Geophysical Hazards - Part 1: Plate Movement Flashcards
Describe Characteristics of the Crust to the Inner Core
Crust - brittle, low density materials. Oceanic = 5km
Land = 30km
Mantle - 2900km
Molten rock causes tectonic movement
Core - Highest density. Combined thickness 3500km.
Outer core - liquid nickel and iron at 4000-5000 degrees Celsius.
2300km.
Inner core - solid nickel and iron 5000-7000 degrees and 1200km.
Explain convection currents in the mantle. (6 steps)
- Heating occurs between the inner core to the mantle.
- Denser, cooler magma in the lower mantle heats and rises making it less dense.
- When it reaches the crust it’s forced to move in opposite directions.
- The movement causes cooling making the liquid rock sinks back down towards the core.
- The friction between the convection current and the crust causes the tectonic plates to move.
- The direction of movement of tectonic plates is determined by which way the convection currents are flowing.
What are earthquakes? (4 features)
- Series of seismic vibrations or shock waves originating from the focus.
- Epicenter = point on surface above the focus.
- Foreshocks and aftershocks.
- Seismic waves can travel along surface or through the body.
Convergent/Collision/Destructive plate boundary.
Continental-Continental and Oceanic-Continental
Continental-Continental:
- Two continental plates move towards each-other.
- Little subduction due to low density rock.
- Rock gets folded, faulted, thickened.
- Magma cannot penetrate thick crust –> forms granite.
Example: The Himalayas and the Tibetan Plateau = 50 mill. yrs of collision between Indian-Eurasian plates.
Oceanic-Continental:
- Oceanic plate subducts continental
- Volcanic arcs arise
- Oceanic = densert = high subduction potential
- Volcanoes eurpt
Example: Cascade mountains of Western North America // The Andes of Western South Africa
Divergent/constructive plate boundaries:
Oceanic and Continental
Oceanic:
- Tectonic plates are pulled apart
- Slab pull is created when plates SINK into the MANTLE at SUBDUCTION zones.
- Convection currents lift crust
Example: Mid-Atlantic Ridge:
- Fissure eruptions
- Shallow earthquake activity
- New sea floor + widen ocean basin
Continental:
- Pull is not strong enough.
- Currents cause plates to arch upwards
- Plates are pulled thin and fractured into rift-shape
- Faults develop + central rock slides.
- Earthquakes can occur
Example: East Africa Rift Valley or the Dead Sea
Conservative/Passive/Transform
Oceanic and continental
- Horizontal movement only.
- Plates move past each other, rubbing along the edges.
Oceanic:
- Short faults on the sea floor near mid-ocean ridges.
- Plates split at different speeds.
- Space is created between margins
- Once the oceanic plate spreads beyond overlap, a fracture zone extends across seafloor.
Continental:
- Extra forces (compression + extension) create mountainous welts and down-dropped valleys.
- Thick crust = wide deformation zones.
Example: San Andreas, California = 100km of faults.
P versus S waves
P waves can travel at 6km/sec, travel through solids and liquids, and shake vertically.
S waves can travel at 4km/sec, travel through solids and shake horizontally.
Love vs Rayleigh waves
Both travel at slow speeds, both travel through solids on the crust.
L waves = horizontal shake.
R waves = vertical shake.
4 Human causes of earthquakes
Construction of large dams, mining, hydraulic fracturing, testing nuclear weapons.
How do we measure earthquakes?
Richter scale: seismometer produces seismograph. Numbered 1-10 and is logarithmic.
6 Primary Earthquake Hazards
Shaking Ground ruptures Landslides Collapsed buildings Gas and water lines burst Fissures in roads
8 Secondary Earthquake Hazards
Aftershocks Fires Water-borne diseases Road blockage Floods Tsunamis Soil liquefaction
7 Factors effecting severity of earthquake impacts
Magnitude and frequency Pop. density and distribution Type of buildings and ifrastructure Time of day Distance from epicentre Types of rock and sediment (urbanisation, deforestation) Economic development
Factors for predicting future earthquakes
Time since last large earthquake, the longer ago the more likely it is.
Earthquake patterns
Mathematical models
Outline the year, location, plates, boundary, epicenter, focus, magnitude of the Christ Church Earthquake
Year: 22 Feb 2011
Location: Pacific Ocean on the Oceania Plate.
Tectonic plates: Australian and Pacific
Boundary: Conservative
Epicenter: 6 miles southeast of CC.
Focus: shallow, 3.1 miles.
Magnitude: 6.3
Aftershock: from 2010 earthquke
