Hazardous Environments

Explain the causes of earthquakes (3 steps)

1. Tectonic plates on the surface of the earth’s crust are rafted around by **convection currents** (creating the 3 different types of plate boundaries) powered by **radioactive decay** in the boundary between the mantle and the core
2. As plates diverge, converge, or slide past each other, **pressure** builds up **between faults** that are locked together by irregularities and friction
3. Eventually the **pressure exceeds frictional resistance** and a **fault fails**, triggering in earthquake, and aftershocks **if other faults are stressed**

Convergent boundaries:

* Crust is **compressed**, creating **thrust** or **reverse** faults
* Hypocenter depth **varies**, **infrequent** earthquakes but **high magnitude** and **destructive**

Divergent boundaries:

* Crust is under **tension**, creating **normal faults**
* **Shallow** hypocenter, frequent earthquakes but low magnitude and **not** very destructive

Conservative boundaries:

* **Transform** faults
* **Shallow** hypocenter, failure usually happens along a **large section** of the fault so very destructive

Case Study - The Haiti Earthquake

* Magnitude 7
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Explain the causes of volcanoes

1. Tectonic plates on the surface of the earth’s crust are rafted around by **convection currents** (creating the 3 different types of plate boundaries) powered by **radioactive decay** in the boundary between the mantle and the core

Divergent boundaries:


1. Thinning crust causes **decompression melting of the upper mantle**, which creates **fluid**, **low silica** magma
2. Causes **basaltic eruptions**, creating **rift** or **shield** volcanoes that erupt **frequently** but not **explosively**

Convergent boundaries:


1. (Must have subduction) Water lowers the melting point of rock, forming **molten plutons** of **viscous**, **high silica magma**
2. Causes **andesitic** eruptions, creating **cone** volcanoes that erupt **infrequently** but **explosively**

Hotspots:


1. Above **isolated mantle plumes**, plume **elevates** and **thins** crust
2. Causes frequent, non-explosive eruptions of **large volumes** of **fluid, low silica** magma

Explain the causes of tropical storms and hurricanes

* **Convection currents** over equatorial oceans with a minimum temperature of **27c**
* **High humidity** allowing earlier condensation r**eleasing latent heat**, accelerating convection
* **Low pressure** is created, so **surface winds converge**
* **An absence of wind shear** allows for the vertical development of convectional clouds
* Due to **Coriolis Effect**, converging winds rotate **anticlockwise in the northern hemisphere** and **clockwise in the southern hemisphere**
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