Week 2 - Internal Structure of the Earth, Isostasy, Continental Drift
Week 2 - Internal Structure of the Earth, Isostasy, Continental Drift Sunday, 29 June 2014 3:10 PM
Earth Structure and Plate Tectonics
Seismographs
• Earths radius is 6400km • Only upper 9km has ever been penetrated • Seismic shock waves provide most compelling evidence of the composition of earths crust ○ Waves move at different velocities depending on the composition of the medium involved ○ :. Measuring the velocities of the waves = idea of earths composition Types of Earthquake waves 1. Body Waves - travel through the interior of the earth a. P Waves - push - pull waves Travel through solids, liquids and gasses Consist of concessive rarefactions and compressions of the material through which they travel Similar to sound waves b. S Waves - shake waves - can only travel through materials capable of resisting change in shape (solids) Particles vibrate at right angles to the direction of wave advance
Instrument capable of measuring various earthquake waves Connected to earth by springs and pendulums - weighed down by a great mass Relative movement between the earth and the mass five rise to a trace of the earthquake waves passing through a given location. Seismographic Records Different types of waves travel at different velocities :. Picked up at different times by seismograph P waves travel faster than S waves :. The farther away the earthquakes centre (focus) the greater the distance of the two waves.` ○ Epicentre: the point on the ground directly above an earthquakes focus ○ Provided records from at least 3 stations are available the epicentre can be located with reasonable precision.
Screen clipping taken: 18/07/2014 5:54 PM
Screen clipping taken: 18/07/2014 5:46 PM
2. Surface waves - travel along the earths surface
Boundaries within the Earth - Based upon our knowledge of seismic records The Mohorovicic discontinuity ○ In 1908 Mohorovicic examined P and S waves associated with Croatian earthquakes ○ Noticed that some waves strike something deep in the earth that causes them to reflect some energy back to the surface ○ Also noted that wave velocity abruptly increases for depths greater than around 50km The Oldham discontinuity ○ In 1906 Oldham correctly inferred that the Earth has a core which acts like a spherical lens by refracting the deeper waves inwards and so concentrating them in the antipodal region at the expense of the surrounding 'zone of shadow' Antipodal Region: The diametrically opposite point on a sphere Zone of Shadow: Area of the mantle surrounding the core ○ P waves enter the core - velocity decreases sharply ○ S wave Donnot enter the core ○ Boundary mantle and core is also a boundary between two different states of matter (liquid / solid) - Low velocity zone in upper mantle - Aesthenosphere - Inner core solid, outer core liquid - Earths magnetic field is generated by the movement of the two cores relative to one another Flipping of the Magnetic Fields… When the N and S poles reverse Geomagnetic Reversal has occurred 9 times in the last 4 million years Evident because the minerals in the earths new crust are aligned differently in some regions - highlighting that during their formation the poles were flipped Estimated to be around 1000 years away from the next phase of field changes CAUSE UNKNOWN - possibly due to earths core.
Essential Characteristics of 3 Major Internal Zones CORE ○ Radius of 3370km - extremely dense ○ Consists predominately of iron and nickel ○ Inner solid, outer liquid ○ Magnetic field thought to be generated by movements of the liquid outer core
GEO164 Page 2
Screen clipping taken: 18/07/2014 9:37 PM - example of epicentre ( E ) location , known distanced from E to three recording centres are plotted as circles. The point of intersection of the circumfrance is unique and gives the location of E.
