Tectonics Flashcards
(66 cards)
Inner core
At the very centre of the Earth and its hottest part (about 6000 degrees) it is solid and mostly consists of iron
Outer core
Is semi-molten and mostly consists of liquid iron and nickel temperatures range from 4500-6000 degrees
Mantle
Surrounds the core and is the widest layer the upper part is solid but below it the rock is semi-molten - forming the asthenosphere on which the tectonic plates ‘float’
Crust
Oceanic crust is a thin dense layer which lines the ocean floor and continental crust is an older thicker layer
What did Francis Bacon notice?
One of the first people to note that the West coast of Africa and the East coast of South America seem to have a “jigsaw fit” with the Eastern seaboard of North and South America.
Convection currents
Heat produced by the decay of radioactive elements in the Earth’s core heats the lower mantle - creating convection currents. These hot, liquid magma currents are thought to move in circles in the asthenosphere.
Slab pull
Newly formed oceanic crust at mid-ocean ridges becomes denser and thicker as it cools. This causes it to sink into the mantle under its own weight - pulling the rest of the plate further down with it.
Seafloor spreading
In the middle of oceans are huge mid-ocean ridges that are formed when hot magma (molten rock) is forced up from the asthenosphere and hardens - forming a new oceanic crust. This new crust pushes the tectonic plates apart in a process called seafloor spreading
Paleomagnetism
Every 400,000 years or so, the Earth’s magnetic fields change direction - causing the magnetic north and south poles to swap. When lava cools and becomes rock, minerals inside the rock line up with the Earth’s magnetic direction (polarity) at the time. Scientists studying mid-ocean ridges found the same pattern on either side of the ridges (something
that could only happen if new rock was being formed at the same time on both sides).
P-waves
P-waves (Primary waves or pressure waves) - these are the fastest and first to reach the surface. They travel through both solids and liquids. They are only damaging in the most powerful earthquakes.
S-waves
S-waves (Secondary or shear waves) - these are slower they only travel through solids and move with a sideways motion, shaking at right angles to the direction of travel. They do more damage than P waves.
L-waves
L-waves (surface Love waves) - these are the slowest but they cause the most damage shaking the ground from side to side. They are larger and focus their energy on the earth’s surface.
Moment Magnitude Scale
Measures the total energy released by an earthquake at the moment it occurs using the:
Size of the seismic waves
Amount of slippage or rock movement
Area of the fault surface broken by the earthquake
Resistance of the affected rocks
The scale goes from 1 (the smallest) to 10 (the largest)
Mercalli scale
Measures the intensity of the earthquake; this uses the amount of damage caused by the earthquake using roman numerals 1-12
Richter scale
Is a measure of the strength of the earthquake by calculating its magnitude 1-10
Liquefaction
The violent shaking during an earthquake causes surface rocks to lose strength and become more liquid than solid. The subsoil loses its ability to support building foundations so buildings and roads tilt or sink.
Intra-plate boundaries
These earthquakes occur in the middle of plate; some scientists think that they occur when stress builds up in ancient faults - causing them to become active again.
Hot spots
While most volcanoes are located along plate margins there are some exceptions these are called hot spots. Volcanoes form where plumes of hot magma rise upwards and erupt onto the sea floor. As the tectonic plate moves over a hot spot, the volcano is carried away with it, and a new one forms. Eventually this will create a chain of volcanic islands.
Basaltic lava
- Hottest type of lava
- Low gas content 0.5-2%
- Low viscosity
- Formed in gentle effusive eruptions
Andestic lava
- Middle temperature
- 3-4% gas content
- Intermediate viscosity
- Formed at violent, moderately explosive eruptions
Rhyloitic lava
- Coolest
- 4-6% gas content
- High viscosity
- Formed at very violent, cataclysmic eruptions
Volcanic Explosivity Index
Scientists use the Volcanic Explosivity Index (VEI) to describe and compare the size or magnitude of volcanic eruptions. It uses a scale from 0 (non-explosive) to 8 (extremely explosive).
The VEI uses several factors to assign a number including:
The amount and height of the volcanic material ejected (tephra and ash fall)
How long the eruption lasts
Quantitative descriptive terms such as ‘gentle’ and ‘explosive’
How are tsunamis formed?
Most are caused by large underwater earthquakes along subduction zones. Energy released during the earthquake causes the seafloor to uplift - displacing the water column above. This displaced water forms tsunami waves
A tsunami wave moves fast. When the wave’s crest reaches the shore it produces a vacuum effect sucking the water back out to sea and exposing a large amount of sea floor. The suddenly retreating water and exposed sea floor is key early warning sign of an approaching tsunami
Tsunamis can also be caused by underwater landslides and asteroids but this occurs less frequently
Explain the DART system
The DART system uses seabed sensors and surface buoys to monitor changes in sea level and pressure. When tsunamis are detected the system sends information via a satellite to tsunami warning stations
