Midterm Flashcards
Internal Structure of Earth
Earth is layered and dynamic
Internal structure of earth can be considered in 2 fundamental ways
By composition, state, and density
By strength
Structural Layers of Earth
Inner Core Outer Core Mantle Crust
Inner Core
Solid
1300 km thick
high temp
composed of iron (90% by weight), as well as Sulphur, oxygen, nickel
Outer Core
Liquid
2000 km thick
composition similar to inner core
Mantle
Solid
3000 km thick
Composed of iron-rich and magnesium-rich silicate rock
Crust
Outer rock layer of earth
Mohorovicic discontinuity separates lighter crustal rocks from the denser mantle
Lithosphere
Cool, strong outermost layer of earth
Asthenosphere
Constitutes all but the uppermost part of the mantle
Hot, slowly flowing layer of relatively weak rock
100 km down from lithosphere
non-brittle, never cracks just slowly deforms
Continents and Ocean Basins and Convection
Crustal rocks are less dense than the mantle rocks below good bc if not they would sink
Continental crust is less dense than oceanic crust, oceanic crust gets dragged under continental crust when they run into each other
Oceanic crust is relatively thinner oceanic crust is always being made and destroyed
Oceanic crust is 200 myo and continental crust is up to 1 byo
Earth’s internal heat causes magma to heat up and become less dense
The less dense magma rises while the cooler magma falls to side and further downward
How do we Know about Earth’s Internal Structure?
Most knowledge comes from seismology study of earthquakes
Earthquakes cause seismic energy to move through the earth
Some waves can move through solids, but not liquids
Some waves are refracted (change direction), some are reflected
What we have learned about earth from earthquakes
Where magma is generated in the asthenosphere
The existence of slabs of lithosphere that have sunk deep into the mantle
The variability of lithosphere thickness, reflected differences in its age and history
What is liquid and what is solid
Plate Tectonics
Tectonic refers to the large –scale geologic processes that deform earths lithosphere
It produces ocean basins (separating), continents (colliding), and mtns (colliding)
Driven by forces deep within the earth
Lithosphere is broken into pieces
Lithospheric plates- all moving in slightly diff speeds and directions
Can cause sudden displacements that are forceful resulting in a natural disaster
Plates move relative to one another
Plate Boundaries
These are delineated by earthquakes and active volcanos (geologically active areas)
They are defined by areas of seismic activity
Dynamic events on earth surface occur when the plates move
Diverge, converge, or slide past one another (transform)
Sliding causes most damage
Seafloor Spreading
This is the mechanics for plate tectonics
At mid-ocean ridges, new crust is added to edges of lithospheric plates
Continents are carried along plates
Crust is destroyed along other plate edges
Called subduction zones
The rate of production of new lithosphere at spreading centres is balanced by consumption of lithosphere at subduction zones
The lithosphere grows at the same speed it subducts
Types of Plate Boundaries
Divergent
Convergent
Transform
Divergent Plate Boundaries
Where two plates move away from one another
New lithosphere is created at these boundaries
Divergence b/w two ocean plates
Causes mid-ocean ridges
Seafloor spreading
ex. Mid-Atlantic Ridge
Convergent Plate Boundaries
Where two plates collide head-on
Oceanic-continental collisions results in subduction zones
More dense ocean plates sink and melt cause problems along coast
Melted magma rises to form volcanos
Collisions b/w 2 continental plates results in a continental collision boundary
Neither plate subducts, instead the plates crumple together
Large mtns form such as the Himalayas
These aren’t as common
Transform Plate Boundaries
Where the edges of two plates slide horizontally past one another
Most common on the ocean floor but some occur within continents
When they come on land they can be extremely powerful lack of water matters
Ex. San Andreas Fault separates the Pacific plate and the North American plate
Rates of Plate Motion
Plates move a few centimetres per year
Although the central portions of plates move at a steady slow rate, movement may not be steady at plate boundaries
Plates can displace by several metres during great earthquakes through stored up energy
Magnetic Stripes
Two possible driving mechanisms for plate tectonics
Ridge push and slab pull
Ridge push is a gravitational push away from crests of mid-ocean ridges
Slab pull occurs when cool, dense ocean plates sinks into the hotter, less dense asthenosphere
Weight of the plate pulls the plate along
Evidence suggests that slab pull is more important process
But can’t happen without both ridge push and slab pull
Hot Spots
Weaknesses in the crust
Concept developed by Canadian geophysicist J. Tuzo Wilson
Volcanic centres away from plate boundaries resulting from hot material from deep in the mantle
Magma moves up through the mantle and overlying plates
Found under both oceanic and continental crust
Plates move over hot spots creating a chain of volcanos ex. Hawaiian Islands
Plate Tectonics and Hazards
Divergent plate boundaries (mid-Atlantic ridge) exhibit earthquakes and volcanic eruptions
Transform plate boundaries that slide past each other (San Andreas Fault) have appreciable earthquake hazards
Convergent plate boundaries where one plate sinks (subduction zones) contain explosive volcanos and earthquake hazards
Convergent plate boundaries where continents collide (Himalayas) have high topography and earthquakes
Earthquake
