Geography of Natural Hazards

Overview and the Earth’s Interior

Dr. Michael Harrison Summer I, 2008

Eruption of Nevado del Ruiz, Columbia: 1985

Lahar buried town of Armero, killing21,000 dead

Mt. Rainier, WA

Today, 10s of thousands of people live on lahar deposits nearMt. Rainier

Sichuan Quake: May 12, 2008

7.9 magnitude quakehas killed at least 55,000—likely to reach >80,000

Weather Events

Iowa tornadoes: May 26, 2008(at least 6 dead)

Hurricane Katrina: August 29, 2005(~2000 dead; >81 billion in damage)

Myanmar cyclone: May 2, 2008(75,000-100,000 dead)

Learning Objectives• Know the difference between a disaster and a catastrophe

• Know the components and processes of the geologic cycle

• Understand the scientific method

• Understand the basics of risk assessment

• Recognize that natural hazards that cause disasters are generally high-energy events caused by natural Earth processes

Learning Objectives, cont.• Understand the concept that the magnitude of a

hazardous event is inversely related to its frequency

• Understand how natural hazards may be linked to one another and to the physical environment

• Recognize that increasing human population and poor land-use changes compound the effects of natural hazards and can turn disasters into catastrophes

Processes & Natural Hazards• Internal forces within Earth

– Plate tectonics

• External forces on Earth’s surface– Atmospheric effects– Energy from the sun

Some Important Definitions• Hazard

– Naturally occurring– Effects on human interests

• Disaster– Effect of hazard on society

• Property damage, injury, loss of life

• Catastrophe– Massive disaster

Figure 1.4

Natural Hazards & Catastrophes

History & Natural Hazards• Natural hazards are repetitive.

• History of an area gives clues to potential hazards.– Maps, historical accounts, climate, and weather data– Rock types, faults, folds, soil composition

Rock Cycle• Different rocks are formed by different processes.

• Rock types in a location give clues to geological past and present.

Figure 1.12

Hydrologic Cycle• Solar energy drives movement of water between

atmosphere and oceans and continents.

Figure 1.13

Biogeochemical Cycle• Combines the previous cycles with the cycling of

nutrients needed for live, like carbon, nitrogen, and phosphorus

• Transfer of chemical elements through a series of reservoirs.

Fundamental Concepts for Understanding Natural Processes as

Hazards1. Hazards are predictable from scientific evaluation.

2. Risk analysis is an important component in our

understanding of the effects of hazardous processes.

3. Linkages exist between different natural hazards as well

as between hazards and the physical environment.

4. Hazardous events that previously produced disasters

are now producing catastrophes.

5. Consequences of hazards can be minimized.

1. Hazards are Predictable• Basis of science is explanation.

• Geologists observe hazardous events and form a possible explanation.– From this explanation, a hypothesis is formed.– Data is taken to test a hypothesis.

• This is the basis of the scientific method.

Hazards are Natural Processes• They are a result of natural forces.

• They become hazardous when they interfere with human activity.

• These process are NOT within our control.

• Best solution is preparation.

Earth’s Long History: 4.56 Billion Years Worth

Forecast & Prediction

• Prediction– Specific date, time, and magnitude of event

• Forecast– Range of probability for event

• Some hazards can be predicted, most can be forecasted.

Flow Chart for Hazard Preparation

Hazard Reduction• Identify the location of probable event

• Determine probability of event

• Observe precursor events

• Forecast or predict event

• Warning the public

2. Risk Assessment• Risk = (probability of event) x (consequences)

• Consequences: damages to people, property, economics, etc.

• Acceptable risk is the amount of risk that an individual or society is willing to take.

3. Links• Hazards are linked to each other.

– Some events may cause others.• Example: Earthquakes and landslides

• Physical environment is linked to hazards.– Example: Some rock types are prone to subsidence.

4. Disasters are Now Becoming Catastrophes

• Concentration of population creates greater loss of life in disaster.

• Human population growth puts greater demand on Earth’s resources.

• Land use affects magnitude and frequency of events.

Concentration of People (world population today >6.7 billion)

5. Consequences Can Be Minimized

• Move from reactive response: Recovery and restoration

• To an anticipatory response: Avoiding and adjusting to hazards– Land-use planning– Building codes– Insurance– Evacuation– Disaster preparedness– Artificial control

Benefits of Hazards• There are some benefits to hazards.

• Examples: – Flooding provides nutrients for soil.– Landslides form dams to create lakes.– Volcanoes create new land.

