Hazards Flashcards
Wegeners theory
Continental drift
During the Carboniferous period Pangea existed and over time the continents separated
Geological evidence of continental drift
Fit of the continents
Mountain chains and some rock sequences on either sides of the oceans show great similarity eg northeast Canada and northern Scotland
Evidence from 290 million years ago of the effects of glaciatiob in South Africa, South America, Australia and India and Antarctica suggest that these land masses were once joined
Biological evidence for continental drift
Similar fossil branchiopods (marine shellfish) found in Australian and Indian limestones
Similar fossil reptiles found in South America
Fossils from rocks younger than the Carboniferous period in places such as Australia and India showing fewer similarities suggesting they followed different evolutionary paths
What is the lithosphere
Consists of the crust and the rigid upper section of the mantle
Approximately 80-90km thick
Divided into plates
Characteristics of the mantle
Surrounds the core Composed of silicate rock rich in iron and magnesium Semi molten Less dense rock Temperature reaches 5000 degrees Generates convection currents
Characteristics of the crust
Rocks composed of silicon, oxygen, aluminium and potassium
Thinnest layer
Is made up of two types - oceanic and continental
Oceanic - made of basaltic rock
Continental - made of igneous, metamorphic and sedimentary rock
Characteristics of the asthenosphere
A layer in the earth mantle in which rocks are easily deformed
It is several hundred km thick
Lies between the lithosphere which has a high viscosity and is much more resistant to deformation
Tectonic plates move on the semi molten asthenosphere
Characteristics of the core
Solid inner part
Densest part of the planet
Rich in iron and Nickel
Outer core - semi molten
How do tectonic plates move
The most likely cause of plate movement is the existence of convection currents in the asthenosphere
Heat comes from a combination of radioactive decay in the core and residual primary heat
The convection currents cause magma to circulate and this moves the lithosphere
Tectonic plates
Pacific, African, Eurasian, North America, South America, Antarctic and I do-Australian
Difference between continental and oceanic crust
Continental - less dense, 30-70 km thick, over 1500 million years old
Oceanic - more dense, 6-10 km, less than 200 million years old
Example of tectonic plates
African plate - convergent margin, moves around 21.5mm per year
Example of oceanic plates
Pacific plate - convergent, divergent and transform margins, moves around 50-100mm per year
Oceanic/continental convergent
Where oceanic and continental plates meet the dense oceanic plate is forced under the lighter continental one - subduction. The continental crust being more buoyant is not subduction but uplifted, buckled and folded forming fold mountains
Ocean trench formed at point of subduction
Subduction plate is heated and melts causing it to rise
Oceanic oceanic convergent
When oceanic crust converges with oceanic crust, subduction still occurs, as one plate is likely to be slightly older and slightly sensed than the other
This creates a deep ocean trench
On the non subduction plate, a volcanic island arc forms from the rising magma generates from the subduction plate
Continental continental convergent
Neither is subducted because of their thickness
The plates are initially separated by an ocean floor that is being subducted under one continent
As the ocean floor continues to be subducted the two continents come closer together until the eventually collide
This causes food mountains eg the Himalayas
Divergent plate boundaries
Plates are separating and new lithosphere is forming
Divergent boundaries are places where crust is extended, thinned and fractured as magma rises to the surface where it appears as pillow lava
Most commonly occur along the crests of oceanic ridges - mid Atlantic ridge
Where else are divergent boundaries formed
Under continents in the early stages of continental break up
When magma wells beneath a continent the crust is initially elevated, stretched and thinned producing fractures, faults rift valleys and volcanic activity - Red Sea separating the Arabian peninsula from Africa and the gulf of California
P waves
Primary - travel fastest and arrive first - can produce warning for heavy ground shaking to follow
Can travel through any type of wave
Rarely causes any damage
L waves
Longer the wavelength than p and S waves
Surface waves
Travel along the surface of the earth
Cause the most damage
S waves
Secondary - travel slower than p waves
Do not travel in liquid
Warning time
The warning time is about one second for every five miles of distance to the focus of the earthquake, which gives automated systems plenty of time to respond and enough time to react
Tsunamis
Larger version of a normal wave with a peak and trough but consist because of the movement of energy through water
Tsunamis are caused by energy originating under water from a volcanic eruption, landslide of most commonly an underwater earthquake
Vulnerability
Concerned with the ability of a person or community to withstand exposure to and risks from a hazard such as an earthquake. People are most vulnerable when relatively small physical changes have major socio-economic implications