When waves pass from one material to another they are refracted (bent), reflected and may undergo a change in velocity Body waves travel directly through the earth :. Distance from the epicentre can imply how deep the wave penetrate the earth ○ e.g. for a body wave to reach a distance of 2400km from the epicentre it must penetrate to a depth of 480km
Isostasy Isostasy: the study of forces exerted on the underlying mantle by crust materials Deducted that mountain ranges contain less mass ○ Current explanation is that mountain ranges are underlain by deep roots of siliac material →less than adjacent sima. ○ :. Even though there is more crust in mountainous areas →mass is deficient by the virtue of the great underlying depth of low density
Earth Structure and Plate Tectonics
Seismographs
• Earths radius is 6400km • Only upper 9km has ever been penetrated • Seismic shock waves provide most compelling evidence of the composition of earths crust ○ Waves move at different velocities depending on the composition of the medium involved ○ :. Measuring the velocities of the waves = idea of earths composition Types of Earthquake waves 1. Body Waves - travel through the interior of the earth a. P Waves - push - pull waves Travel through solids, liquids and gasses Consist of concessive rarefactions and compressions of the material through which they travel Similar to sound waves b. S Waves - shake waves - can only travel through materials capable of resisting change in shape (solids) Particles vibrate at right angles to the direction of wave advance
Instrument capable of measuring various earthquake waves Connected to earth by springs and pendulums - weighed down by a great mass Relative movement between the earth and the mass five rise to a trace of the earthquake waves passing through a given location. Seismographic Records Different types of waves travel at different velocities :. Picked up at different times by seismograph P waves travel faster than S waves :. The farther away the earthquakes centre (focus) the greater the distance of the two waves.` ○ Epicentre: the point on the ground directly above an earthquakes focus ○ Provided records from at least 3 stations are available the epicentre can be located with reasonable precision.
Screen clipping taken: 18/07/2014 5:54 PM
Screen clipping taken: 18/07/2014 5:46 PM
2. Surface waves - travel along the earths surface
Boundaries within the Earth - Based upon our knowledge of seismic records The Mohorovicic discontinuity ○ In 1908 Mohorovicic examined P and S waves associated with Croatian earthquakes ○ Noticed that some waves strike something deep in the earth that causes them to reflect some energy back to the surface ○ Also noted that wave velocity abruptly increases for depths greater than around 50km The Oldham discontinuity ○ In 1906 Oldham correctly inferred that the Earth has a core which acts like a spherical lens by refracting the deeper waves inwards and so concentrating them in the antipodal region at the expense of the surrounding 'zone of shadow' Antipodal Region: The diametrically opposite point on a sphere Zone of Shadow: Area of the mantle surrounding the core ○ P waves enter the core - velocity decreases sharply ○ S wave Donnot enter the core ○ Boundary mantle and core is also a boundary between two different states of matter (liquid / solid) - Low velocity zone in upper mantle - Aesthenosphere - Inner core solid, outer core liquid - Earths magnetic field is generated by the movement of the two cores relative to one another Flipping of the Magnetic Fields… When the N and S poles reverse Geomagnetic Reversal has occurred 9 times in the last 4 million years Evident because the minerals in the earths new crust are aligned differently in some regions - highlighting that during their formation the poles were flipped Estimated to be around 1000 years away from the next phase of field changes CAUSE UNKNOWN - possibly due to earths core.
Essential Characteristics of 3 Major Internal Zones CORE ○ Radius of 3370km - extremely dense ○ Consists predominately of iron and nickel ○ Inner solid, outer liquid ○ Magnetic field thought to be generated by movements of the liquid outer core
GEO164 Page 2
Screen clipping taken: 18/07/2014 9:37 PM - example of epicentre ( E ) location , known distanced from E to three recording centres are plotted as circles. The point of intersection of the circumfrance is unique and gives the location of E.
When waves pass from one material to another they are refracted (bent), reflected and may undergo a change in velocity Body waves travel directly through the earth :. Distance from the epicentre can imply how deep the wave penetrate the earth ○ e.g. for a body wave to reach a distance of 2400km from the epicentre it must penetrate to a depth of 480km
Isostasy Isostasy: the study of forces exerted on the underlying mantle by crust materials Deducted that mountain ranges contain less mass ○ Current explanation is that mountain ranges are underlain by deep roots of siliac material →less than adjacent sima. ○ :. Even though there is more crust in mountainous areas →mass is deficient by the virtue of the great underlying depth of low density
