Week 2 Flashcards
How do we know that the earth is 4.6 billion years
The oldest Rock/ Continental Crust is 4.6 Billion yrs old
Australia and Canada has the oldest overriding tectonic plates and this is also the oldest rock. (Found in canada)
Hence the age of the earth is derived from the age of the oldest rock
What is the oldest rocks called
Cratons
What are the four major structure
- Crust (Oceanic -5km and Continental- 30 to 40km)
- Mantle- 2885km
- Outer Core- 2270km
- Inner Core- 1216 km
each layer has a unique chemical composition, physical state and can impact life on earth’s surface
The internal dynamics of the earth
The lithosphere is the solid, outer part of the Earth
The lithosphere includes the brittle upper portion of the mantle and the crust, the outermost layer of Earth’s structure
It is bounded by the atmosphere above and the asthenosphere below.
What was Alfred Wegener Concept of Continental Drift
He believe that earth’s continents had moved over geologic time relative to each other, thus appearing to have drifted across the ocean bed
- the continents seems to fit together like puzzle pieces
- if you fit them together, you can match fossils of animals and plants across the continental edges
- Glacial striations also match across the continental edges which suggested a common origin point
Why were people skeptical of Wegener’s idea of continental drift
- they knew that most of the earth is solid rock
- only the outer core is molten rock
- A slice into the earth shows a solid inner core and a liquid outer core, both composed of nickel and iron
- The earth’s solid mantel makes up most of the volume of the earth
- The earth’s crust on which we live on is as thin as a line at this scale
HENCE HOW CAN CONTINENTS DRIFT ON A SEA OF SOLID ROCK? - There was a lack of mechanism for drift
Why is plate tectonics a scientific theory
it explains how major landforms are created as a result of Earth’s Lithosphere movements which can explain many phenomena, including mountain building events. volcanoes and Earthquakes
How was Wegener’s Idea revived
By Harry Hess
By Bruce Heezen and Marie Tharp
What did Harry Hass do
Mapped the ocean floor bathymetry during world war 2, which provided one of the most important evidence to support plate tectonics theory
What did Bruce Heezen and Marie Tharp do?
Mapped the Mid-Atlantic ridge and found that the mid ocean ridge was higher
Magnetization patterns on sea floor
Ocean crust is striped with parallel bands of magnetized rock with alternating polarities
Stripes are parallel to the mid-ocean ridge and the pattern of the stripes are symmetrical
Concept of Geodynamo
The earth contains a liquid iron-nickel outer core
The convection and rotation of this conductive fluid creates the geodynamo which is a self sustaining magnetic field that encompasses the entire Earth. The magnetic field of the earth can flip its polarity
The earth’s magnetic field
Inclination - tilt of the compass needle from horizontal. Depends upon
1. Normal or reversed polarity
2. Latitude
Process of magnetization
- When a volcanic rock erupts, it contains particles of magnetic minerals and these minerals would want to align with the Earth’s magnetic field
- But the magnetic particles can only move when the rock is hot as once it cools down, they are frozen in place
- It turns out that the earth’s magnetic field periodically reverses
- When this occurs, we get some rocks with magnetic fields frozen in current orientation and some frozen in reversed orientation
Ocean spreading centers
- Linear boundary between two diverging lithospheric plates on the ocean floor
- Two plates move apart from each other
- Molten rock wells up from the underlying mantle into the gap between plates
- Solidifies into new oceanic crust
- Found at the crest of oceanic ridges
Summary of the Evidences (in addition to Wegener’s evidences )
- Global Seismicity delineate the plate boundary
- Deep seismicity indicate the slab (old, heavier plate) has been subducted into the deeper part of the earth
- The magnetic anomaly across the mid-ocean ridge supporting the spreading of the seafloor
- The mid-ocean ridges are higher than the nearby seafloor
What are the three types of plate boundaries
- Convergent
- Divergent
- Transform
What are divergent boundaries
- Where two plates are moving apart
- The space created can be filled with new crustal material sourced from the molten magma that forms below
- Divergent boundaries can form within continents but eventually they will open up and form oceanic basins
- On land they produce rift valleys
Under the ocean they produce mid oceanic ridges
Why are some young oceanic floor wider than other young oceanic floors
The speed that it moves apart from the mid oceanic ridge is faster hence wider
Why is oldest ocean floor the deepest
The rocks become cooler, hence they become denser and slowly sink lower and lower into the mantle
Driving force in plate tectonics
Convection of the Earth’s solid mantle raises hot rocks beneath mid-ocean ridges. These rocks spread laterally, cool, and sink back into the interior of the mantle, driving plate tectonics
Convergent boundaries
- Where two plates are colliding
- Subduction zones occur when one or both of the tectonic plates are composed of oceanic crust
- The denser plate is subducted underneath the less denser plate
- The plate being forced under is eventually melted and destroyed
- Subduction zones are responsible for the largest earthquakes and also form chains of volcanoes in the overriding plate
- Subducting zones can also exist between two oceanic plates and in this case we will get a chain of volcanoes
Transform Boundary
- where two plates slide pass each other
- the relative motion of the plates is horizontal
- they can occur underwater or on land
- crust is neither destroyed nor created
- Because of friction, the plates cannot simply glide pass each other
- Rather stress builds up in both plates and when it exceeds the threshold of the rocks, the energy is released and results in earthquakes
- Some transform boundaries are associated with oceanic spreading ridges which are irregular and jagged (compsed of segments of spreading ridge connected by segments of transform faults)
- Transform boundaries are on the edges of plates
Summary of plate tectonics theory
- the lithosphere is broken into tectonic plates (seven or eight major plates)
- Earthquakes, volcanic activity, mountain-building and oceanic trench formation occur along the plate boundaries
- Tectonic plates are composed of oceanic lithosphere and the thicker continental lithosphere, each topped by its own kind of crust
