Density Structure of Earth -
Sea-Floor Spreading -
Mechanical Structure of Earth -
Driving Mechanism of Plate Tectonics -
Plate Tectonics and the Depth of the Seafloor -
The density structure of Earth is based on density and composition.
Using this classification system, Earth is divided into three layers with different densities and compositions:
The crust is the outside of Earth, the mantle underlies the crust, and the core is in the center.
Early in Earth's history, it segregated by density.
Most likely Earth melted because of the energy released during its formation by aggregation.
The more dense elements sank to the core, and the least dense elements rose to the crust.
The segregation resulted in density differences:
The process of segregation also resulted in compositional differences.
Each layer has a different average elemental composition:
Silicon (Si) and oxygen (O) are the two essential elements in silicates.
The shorthand notation for silicate is SiO2
Al - aluminum
Fe - iron
Mg - magnesium
Why do oceans and continents exist?
Some parts of the crust are high, above sea level, and others are low, below sea level.
Earth has two types of crust:
Continental crust has a higher average elevation - 840 m above sea level.
Oceanic crust has a lower average elevation - 3800 m below sea level.
The difference in average crustal elevations between continental and oceanic crust is:
Currently Earth has sufficient surface water to fill the low parts of the
crust and barely lap up on to the high crust.
Although the shoreline does not coincide with the boundary between continental and oceanic crust, the distribution of land and ocean approximates the extent of continental crust and oceanic crust.
Scientists used the theory of isostasy to explain the large elevation difference, an average of almost 5000 m, between the two types of crust.
Isostasy - a condition of equilibrium (comparable to buoyancy) in which the
crust floats on the mantle.
The crust and the mantle are almost entirely solid rock; however, the behavior of the crust indicates that it floats on the mantle.
The theory of isostasy was developed because mountains have deep roots.
These roots form as mountains grow and force the crust to sink into the underlying mantle rock.
Isostasy implies two concepts:
The Crust Is Buoyant
Crust moves up and down in response to changes of mass.
When mass is added to the crust, such as the formation of mountains, the crust sinks under the additional mass.
As mountains erode, the crust once again rises.
The Mantle Behaves Like a Fluid
To allow the crust to sink, the mantle must flow out of the way.
Where the crust rises, the mantle flows back under the crust.
Solid matter flows under certain conditions.
In particular, when a solid is very hot and close to its melting point, it can lose strength and begin to flow.
The different elevations of continental crust and oceanic crust can be explained
using the theory of isostasy.
Continental crust float higher on mantle rock than oceanic crust because it is:
The fundamental difference between continental and oceanic crust is compositional:
Fe, Mg silicate is more dense than Al silicate.
The compositions of continental crust and ocean crust are different because
each type of crust is formed by a different process.
The source of the magma that forms oceanic crust is melted mantle rock.
Humans have long believed that the continents of South American and Africa
were joined once.
200,000,000 years ago, most of Earth's landmasses were joined in the supercontinent Pangaea.
In 1912, Alfred Wegener wrote a book in which he provided evidence that the continents drift (move).
However, many geophysicists considered Wegener's theory of continental drift preposterous.
Bathymetry - the measurement of ocean depth
They asked what the driving mechanism would be?
What makes the continents move?
Mid-Ocean Ridges and Ocean Trenches
Until the early 1900s, humans knew little about the bathymetry of the sea floor.
The development of sonar led to the discovery of a submerged mountain range.
It is the longest geological feature on Earth and runs through the Atlantic, Indian, and Pacific oceans.
In the Atlantic Ocean, the ridge conforms to the edges of the continents.
The submarine mountain range is called the mid-ocean ridge - MOR
The depth of the summit of the MOR averages 2 km below sea level, and it is a volcanic region.
Scientists also discovered long, linear ocean trenches.
These are the deepest parts of the crust, up to 11 km below sea level.
Commonly these trenches are associated with the edges of continents.
Most trenches surround the Pacific.
Theory of Sea Floor Spreading
Theory that the sea floor spreads.
This theory states that the sea floor is created at the MOR.
From the MOR the sea floor spreads out in opposite directions, perpendicular to the trend of the MOR.
Ultimately the sea floor is subducted and destroy in ocean trenches.
To understand how the continents can drift and the sea floor can spread, scientists
view the structure of the earth in a different way.
They use a structure based on strength and viscosity:
The lithosphere is the outer most shell of the earth.
It includes all of the crust and uppermost mantle.
It averages approximately 80 km thick
It is relatively strong compared to the asthenosphere.
It has a relatively high viscosity compared to the asthenosphere.
It has a relatively low temperature compared to the asthenosphere.
The asthenosphere underlies the lithosphere
It extends from 80-350 km deep.
It is relatively weak compared to the lithosphere.
It has a relatively low viscosity compared to the lithosphere.
It has a relatively high temperature compared to the lithosphere (the upper part is near its melting point).
A small percentage, 1-2%, of the upper asthenosphere is magma.
This region is the source of magma that forms the sea floor.
Mesosphere underlies the asthenosphere:
It extends to the core and contains most of the mantle
It is more viscous than the asthenosphere, but less viscous than the lithosphere
It likely flows when stress is applied
Core at the center:
The inner core is solid, outer core is liquid
The originally-accepted driving mechanism for plate motions was thermal convection.
Part of the mantle become hotter than surrounding material.
This solid rock expands, becomes less dense, and rises to the base of the lithosphere.
Once it reaches the base of the lithosphere, a small percentage melts to form the magma that creates new sea floor at the summit of the MOR.
However most of the solid rock spreads out laterally from the MOR.
Eventually the convecting rock cools, contracts, becomes more dense, and sinks.
Oceanic lithosphere subducts at the deep ocean trenches.
The circulation of mantle rocks forms convection cells in the asthenosphere.
In this hypothesis, the lithosphere moves because it sits on top of convecting (flowing) asthenosphere.
Features seen with increasing distance from the MOR:
Seafloor spreading can account for these characteristics.
Increasing Age of the Seafloor
Seafloor is created at the MOR and moves away in opposite directions
Therefore, the seafloor is young at the MOR and becomes progressively older with increasing distance from MOR.
Decreasing Rate of Heat Flow
Seafloor is created at the MOR by volcanic processes.
Scientists measure relatively high rates of heat flow from the volcanic region at the summit of the MOR
As the newly created seafloor is carried away form the MOR, it cools with increasing age.
Seafloor complete cools in about 100 million years. (100,000,000 yrs = 108 yr)
Increasing Ocean Depth
At the MOR, the seafloor is young and relatively hot.
The higher temperature results in oceanic lithospheric rock that has a relatively low density.
As the newly formed lithosphere spreads away from the MOR, it get older, colder, contracts, and becomes more dense,
The progressively increasing density results in the oceanic lithosphere sinking lower into the asthenosphere.
This process is explained by the theory of isostasy.
The depth of the ocean is a function of its age: the older it is, the deeper its depth.
To calculate the average rate at which the seafloor subsides, one needs the distance and time of the sinking:
The total increase in depth:
Therefore one can say that the rate at which the ocean generally increases
in depth as it ages is 4 km/100,000,000 yr.
However, scientists like to see rates with distance units that are easy to visualize (e.g. centimeters per a number of years), which are easier to visualize.
To convert this rate, change kilometers to meters and, then, meters to centimeters.
4 km/100,000,000 yr * 1000m/1 km *100 cm/1 m = 400,000 cm/100,000,000 yr = 4 cm/1000 yr
4 cm/1000 yr would be the answer to the question, "Calculate the average rate of subsidence of the sea floor in cm/1000 yr."
The first exam notes continue in