An earthquake is a motion/trembling of the ground caused by the sudden displacement of rock
Consequences of Earthquake
depend on: Magnitude Depth Distance from populated areas Direction of fault rupture Nature of the local earth materials Engineering and construction practice Population density
Earthquake Magnitude
Measured by moment magnitude (MW)
The scale is logarithmic and based on powers of ten
Ground motion for a M 3 is 10 times that of a M 2
Amount of energy released for a M 3 is 32 times that of a M 2
Smaller earthquakes are more frequent than larger ones
Earthquake Intensity
Measured by the Modified Mercalli Intensity Scale
The scale is qualitative and based on damage to structures and people’s perceptions
Modified Mercalli intensity maps show where the damage and perceived shaking is most severe
Shake maps use seismograph data to show areas of intense shaking
Earthquake Processes
Earthquakes commonly occur at or near plate boundaries
Plate boundaries may contain many faults (where movement and friction can occur)
Friction along plate boundaries exerts strain or deformation
When stress on rocks exceeds their strength, the resulting rupture produce seismic waves
Dip Slip
Vertical movement
Three types of dip-slip faults based on which way the bounding earth materials move
Walls on an incline are defined as:
Hanging wall block- higher surface going down
Footwall block- lower surface going up
Normal Fault: the hanging wall has moved downward relative to the footwall (usually happens where things are diverging)
Reverse Fault: the hanging wall has moved up relative to the footwall
if the fault plane angle is 45 degrees or less, it is a thrust fault
Blind Faults: do not extend to the surface
Strike Slip
horizontal movement: plates don’t move up or down just sideways
Fault Activity and Tectonic Creep
Active faults: movement during the past 11 600 years
Potentially active faults: movement during the past 2.6 million years
inactive faults: no movement during the past 2.6 million years
Tectonic creep: occurs when movement along a fault Is so gradual that earthquakes are not felt
Can slowly damage infrastructure
Such slow movement that doesn’t really cause earthquakes but slowly damages infrastructure (ripping away at sewer lines and building layouts)
Seismic Waves- Body Waves
travel within the body of the earth 2 types: P-Waves (primary or compressional waves) Move fast with push/pull motion Can travel through solid, liquid, and gas S-Waves (secondary or shear waves) Second fastest wave Move slowly with a back-and-forth motion at right angles to the direction the waves are moving Can travel only through solids
Seismic Waves- Surface Waves
Travel along Earth’s surface horizontally and vertically and can produce rolling motion Move more slowly than body waves Are responsible for damage near the epicenter 2 types: Love waves Cause horizontally shaking Rayleigh waves Rolling waves, elliptical motion
Earthquake Shaking: Distance to the Epicentre and Focal Depth
Seismographs record arrivals of waves to station sites
P waves and S waves travel at different rates and arrive at each station at different times
Distance to the epicentre can be found by comparing travel times of the waves using triangulation
Focal depth influences amount of shaking due to attenuation
You need 3 stations at least to determine the earthquake epicentre
Local Soil and Rock Conditioning
Local geology influences the amount of ground motion
Dense rocks (eg. Bedrock) transmit earthquake energy quickly
dissipates quickly) and slows down in sediment (loose sediment)
Seismic waves slowdown in the heterogeneous rocks, unconsolidated sediment and sediment with high water content
Slowing it down can allow for amplification of the energy
Heterogeneous rocks- rocks that are different
Amplification occurs when energy is transferred from P waves and S waves to surface waves
More damage can occur in areas farther away from the epicentre depending on local ground conditions
Mexico City has the potential for catastrophic earthquakes (bc of the sediment)
The Earthquake Cycle
First part is inactive period where stress builds in the fault (some sort of movement occurring but it isn’t showing in the rocks)
Period of small earthquakes where stress begins to release, causing strain
Foreshocks: occurs prior to a major release of stress (this stage does not always occur)
it’s a smaller earthquake before the mainshock (largest earthquake)
Mainshock: when the fault releases the majority of the stress
Aftershock: releases of stress after a major earthquake
Geographic Regions at Risk from Earthquakes
Earthquakes are not randomly distributed
Most occur along plate boundaries
“Pacific Ring of Fire”, Japan, Western US/Canada, Indonesia, New Zealand
Himalayan Mountains, Middle East
However, not all areas are at risk near plate boundaries
Plate Boundary Earthquakes
Strike-slip earthquake
Occur on transform faults, where plates slide past each other
Ex. San Andreas and Queen Charlotte faults
Thrust earthquakes
Faults separating converging plates
Ex. Cascadia subduction zone
Normal fault earthquakes
Occur on diverging plate boundaries- tend to be a lesser magnitude
Ex. Mid-Atlantic ridge
Intraplate Earthquakes
Earthquakes that occur within plates, happens at zones of weakness
Ex. New Madrid seismic zone in Missouri, Southern Quebec and eastern Ontario
Intraplate earthquakes are often smaller than plate boundary earthquakes
However, they can cause considerable damage due to the lack of preparedness and because they can travel greater distances through dense continental bedrock
Effects of Earthquakes and Linkages
Shaking
Causes damage to buildings, bridges, dams, tunnels, pipelines, etc.
