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Transcript of Unit 3
The Theory of Plate Tectonics
Created based on the evidence of sea-floor spreading and continental drift
States: The lithosphere is broken into several pieces of crust that move on top of the asthenosphere
Studying the ocean floors, earthquakes and volcanoes, scientists now have determined that (as of now) there are 12 major plates
Over 1,000,000 earthquakes recorded every year
Out of this number, 3,000 are felt
Roughly 20 of these cause serious damage
Despite the high temperature in the mantle, most of the rock remains solid. Occasionally, solid mantle and crust melt to form
- liquid, molten rock.
How mountains form
: a large mass of rock that rises a great distance above it's base
Minerals and Rocks
A mineral is a
solid that must have:
1. A characteristic chemical composition
2. An orderly internal structure
3. A characteristic set of physical properties
But how do these plates move?
Remember...things that are more dense will sink, and things that are less dense will rise.
This rising and falling current called a
Difference Between Magma and Lava
- liquid, molten rock that is on the surface of the earth
- liquid, molten rock that is INSIDE the earth
How we know what we know
Alfred Wegner 's
Theory of Continental Drift
Africa and South America look like they fit together
Rock and Mountain formations matched up
paleoclimatology (the study of old climates) show evidence of matching climates across continents
The "smoking gun" of Continental Drift
1947: Scientists decide to map out the ocean floor and mountain ranges and trenches
Dust and other sediments that were on the ocean floor were thinner at the ridges, and got thicker the further away they got
The oldest rocks found in the ocean were around 175 million years old. The oldest rocks found on land were 3.8 BILLION years old
Scientists record the temperatures of the sea floor and also found that the further they moved away from the ridge, the cooler the floor got.
The driving force behind
Late 1950s, two geologists working on mid-ocean ridges proposed two new hypotheses
1. The ridge actually was a crack in the Earth's crust where molten rock (magma) bubbled up from the inside of our planet and forms new ocean rock
2. When new rock formed, older rock gets pushed out of the way, moving continents
The final piece of the puzzle
Using sonar to track soviet union nuclear subs, scientists see stripes on the sea floor
What happens at the edge of all these plates?
Some plates are purely made of oceanic crust, some are purely continental crust,
and others are both
Divergent: Plates move away from each other
They are found at
, like MORs (Mid Ocean Ridges/Rises)
While most divergent plates are found in oceans, some are found on land
Divergent boundaries can push continents together but they can also split them apart.
Convergent Boundaries form where two plates collide
Three things can happen at convergent boundaries
Two plates slide past each other, neither one subducting or colliding into the other.
Many major fault lines occur at transform boundaries.
Also occurs along mid-ocean ridges.
These transform boundaires are called fracture zones
MOST COMMON REASON EARTHQUAKES HAPPEN:
Pressure buildup along faults/plate boundaries (caused by CONVECTION CURRENTS)
The Elastic-Rebound Theory
Faults accumulate stress
Pressure begins to build up.
Pressure hits a tipping point, which releases a energy.
Earthquakes are the result of the sudden elastic rebound of previously stored energy.
FOCUS POINT VS. EPICENTER
the EXACT first point of movement- typically well below the surface of the Earth.
: (We hear more about this on the news) is the point DIRECTLY ABOVE THE FOCUS
CHARACTERISTICS OF SEISMIC WAVES
Primary Waves, twice as fast as S-waves, can move through solid rock, magma, water and air
Sheer Waves (secondary waves), moves at right angles to the direction of movement, can travel through solid material but NOT through liquid or gas
Cause the most damage, only travel through the crust, originate from the Epicenter
How Scientists Measure Earthquakes
A seismograph: An instrument scientists use to record the movement of earth (seismic vibrations from P waves, S waves and Surface waves)
How to interpret SEISMOGRAMS
1. P waves arrive first- the frst major zigzag pattern we see indicates its arrival
2. S waves arrive second and show a significant pattern difference, normally in it's AMPLITUDE
3. The final waves to arrive are the surface waves- most of the time, these are harder to detect
THE LANGUAGE OF WAVES
A disturbance that carries energy through matter or space
Two kinds of waves:
- require a medium to move energy
- do not require a medium to move energy
Waves travel in three ways:
3. Rolling, Surface waves
Steps to finding the epicenter of an earthquake from a seismogram
1. You need to determine (figure out) the arrival time of P and S waves
2. You need to determine the difference in those times
3. Use the difference in arrival times, read the graph's x-axis to determine
the distance the seismograph was to the epicenter of the earthquake.
1. What time did the P-waves arrive?
2. What time did the S-waves arrive?
3. What is the difference between those times?
1. What time did the P-wave arrive?
2. What time did the S-wave arrive?
3. What is the difference between those times?
Though seismograms can tell us the magnitude of
an earthquake and the distance the seismograph is
from the epicenter, it cannot tell us the direction of
We need a MINIMUM of three seismograms from 3 respectively different seismographs to locate the epicenter of an earthquake.
How do we measure the amount of energy released in an earthquake?
