Saturday 7th July
The fourth lecture/demonstration was “Earth Science: The Seismology story for ages 14 to 19” given by Stephen Davies, ESEU facilitator.
Looking inside the Earth with physics is needed to locate the materials (monitoring and metal detecting). Physics is needed for the mineral exploitation. Physics is even needed in archaeology. Physics produces the instruments for location, investigation and processing.
Steve is illustrating how little of the Earth we use. The radius of the Earth is 6300km but the deepest commercial mine is less than 12km.
The Earth has a magnetic core. A pivoting magnet can show the magnetic field in three dimensions. Magnetic rocks have produced evidence of sea floor spreading.
Coarse grain basalt (dolerite like) contains magnetism. It is a silica rich igneous rock. Coarse grain gabro does not contain individual grains of iron.
Iron occurs in many types of rocks, not usually in its metallic form, but as an oxide mineral called magnetite, which as the name suggests is magnetic. Just as all the molecules in the compass needle align themselves along the earth’s magnetic field, so do the molecules in the magnetite grains in rocks.
This will happen at the time a magnetite particle in a sediment or volcanic ash comes to rest, or in a lava (hot volcanic rock) as it cools to 500°C. Once the sediment layer is deposited and buried, or the lava flow has cooled below 500°C, the direction of the earth’s magnetic field as recorded by magnetite grains in these rocks cannot usually be changed by subsequent geological events (except for metamorphism—the process of changes to rock under the influence of elevated pressures and temperatures), even if the direction of the earth’s magnetic field has subsequently changed. This magnetism in the rocks is thus in essence ‘fossilised’, and so is usually called palaeomagnetism (fossil magnets). The existence of this palaeomagnetism in the rocks has claimed a lot of attention since the 1960s. At that time it was discovered that there were what appeared to be magnetic ‘stripes’ in the rocks on the ocean floor. The stripes represented sections in the rock of normal (the same as today) and reversed directions of the earth’s magnetic field, and this has been used as evidence for so-called sea-floor spreading and continental drift.
The lecture continued with seismic waves. There are two main types of wave (body waves): P or primary waves are longitudinal waves and S or secondary waves are transverse waves. Secondary waves don’t travel through liquids. Other types of waves are surface waves, Rayleigh, Love, and Stoneley which cause the most damage.
The springs are being used to show the main types of seismic waves and how one P waves can set up transverse waves. This explains how S waves stop at the liquid boundary but then re-start. The springs show incompressibility is important for the speed not the density.
A seismograph, or seismometer, is an instrument used to detect and record earthquakes. Generally, it consists of a mass attached to a fixed base. During an earthquake, the base moves and the mass does not.
Demonstrating how seismic waves can occur.
Demonstrating how Tsunamis form.
The sticks are placed on a vibrating plate. It is resonance that causes damage to buildings to occur. When an object is forced to vibrate at its natural frequency resonance occurs. It vibrates violently. This demonstration showed this and it also showed that you can reduce the likelihood of resonance by not using specific heights for the buildings.
Density results indicate that the core of the Earth is made up of iron. Types of meteorite have types of iron similar to the core. The Magnetism of the core must be constantly generated as the temperature is above the Curie temperature (the temperature at which the material loses its magnetism),
The crust is about 5km under the ocean. The mantle is solid with liquid properties. It is not rigid. The lithosphere (comprising the crust and the upper portion of the mantle) is the source of earthquakes. The asthenosphere is part of the upper mantle that shows liquid like behaviour (like plasticene). Rigid bits move it on. It is a low velocity area.
References: http://www.earthscienceeducation.com/index.html http://hypertextbook.com/facts/2003/YefimCavalier.shtml http://en.wikipedia.org/wiki/Seafloor_spreading http://en.wikipedia.org/wiki/Basalt http://www.geographyinaction.co.uk/Geology%20files/Dolerite.html http://en.wikipedia.org/wiki/Gabbro http://www.answersingenesis.org/articles/cm/v13/n3/fossil http://en.wikipedia.org/wiki/Seismic_wave http://en.wikipedia.org/wiki/Curie_temperature http://en.wikipedia.org/wiki/Structure_of_the_Earth http://en.wikipedia.org/wiki/Lithosphere http://en.wikipedia.org/wiki/Asthenosphere http://earthquake.usgs.gov/learn/glossary/?termID=167