed as a
result of volcanic activity;
1.Fractures along central valley of a mid-ocean
ridge fill with magma that wells up from the hot
mantle below. Gradually magma cools produce
new slivers ( strips of loose pieces) of ocean
floor. Spreading and upwelling ( flowing upward)
of magma continuously adds new ocean floor.
2. The process can also begin on land when a rift
valley forms and splits a continental landmass.
Over a long period of time, the rift valley widens
to form a new oceanic basin like the Red Sea.
Subduction at Deep-ocean Trenches
• Erath’s total surface area remains the same,
although new ocean floor is constantly being
added at the mid-ocean ridges. This happens
by Subduction ; it is returning the older
portions of the ocean floor back to the mantle
as it sinks beneath a deep ocean trench.
• The area where subduction occurs are called
subduction zones.
Evidence for sea floor spreading
• Evidences that support Hess’s hypothesis;
1. Magnetic strips in ocean-floor rocks.
Earth has its magnetic field. This field could
reverse its polarity ( the north magnetic pole
becomes south pole). Earth is said to have a
reverse polarity. Normal polarity means when
Earth magnetic field lines up in the same
direction as present magnetic field.
• When some rocks form, they acquire the
polarity of Erath’s magnetic field at that time.
They posses Paleomagnetism.
• Igneous rocks contain magnetite, iron-rich
mineral. As the rock cools and hardens, the
iron rich mineral grains become magnetized in
the same direction of as the existing magnetic
field.
• When the rock has been formed, its polarity
remains frozen unless it is reheated above
certain temperature. The rock’s
paleomagnetism does not change.
• Using magnetometers to study the
paleomagnetism of Basalt rock that makes up
the ocean floor. The data showed alternating
strips of normal and reverse polarity of
magnetized rock. Scientists inferred that a
new basalt rock forms along the mid-ocean
ridges became magnetized according to the
Earth’s polarity of that time.
2. Earthquake patterns
• The depth of certain earthquakes
• Scientists, Kiyoo Wadati and Hugo Benioff, found
patterns between the depth of earthquakes in
relation to their distance from deep-ocean
trenches
• Shallow-focus earthquakes occur in and around a
trenches
• Intermediate-focus and deep-focus earthquakes
occur in a belt 50 kilometers thick.
• The pattern was what scientists expected would
result from subduction of the ocean floor. At
subduction zones, the ocean floor returns to the
mantle (destroying crust.
3. The age of the ocean floor
• Drilling into sediments on the ocean floor and the
crust below it
• Data confirmed:
• Ocean floor is youngest along the central valley of the
mid-ocean ridge
• Ocean floor is the oldest in subduction zones or near
the edges of continents far from the ridge.
• Ocean floor is not older than 180 million years old.