Hazardous Earth
There is a Variety of Evidence for the Theories of Continental Drift and Plate Tectonics
BASIC STRUCTURE OF THE EARTH
• 3 primary concentric layers- the core, the mantle and the crust
• Core and Mantle separated by a sharp boundary at a depth of around 2900km
• Mohorovicic (Moho) Discontinuity- lies around 35km below continental plate and 10-15km below oceans
• Upper mantle consists of two layers
◦ Upper layer called the lithosphere (down to 100km below ground level)- rigid structure, moved by the
liquid layers below it- varies in thickness and boundary
◦ Asthenosphere (100km-300km)- semi-molten and viscous, capable of flowing slowly
◦ Lithosphere starts to melt and becomes incorporated in asthenosphere
◦ Lithosphere and Crust makes up tectonic plates
• Within the asthenosphere, convection currents exist, caused by the vast amounts of heat generated in the
mantle- semi-molten ASP therefore carries with it the solid LSP and crust
EVIDENCE FOR CONTINENTAL DRIFT
The Idea of Alfred Wegener
• In 1912, Wegener made a case for continental drift, proposing that a large single continent, Pangaea,
existed during the Carboniferous period, which later separated into Laurasia and Gondwanaland
Geological Evidence:
◦ South America and Africa fit side by side
◦ Evidence from about 290m years ago show the effects of contemporaneous glaciation in Southern
Africa, Australia, South America, India and Antarctica, indicating the land masses were joined at the
time
◦ Mountain chains and rock sequences are similar on either side of oceans, such as Appalachians and
Highlands
Biological Evidence:
◦ Similar fossil brachiopods found in Australian and Indian limestones
◦ Similar fossil reptiles found in South America and South Africa
• However, Wegener's theory failed to provide a mechanism to explain the movement of continents, hence
there were still many sceptics
, Paleomagnetism and Sea Floor Spreading
• Trackers used on submarines picked up tiny variations in oceans' magnetic field
• Explanation begins with igneous rock- previously lava flows and contain iron particles
◦ As lava erupts, ice cools and magnetic orientation of iron particles locked into rock, depending on
Earth's polarity at the time
◦ Every 400-500k years, the Earth's polarity changes, recorded by the rock on the ocean's floor- record
of this is called paleomagnetisms
• Idea of Sea Floor Spreading brown as a result of paleomagnetism
◦ The width of each strip of ocean bed with same magnetic orientation found to correspond with time
scale of each magnetic reversal
◦ Symmetrical pattern of geomagnetic reversals indicate that as fresh molten rock from asthenosphere
reached Earth's surface, older rock was pushed away from the ridge
There are Distinctive Features and Processes at Plate Boundaries
PATTERN AND LOCATION OF PLATE BOUNDARIES
• E.g. Eurasian, North America, Indo-Australian, African
• Most earthquakes were spatially concentrated in narrow bands, suggesting that rigid lithosphere and
crust were broken into tectonic plates
DIVERGENT PLATE BOUNDARIES
• Areas where plates are moving apart, with magma rising through the asthenosphere and forcing its way to
the surface
• Mostly takes place at Mid-Ocean ridges, which are broken into segments at frequent intervals by
transform faults
◦ Volcanic activity absent along these faults but can cause earthquakes as they slip
• The eruption of magma mostly occurs underwater- cooled rapidly onto sea bed, forming pillow lavas
• As magma rises to the surface, overlying rocks can be forced up into a dome, forming underwater rift
valley (more later on in notes)
There is a Variety of Evidence for the Theories of Continental Drift and Plate Tectonics
BASIC STRUCTURE OF THE EARTH
• 3 primary concentric layers- the core, the mantle and the crust
• Core and Mantle separated by a sharp boundary at a depth of around 2900km
• Mohorovicic (Moho) Discontinuity- lies around 35km below continental plate and 10-15km below oceans
• Upper mantle consists of two layers
◦ Upper layer called the lithosphere (down to 100km below ground level)- rigid structure, moved by the
liquid layers below it- varies in thickness and boundary
◦ Asthenosphere (100km-300km)- semi-molten and viscous, capable of flowing slowly
◦ Lithosphere starts to melt and becomes incorporated in asthenosphere
◦ Lithosphere and Crust makes up tectonic plates
• Within the asthenosphere, convection currents exist, caused by the vast amounts of heat generated in the
mantle- semi-molten ASP therefore carries with it the solid LSP and crust
EVIDENCE FOR CONTINENTAL DRIFT
The Idea of Alfred Wegener
• In 1912, Wegener made a case for continental drift, proposing that a large single continent, Pangaea,
existed during the Carboniferous period, which later separated into Laurasia and Gondwanaland
Geological Evidence:
◦ South America and Africa fit side by side
◦ Evidence from about 290m years ago show the effects of contemporaneous glaciation in Southern
Africa, Australia, South America, India and Antarctica, indicating the land masses were joined at the
time
◦ Mountain chains and rock sequences are similar on either side of oceans, such as Appalachians and
Highlands
Biological Evidence:
◦ Similar fossil brachiopods found in Australian and Indian limestones
◦ Similar fossil reptiles found in South America and South Africa
• However, Wegener's theory failed to provide a mechanism to explain the movement of continents, hence
there were still many sceptics
, Paleomagnetism and Sea Floor Spreading
• Trackers used on submarines picked up tiny variations in oceans' magnetic field
• Explanation begins with igneous rock- previously lava flows and contain iron particles
◦ As lava erupts, ice cools and magnetic orientation of iron particles locked into rock, depending on
Earth's polarity at the time
◦ Every 400-500k years, the Earth's polarity changes, recorded by the rock on the ocean's floor- record
of this is called paleomagnetisms
• Idea of Sea Floor Spreading brown as a result of paleomagnetism
◦ The width of each strip of ocean bed with same magnetic orientation found to correspond with time
scale of each magnetic reversal
◦ Symmetrical pattern of geomagnetic reversals indicate that as fresh molten rock from asthenosphere
reached Earth's surface, older rock was pushed away from the ridge
There are Distinctive Features and Processes at Plate Boundaries
PATTERN AND LOCATION OF PLATE BOUNDARIES
• E.g. Eurasian, North America, Indo-Australian, African
• Most earthquakes were spatially concentrated in narrow bands, suggesting that rigid lithosphere and
crust were broken into tectonic plates
DIVERGENT PLATE BOUNDARIES
• Areas where plates are moving apart, with magma rising through the asthenosphere and forcing its way to
the surface
• Mostly takes place at Mid-Ocean ridges, which are broken into segments at frequent intervals by
transform faults
◦ Volcanic activity absent along these faults but can cause earthquakes as they slip
• The eruption of magma mostly occurs underwater- cooled rapidly onto sea bed, forming pillow lavas
• As magma rises to the surface, overlying rocks can be forced up into a dome, forming underwater rift
valley (more later on in notes)