Measured as ground acceleration
Buildings are damaged due to resonance
Ground rupture
Displacement along the fault causes cracks in the surface and faults scarps (rising of the ground, big problem if it happens under buildings)
Where the ground moves and shakes, destroying highways, railways
Liquefaction
Water-saturated loose sediment turns from solid to liquid, causing buildings and land to subside
Land-level changes
Landslides
Fires
Ground shaking and surface rupture can sever electrical power and gas lines
Disease
A loss of sanitation and housing, contaminated water supplies, and disruption public health service
Natural Service Functions of Earthquakes
Water, oil, and natural gas may be rerouted due to faults
Faults can channel groundwater to the surface at springs
New mineral resources may be exposed
Some minerals are preferentially deposited in veins
Scenic landscapes may form ex. Rocky Mountains
Human Interaction with Earthquakes
The weight form water reservoirs may create new faults or lubricate old ones
Liquid waste disposals deep in the Earth can create pressure on faults
Pumping of oil and gas and hydraulic fracturing can both cause small earthquakes
Nuclear explosions can cause the release of stress along existing faults
Minimizing the Earthquake Hazard
Earthquake Hazard Reduction Programs
Five major goals:
Operate national seismograph networks
Develop and understanding of earthquake sources
Predict effects of earthquakes on buildings and other structures
Communicate research to educate individuals, communities, and governments
Short-Term Prediction
Forecast: specific the probability of an earthquake occurring
Prediction: specific when and where an earthquake will occur
Precursors
Pattern and frequency of earthquakes
Land-level change
Seismic gaps along faults
Physical and chemical changes in earth’s crust
Pattern and frequency of earthquakes (foreshocks and microearthquakes)
Land-level change (uplift or subsidence)
Seismic gaps (areas that have not seen recent earthquakes)
Physical and chemical changes (changes in electrical resistivity and groundwater levels)
Perceptions of and Adjustment to the Earthquake Hazard
Perception
One community’s experience does not stimulate other communities to improve their preparedness
Community Adjustments
Critical facilities must be located in earthquake safe locations
Requires detailed maps of ground response to seismic shaking
Buildings must be designed to withstand vibrations
Retrofitting old buildings may be necessary
People must be prepared through education
Insurance must be made available
Personal Adjustments before, during, and after an Earthquake
Before the shaking starts
Make sure that your home is structurally sound
Secure large objects
Turn off gas, water, and electricity
Make personal plan of how to react to an earthquake
During the shaking
Do not panic
Move away from windows, protect your head and face
When the shaking stops
Leave the building
Check for damage and injuries
even though you only have 15 seconds to do this
Volcanoes
Most active volcanos are located near plate boundaries
Volcanos happen along plate boundaries just like earthquakes
Mid-ocean ridges and subduction zones are sites where molten rock reaches the surface
Magma is molten rock
Lava is magma on earth’s surface
2/3 of all active volcanos on land are located along the Ring of Fire which surrounds the Pacific Ocean Indonesia around South American plate and up to Japan
It’s impt to understand how volcanos work to predict volcanic hazards and to understand the nature of the earth’s interior
Pyroclastic Debris
Lava and rock fragments ejected in an eruption
Volcanic Bombs
Large material thrown out and is very viscous
Lapilli
Small volcanic bombs
Caldera
Depression >1km wide
How Magma Forms
Most magmas come from the asthenosphere (weak, but not liquid, layer of rock)
Three main ways in which silicate rocks can melt:
Decompression
Pressure exerted on hot rock is reduced
Occurs at divergent boundaries, continental rifts and hot spots
Addition of volatiles
Chemical compounds that lower the melting temperature of the rock
Addition of heat
As magmas rise, they release heat to overlying rocks
Magma Properties
Magma is composed of melted silicate minerals and dissolved gases
Two most abundant elements in magma are silicon (Si) and oxygen (O); when combined they are referred to as silica (SiO2)
Volcanic rocks are named depending on the amount of silica present in the rock
Basalt, andesite, dacite, rhyolite
Affect style of eruption and type of volcano produced
Silica content of magma is critical
The more viscous the magma the higher and thinner your magma will be (silica rich)
Silica-rich (felsic) lavas
Very viscous, flow slowly
Gases cannot escape easily
Violent eruptions, explosive
Ex. rhyolite
Silica-poor (mafic) lavas
Low viscosity, flow easily
Gases escape easily
Quiet eruptions, lava flows
Ex. basalt
