UPPER SIXTH
LONG TERM CHANGE - VOLCANICS

The Development Of The Knowledge Of The Inside Of The Earth

1906        R. D. Oldham suggests that the Earth has a liquid core.
1909        A Croatian seismologist named Adrija Mohorovicic discovered a change in material composition of the mantle at about 6 - 60 km deep, which enables waves to travel more rapidly. This is named the Mohorovicic Discontinuity (the Moho).
1913        Beno Gutenburg calculates the depth of the core-mantle boundary from the sudden change in P-wave velocity at 2900 km. Now called the Gutenburg discontinuity.
1936        Inge Lehemann suggests that the core has a solid ‘inner core’ on the basis of travel times of waves passing through it.
1955        Radioisotopic dating discovered. Scientists are now able to accurately predict how old rocks are.
1963        The Atmospheric Test Ban Treaty forbids surface nuclear testing. Testing continues, but is forced underground.
Sensitive seismographs are developed by the military to analyse shockwaves from the tests. The US seismograph is based at Billings, Montana. It is known as LASA, the Large Aperture Seismic Array. It covers a highly surveyed area, covered in highly sensitive seismic sensors. The overall quality of the seismic data increases.
                Vine and Matthews publish their theory on paleomagnetic strips as evidence for ocean floor spreading.
1969        NASA analysis of a meteorite discovers a 450 million year old rock which is of similar composition to the Sun.
1970        The invention of highly sensitive seismographs confirm the suggestions of Lehemann.

The Evolution of Ideas;
To begin with, no-one really could explain why the continents were where they were. It was not until the 20th century that people really understood what was happening. Although the Navigational charts for the Atlantic in the 16th century had noticed the jigsaw fit between the West African coast and the eastern seaboard of South America. Then, in the twentieth century, a series of geographers published work which led to the overall development of a model which we now know as plate tectonics.

Richard Oldham
·       Was a geologist working in British India at the turn of the twentieth century.
·       He realised that the centre of the earth is denser, since it slows sound waves down.
·       There is a fundamental difference between P and S wave transmission in liquids and solids.
·       He realised that since S waves were not detected in the shadow of the “core” it must be a liquid.
Alfred Wegener, 1912-1915.
·       Lived from 1880 - 1930. Published his work during the years above. He was a German explorer and balloonist, and his work was not well regarded because of this.
·       He is regarded as the “Father of Continental Drift”, because of his work which set out clearly the evidence for drift.
·       The evidence is as follows;
Ž   The idea of the ‘jigsaw fit’ of the major continents, especially West Africa and South America.
Ž   Relic terminal moraines (tillites) from the late carboniferous supercontinent South polar glaciation. The Pangea supercontinent fits together and was covered in an ice sheet. The tillites mark the limits of this glaciation.
Ž   The geological evidence of the fold mountains in NE Brazil and the Gulf of Guinea which have the same trends and axes of folding.
Ž   Fossil evidence. This exists in both animal and biotic form. Fossils of Mesosaurus, a small alligator like reptile, have been found in both Africa and S. America. There is no way it could have swum across the Atlantic. There is also a carboniferous plant named Glycopteris, which is common to both continents. This must only have occurred when they were joined.
Arthur Holmes, 1944.
·       Wrote a book entitled “The Principles of Physical Geology”.
·       It offered support for the theory of continental drift, but suggested nothing in the way of mechanisms or proofs.
Vine and Matthews, 1963.
·       These were students from Cambridge, working on Magnetic data from the North Atlantic.
·       They proposed a mechanism of sea floor spreading, which leads to continental drift.
·       This evolved to Plate Tectonic Theory; “a coherent model of how the outer part of the Earth evolves”.
·       It includes the related ideas of;
Ž   Continental Drift. Ž   Sea floor spreading.Ž   Volcanics.Ž   Seismic activity.Ž   Crustal structures.

Evidence For the Earth’s Structure
It goes without saying that we cannot dig to the centre of the Earth. Even if we had the technology, which we don’t, it would be far too hot for us to cope with. We must therefore predict the content of the Earth by analysing other sources. This evidence is presented here;
·       Volcanoes; analysis of magma gives us an idea of the content of the rocks at depth.
·       Solar System; Big Bang theory states that we are all formed from the same material. Therefore, analysis of meteorites and the Sun will give some idea of the composition of the Earth.
·       Comparisons of the densities of mantle rock and the known density of the Earth suggests that the Earth has a more dense core.
·       Magnetic field of the Earth; Elizabeth 1st’s physician discovered that there was a magnetic field in the Earth. The core acts a self exciting dynamo. The outer core crystallises, releasing heat and generating a magnetic field. The flow of current is aligned with the axis of the Earth. This causes magnetic variation. From 1827 to 1967, the alignment had varied by some 20°.
·       Analysis of seismic wave paths shows which parts are liquid or solid.
Results;·       Mantle             -               Peridotite
·       Outer Core      -               Liquid Iron           Inner Core       -               Solid Iron.

Brief Comparison of Seismic Waves.
Seismic waves are released when an earthquake happens. They propagate through the Earth in several different forms, and
are classified according to their direction of propagation.

Abbreviation

Name

Speed

Description

P

Primary waves

5km/sec

Can pass through anything.

S

Secondary Waves

3km/sec

Cannot pass through liquid

L

Surface (Rayleigh-Love)

Most of the energy of shallow earthquakes.