Chapter 4 – Introduction to Natural Hazards
Most large tsunami’s occur at subduction zones (narrow strip of earth’s lithosphere where one
tectonic plate moves beneath another).
Volcanic eruptions and earthquakes are the result of internal forces explained by plate tectonics.
o Most earthquakes and active volcanoes occur at boundaries b/w plates.
The “processes” we consider to be hazards are derived from the internal heating of the earth
and heat from the sun.
Hazard – Natural process that threatens human life
Risk – Probable severity that destructive event will occur multiplied by the likelihood the event will
Disaster/Catastrophe – Events that cause serious injury or property damage.
Mitigation – Used by scientists and planners to describe efforts to prepare for disasters and minimize
Floods, hurricanes, earthquakes, volcanic eruptions, and large wildfires are most likely to create
Geologic Cycle - Processes that produce the mineral resources, fuels, land, water, and atmosphere we
require for survival. The cycles include:
the Tectonic Cycle
the Rock Cycle
the Hydrologic Cycle
The Tectonic Cycle – The large-scale geologic processes that deform earth’s crust. Driven by forces deep
within earth. Involves the creation, movement, and destruction of tectonic plates.
Earth’s Lithosphere and Crust
Outer most outer layer of earth is the lithosphere. It is stronger and more rigid than the deeper
material. Below the lithosphere is the asthenosphere, a hot layer of low-strength rock that
extends to an average depth of 250km.
The upper part of the lithosphere is the crust.
o Two types: Oceanic and Continental
o Oceanic crust is denser and thinner than continental. Types of Plate Boundaries
Lithosphere is broken into fragments called tectonic plates that move.
Plate boundaries can be divergent, convergent, or transform.
o Divergent – Two plates move away from each other and new lithosphere is produced.
Underwater mountain ridges called mid-ocean ridges are formed by a process called
seafloor spreading. Cracks in the underwater rift zone fill with molten rock/magma and
new lithosphere forms.
o Convergent – Two plates collide head on. A higher density oceanic plate is drawn
beneath a lower density continental plate (usually). This is called subduction, and
convergent boundaries like this are called subduction zones. The oceanic plate heats as
it moves beneath the continental plate. The high temperatures cause lower crustal rocks
to melt and magma moves up through the crust along fractures. Some of the magma