A
The Lithosphere Part 2
Cinder Cone
A cinder cone is a cone-shaped hill surrounding a volcanic vent,
formed by the accumulation of small pieces of fragmented lava
(pyroclastic cinders and ash) that was spewn out of the vent.
Volcanic Features
A dyke is a hardened sheet of magma that cuts across layers of
surrounding rock. A sill is a hardened magma layer that flowed
between two surrounding rock layers. A laccolith is a dome-shaped
lava chamber that forces adjacent rock layers apart. A batholith is a
massive, deep intrusion of magma.
Eroded Batholiths
When surface layers over batholiths are eroded away and the
batholiths are lifted, massive granitic mountains may be formed. The
mountains from Vancouver to Alaska show batholith roots. The Sierra
Mountains of California are batholiths.
Kinds of Lava
Pahoehoe lava is smooth and wrinkled while aa lava is rough and slaglike with broken fragments. Pahoehoe lava runs smoothly because it
contains many dissolved gases.
Cratons
The oldest and most stable parts of continents are called cratons. A
craton is a section of a continent that has not had mountain-building
activity for hundreds of millions of years. Cratons have less surface
relief like valleys and nearby mountains. The Canadian Shield is a
North American craton.
Accretion
Accretion is a process by which bits
of islands or continent collide, stick
together and accumulate into larger
masses. Most of British Columbia
was formed by accretion as pieces
of continental plates collided with
western North America and stuck to
the west coast of North America.
The formerly separate pieces now
stuck together are called terranes.
Accretion and Vancouver Island
Vancouver Island
is thought to be
made up of at
least three major
accretions.
Isostasy
The state of balance between ocean crusts and continental crusts “floating”
on the asthenosphere is called isostasy. For mountains to stay uplifted on
continental crusts, there must be “roots” below the mountains or they will
sink. When continental glaciers covered Canada and the Northern States,
the glacier weight pushed the continental crust downward. With the melting
of the glaciers in recent Geologic times, the Canadian crust is rebounding
upward because of isostasy while the Southwestern US is sinking as the
continental crust seeks an equilibrium or balance.
Earthquakes
Earthquakes can occur anywhere. More earthquakes occur near
subduction zones than anywhere else. Unlike most other
environmental hazards, earthquakes give no warning. Earthquakes
take more lives than any other type of natural disaster. Some effects
of earthquakes are fires, floods, landslides and tidal waves.
Earthquake Causes
Earthquakes are caused by tension built up as rocks are distorted. As
rocks under tension suddenly slip and move, they release shock waves
that move in all directions from the point making the waves.
Focus and Epicentre
The focus of an earthquake is the point below the earth’s surface
where the energy of the moving rocks is released. The epicenter is the
point on the earth’s surface directly above the focus. At the epicenter,
the shock waves from the earthquake are the strongest.
Earthquake Magnitude
Earthquake magnitude measures the energy difference between earthquakes
and is measured by the Richter Scale. The numbers of the Richter Scale are
logarithmic which means that for each higher number, the earthquake is ten
times more energetic. An earthquake of Richter 5 is 100 times more
energetic than an earthquake of Richter 3. The Mercali Scale measure the
intensity of an earthquake on the earth’s surface in terms of an observer’s
judgements. The Mercali Scale is much less precise than the Richer Scale.
Earthquake Effects on Gravels
Unconsolidated deposits (not cemented together) like gravel beds
often behave like liquids during earthquakes which causes objects
resting on these beds to sink in them. This process is referred to as
liquefaction of the gravel beds.
Relative Position of Earthquakes
Most earthquakes happen on the landward side of a subduction
trench. The more inland the earthquake epicentres, the deeper the
foci are found. This is explained by the deeper a plate is subducted as
it moves from a coastal area.
When Earthquakes are Devastating
The devastation of an earthquake is directly related to the density of
the human population in the earthquake area. Most deaths due to an
earthquake are related to side effects of the earthquake like fires
caused by broken gas and electric systems, disease caused by broken
sewer systems, lack of food and water due to broken water mains and
road damage preventing food transport.
Earthquake Sites
Japan has many earthquakes because it is in a subduction zone.
California has many earthquakes due to many transverse faults.
Oklahoma has more earthquakes than any other place in the United
States and these are related to the practice of fracking for natural gas
in which drills sent deep underground release water and chemicals
which breaks apart deep rocks releasing natural gas but also
weakening lower rock layers leading to earthquakes.
Measuring Earthquakes
A seismograph is an instrument used to measure earth movements.
Earthquakes generate three main types of shock waves called primary
waves (p waves), secondary waves (s waves), and long waves (l
waves). P waves travel the fastest, s waves slower, and l waves the
slowest.
P and S Waves
P waves are compression waves which push and pull back and forth as
they move forward. P waves can travel through solids and liquids. S
waves are transverse waves which move sideways to the direction that
the waves travel in. S waves travel slower than P waves and can not
travel in liquids but do travel through solids.
Locating the Epicenter of an Earthquake
Seismographic stations watch for P and S waves from earthquakes.
The stations record P waves first and a bit later they record the slower
S waves. The time between the first P and first S waves tells the
recording station how far away the epicenter is. A circle is drawn
whose radius represents the distance away that the epicenter is.
Three Seismic Stations Needed to Fix Epicentre Location
When three seismic stations share their circles of epicenter distance,
where the circles all intersect is where the epicenter of the earthquake
is.
L Waves
L waves travel only on the earth’s surface and shake rocks sideways as
they move forward.
P and S Waves Through the Earth
P waves travel through the earth in all directions but S waves can not
pass through the earth’s outer core which is proof that the outer core
is liquid because S waves can’t travel in liquids.
A Past Supercontinent
Scientists have evidence that millions of years ago there was a single
supercontinent made up of today’s separate continents. They have named
this supercontinent Pangea. Evidence suggests that this supercontinent
trapped heat energy beneath it, creating hot spots which caused cracks and
rifts to form. These cracks and rifts eventually split Pangea into a “Northern”
part that has been named Laurasia and a “Southern” part called
Gondwanaland.
The Fates of Laurasia and Gondwanaland
Laurasia later split into North America and Asia. Gondwanaland split
into South America, Africa, India, Antarctica and Australia. Dinosaur
fossils have been found on all of the present day continents as well as
tropical fossils, indicating a warm climate for the ages of Pangea,
Laurasia, and Gondwanaland as well as the ability for dinosaurs to
roam over a single land mass.
End of Presentation
A
A