Seafloor Spreading

Henry Hess: 

Earth’s crust and the upper part of the mantle make up the relatively cool, rigid outer layer of the planet = lithosphere

lithosphere is broken into pieces = tectonic plates

The tectonic plates are constantly moving together and apart

the theory of plate tectonics explains how Earth’s surface is recycled over time

Henry Hess was in the marines and studied the ocean floor

Hess theorized that the ocean floor is at most only a few hundred million years old, significantly younger than the continents

while cruising from one battle to the next, Hess kept the transport’s sounding gear (which bounced sound waves off the sea-floor in order to determine the underwater relief or topography) running day and night

led to his discovery of submerged and curiously flat-topped mountains that he named “guyots” in honor of the Swiss founder of the Princeton geology department

In 1962, he proposed a groundbreaking hypothesis that proved vitally important in the development of plate tectonic theory

• If the oceans have existed for at least 4 billion years, why has so little sediment accumulated on the ocean floor? 

• Why are fossils found in ocean sediments no more than 180 million years old? 

• And how do the continents move?

Hess explained how the once-joined continents had separated into the seven that exist today

Hess also theorized that because the continental crust was lighter, it didn’t sink back into the deep earth at trenches as did the oceanic crust

Instead, it scraped rock off the descending ocean crust and piled it into mountain rages at the trenches’ edge

Long story short...Hess determined HOW the continents move

Seafloor Spreading:

plate movements are responsible both for the formation of certain features on Earth's ocean floor and continental crust

and also the destruction of Earth's crust at subduction zones (areas where a denser plate slides beneath a less dense plate)

the denser plate then melts in Earth’s mantle

new oceanic crust is formed at mid-ocean ridges = sea floor spreading

mid-ocean ridges are oceanic rift zones

they consist of long mountain chains along a central rift valley

mid-ocean ridges are divergent plate boundaries (tectonic plates move apart and seafloor spreading occurs)

seafloor spreading = new oceanic lithosphere forms at mid-ocean ridges

as tectonic plates move away from each other, magma rises from Earth’s interior

it then cools and solidifies in the center of the ridge

rising magma pushes up between the plates and drives them further apart

as new crust is forming at the spreading center, it pushes the older crust apart

the oceanic crust contains symmetrical patterns of crustal rock ages. These rocks increase in age as their distance from the mid-ocean ridge  increases.

Evidence of Seafloor Spreading:

Earth’s magnetic field occasionally reverses its polarity

this field is not static—it moves all the time

for this reason, magnetic north and true north (the north end of Earth’s rotational axis) do not always face the same direction

true north is a fixed geographic location, but magnetic north is what attracts the needle in a compass

magnetic north migrates over time in a phenomenon called polar wandering. So, magnetic north does not always match up with true geographic north

Earth’s magnetic poles completely flip

throughout geologic history, Earth’s magnetic field has completely reversed many times

Paleomagnetism is the study of the fixed orientation of a rock's magnetic minerals as originally aligned at the time of the rock's formation (simply, old magnetism)

• igneous rocks as they cool

• As rocks (with iron in them) solidify, the magnetism direction points to the magnetic pole

• Igneous rocks may keep their magnetic orientation they obtain at the time they form (if they are not altered)

• This magnetic signature is preserved

• Paleomagnetism studies are fundamental to the theory of seafloor spreading

• provides evidence of different ages of crust (new crust being close to mid-ocean ridge and oldest crust being the furthest away

these reversals have happened many times throughout Earth’s history, but they do not happen at a predictable rate

scientists can study these magnetic pole reversals by studying the magnetic polarity of rocks in Earth’s crust

the oceanic crust contains symmetrical bands of alternating magnetic polarity on each side of mid-ocean ridges

Effects of magnetic polarity

• it is unlikely that a reversal would be harmful to most life on Earth

• Animals that rely on magnetic fields for navigation and migration would likely be able to adapt since pole reversals seem to occur very slowly in relation to an individual’s life span