1) In the 1960s, Harry Hess proposed the theory of seafloor spreading to explain continental drift, suggesting that new crust is formed at oceanic ridges and pushes plates apart. 2) As a Navy submarine commander, Hess used sonar to map features on the ocean floor, discovering evidence of seafloor spreading and helping develop his theory. 3) Plate tectonics divided the Earth's outer layer into major and minor tectonic plates that move at boundaries due to convection in the mantle, gravity, and the planet's rotation.

1. Harry Hess’ Theory,
Sonar and Plates.
By Tiara, Evelyn and Katelen

2. Hess’s Theory
In 1962, as a result of oceanographic research conducted in the 1950's, Harry
Hammond Hess proposed the theory of seafloor spreading to account for
continental movement. He suggested that continents do not move across oceanic
crust, but rather that the continents and oceanic crust move together. He suggested
that the seafloor separates at oceanic ridges and that new crust is formed by
upwelling magma.

3. Seafloor Spreading
As a result of the discovery of plate tectonics and the mapping of the earth into 12 plates, plus the understanding
of that plate movements create earthquakes, Alfred Wegener’s idea of the continental drift looked less ridiculous
than his superiors believed. There is still no way that continents could plow through the earth’s surface on their
own but there is something else that could explain how the land masses had once been joined.
Harry Hess, who was a geologist and Navy submarine commander during World War II. His mission had been to
study the depths of the ocean floor. In 1946, he discovered that hundreds of flat-topped mountains, that could
have perhaps been sunken islands, shape the floors of the Pacific. In 1960, he proposed that the movement of
continents resulted in sea-floor spreading. Hess added a geologic mechanism to relate to Alfred Wegener’s
account of moving continents. He said that it is possible that molten magma from beneath the crust of earth could
ooze out between the plates. As the hot magma cools in the ocean water, this expands and pushes the plates on
either side of it (North and South to the West and Eurasia and Africa to the East.) As a result of this, the Atlantic
Ocean would grow wider but the coastlines of the landmasses would not change as dramatically.
Harry Hess proved Alfred Wegener’s idea right and clarified the mechanism that explained the separation of a
once-joined continent into the seven individual ones we are now familiar with. The continents are connected to the
plates and do not move independently. The plates however shift and change shape, carrying the continents along
with them.

4. Sonar
Sonar is a technique that uses sound waves to detect object underwater by
emitting waves and measuring their return (echo) after being reflected. Sonar
stands for SOund Navigation And Ranging.
There are two types of Sonar -
Passive Sonar : Listening to the sounds of objects or vessels underwater.
Active Sonar : Emitting pulses of sound and listening for echoes.
In this situation, Hess uses sonar to calculate the distances using the Active
Sonar Method.

6. How did sonar help Hess’ Theory?
Harry Hess was a geology professor and Navy reservist. So it
was not long before Hess became a Navy Submarine
Commander during World War II. Therefore while he was
travelling from destination to another, Hess would leave the
sonar system on, so that it can take measures of the sea
floor. It was at this time that Hess discovered features on the
floor which led him to conclude a theory of seafloor spreading.
Without the help of sonar, Hess would have possibly not have
come up with his theory.

7. Theory of plate tectonics
The lithosphere is the firm outer shell of a planet, in this case- Earth. It is
divided into various tectonic plates. On Earth, there are 7 major tectonic
plates:
African plate
North American plate
Antarctic plate
Pacific Plate
Eurasian plate
South American Plate
Indo-Australian Plate
There are many other minor plates. At the point where plates meet defines
what type of boundary is formed: convergent, divergent or transform.

9. Three main forces that drive the
movement of the plates: mantle
convection, gravity & the Earth’s
rotation.