Earth’s Interior and Exterior

Formation of Earth’s Layers

• Early Earth was fairly homogeneous in composition (known from meteorites)

• Early Earth was hot (heat from gravitational compaction, meteor impacts, and mantle radioactivity)

• Heat within Earth caused rock to melt (magma oceans); lighter (less dense) elements floated to surface (Si, Al, K, Na, O); heavier elements (more dense) sank to core (Fe, Ni)

• This differentiation formed Earth’s distinct compositional layers: core, mantle, and crust

Earth’s Differentiation

Iron droplets from magmaocean pond on top of mantle,accumulate into large blobs,and descend toward core

Mantle cools upwardand downward Magma oceans solidify

at top and bottom ofmantle

(Nature: 12/6/07)

Earth’s Layers

Average diameter is 12,742 km with a density of 5.5 g/cm3

Deeper layers aredenser and hotter

Temperature increasesat ~25 degrees C/km(geothermal gradient)

Earth’s Layers• Crust-thin, rocky outer layer (less dense than mantle);

composed of: (1) continental crust 15-70 km thick (granite) and (2) oceanic crust 7-10 km thick (basalt)-granite has a density of ~2.8 and basalt is ~2.9-the Moho (a surface) separates the crust from underlying mantle

Continental crust covers ~40% Earth’s surface; oceanic crust covers ~60%

Most continental crust at sea level; most oceanic crust at 5 km below sea level

Composition (wt.%): O (45%), Si (27%), Al (8%), Fe (6%), others (14%)

Earth’s Layers• Mantle-middle layer of rock (less dense than core); 82%

Earth’s volume and most of its mass. Rock type is peridotite; mantle is 2900 km thick with a density of 4.5; geothermal gradient = 10 degrees C/km

• Outer core-molten iron in motion—generates magnetic field; 2300 km thick

• Inner core-solid iron and nickel; 1200 km thick; pressure ~50 million lbs/in2 at 5000 degrees C

• The core (outer and inner) is Fe (85%), Ni (5%), S (2%), Si (4-5%) and O? (>1%); density is 10.7

Earth’s Magnetic Field• Established by 3.2 Ga (magnetite inclusions in silicate minerals

show remanent magnetism, Nature: 4/5/07)

• (1939) Electro-Dynamo hypothesis: convecting liquid iron induces EM field

Global scale: Earth’s magnetic field dominated by the core

Local scale: magnetic field dominated by remanent magnetism in crust

Remanent magnetism in crustextends from surface to Curieisotherm (depth = to a temperatureof 580 degrees C)

Earth’s Physical Layers(How strong are Earth’s layers?)

Earth’s Physical Layers

• Lithosphere-outermost zone of cooler, strong rock ~100-250 km thick; includes crust and upper mantle

• The base of the lithosphere is near the melting temperature of peridotite (~1280 degrees C)

• Asthenosphere-hotter, weak mantle rock that flows (not a liquid—it’s solid, but like taffy)

• The lithosphere is stronger than asthenosphere because the rocks are cooler

• Mesosphere-hot, strong mantle rock (stronger than asthenosphere because of higher pressure)

How do we know about Earth’s layers?• Deepest well (Kona Peninsula, Russia) is only 12 km1. Earthquake energy (type and velocity reveal rock

properties)2. Mantle rocks from volcanoes (xenoliths)3. Mantle rock from the oceans now on land (ophiolites—

Newfoundland and Cypress)4. Meteorites-most from asteroid belt between Mars and

Jupiter—represent raw, planet-forming material (iron meteorites resemble core; stony meteorites resemble mantle)

5. Laboratory experiments (e.g., diamond-anvil cell)

Mantle xenoliths (8 – 300 km)

Kola borehole: 12.6 km

Radius of Earth: 6378 km

Seismic tomography

Earth’s Surface: Continents and Oceans• Continents ~0.8 km above sea level; oceans ~2.4 km

below sea level. Why?• Continents are composed of granitic rock about 40 km

thick with a density of 2.8• Oceans are composed of basaltic rock about 8 km thick

with a density of 2.9• Continental crust is thicker and less dense than oceanic

crust therefore it is buoyant—it “floats” atop the mantle

Features of Continents• Mountain belts-linear uplifted areas (higher elevations)• Craton (platform and shield)-interior of continent; stable

Features of Oceans• Continental shelf -continental crust below sea level• Abyssal plains -flat seafloor, deep ocean (2-2.5 km below

sea level)• Trenches -deep, linear depressions (some >11 km below sea

level)• Mid-ocean ridge -linear mountain range (cumulative length =

~70,000 km) below sea level (e.g., Mid-Atlantic Ridge). Iceland is an exposure of these mountains above sea level

• Volcanic island arcs -chain of oceanic islands next to trenches (e.g., Aleutian Islands, AK)

• Seamounts -submarine volcanoes that may form chains (e.g., Emperor seamount chain)

Features of Oceans