The measure of energy an earthquake produces
The Richter Scale:
Measures the intensity of the SURFACE movements
Measures the energy released at the FOCUS, accurately measures the total energy involved in a quake
Scientists have been using the Moment Magnitude since 1993...but because it's not as easily understood by nonscientists, media still uses the Richter Scale.
Richter Scale Info:
Uses a scale of 1-10
Every increase of 1 whole number indicates:
-An increase of 10x the total size of the earthquake
-An increase of 31x the total energy of the earthquake
Damage from Earthquakes
Buildings can withstand ground shaking that is primarily up and down.
Earthquakes, unfortunately, have movement that is both up and down AND side to side.
Damage also comes from:
Fire ** Aftershocks ** Liquefaction ** Tsunamis
Tsunami's predominantly occur at deep ocean subduction zones.
When an earthquake occurs at these undersea boundaries, the energy released causes the water ABOVE the epicenter to rise up and radiate outward.
How we use seismic waves to determine the interior of the crust
Locations on Earth's surface where no body waves can be detected
How Magma Forms
Magma forms under three conditions:
1. A decrease in pressure
2. If the T rises above the melting point of all minerals the rock is made out of
3. Increase of certain fluids, such as water
Any activity that includes the movement of magma onto Earth's surface is called
Many volcanoes (not all) are located along subduction zones.
As oceanic crust subducts, the increase in temperature from friction and the increase of fluid in the trench forms magma- which breaks through the crust- building a volcano
Other Volcanic Zones
(cracks) through which lava flows
Areas of volcanism within the middle of lithospheric plates; originates from something called a mantle plume.
Types of Eruptions
Two types of magma:
1. Felsic- magma that is stickier, less dense and typically very viscous. Found more in the land.
2. Mafic- thinner, less viscous and is more dense. Found more in the oceans and on land.
Viscosity: The resistance of a substance to flow easily
A'a lava is a type of Felsic lava
rough, jagged lava
Pahoehoe is a type of Mafic lava
rope like, "traditional" lava
The type of lava (mafic/felsic) also indicates what type of eruption
Mafic- hotter, less gassy, less explosive eruptions
Felsic- cooler (but still hot,) explosive, a lot of gas
Felsic eruptions can create
Best known pyroclastic flow?
Materials in a pyroclastic flow can be classified by their size:
1. Ash (particles <2mm)
2. Lapilli (particles up to 64mm)
3. Blocks/Bombs (particles >64mm)
HOT SPOTS DON'T MOVE. THE PLATE MOVES OVER THE PLUME!
Types of MAGMA
low silica content, least gassy, least viscous, rarely explosive, highest melting temperature
intermediate silica content (~60%), intermediate gassiness, intermediate viscosity, sometimes explosive, intermediate melting temperature
Super high silica content (>70%), REALLY gassy- gross, most viscous, most explosive, lowest melting temperature
A little basic chem...
Scientists call all the "stuff" of which objects are made from
-Matter is anything that has mass and volume
Atoms: The smallest particle
Made up of:
Cloud of Electrons
Combinations of Atoms
Elements rarely occur by themselves
The forces that hold atoms in molecules together are called chemical bonds.
The way chemical bonds hold the compound together (and the type of bonds) indicate how a mineral crystal grows!
There are about 3800 known minerals today- crystals of various chemical composition
Of these- about 20 are common:
Common Minerals are called
ROCK FORMING MINERALS
because they make up the Earth's crust.
Minerals are broken down into 2 groups:
1. Silicate minerals
2. Nonsilicate minerals
is a solid mineral whose particles are arranged in a repeating, regular pattern
minerals make up 96% of the Earth's crust
and while there are many kinds of silicates, they all have the same basic simple structure of a:
Color is a property of a mineral that is easy to see. Some minerals have very distinct colors. But color alone isn't a good indicator.
Light that is reflected from a mineral's surface
Metallic (like metal)
1. Glassy Luster
2. Waxy Luster
A mineral crystal forms in one of 6 basic shapes
But crystal shape is complex- because growth depends on the environmental conditions (temperature and pressure)
Besides luster, crystal shape and color- there are 4 other properties that can help identify a mineral
Sometimes scratching a mineral against a ceramic
plate can leave a colored streak. Metallic minerals,like goldish
pyrite for example, leave a dark streak.
Minerals break in one of two ways
-Minerals that break along flat planes
-Uneven or irregular fractures
-conchoidal fractures (like glass)
The measure of the ability of a mineral to resist scratching
-Hardness DOES NOT mean "resistance to cleaving or fracture" Only scratching!!
Can be determined using Moh's hardness scale
Special Properties of Some Minerals
Magnetism: Some minerals act as magnets, minerals like magnetite!
Radioactivity: Minerals made out of elements like Uranium and Radium are radioactive. A geiger counter is used to detect how radioactive they are!
Double Refraction: Light rays can penetrate through some clear minerals. Crystals of some minerals, like calcite, bend light in a way that make two images appear.
Fluorescence: In ultraviolet light, some minerals have the ability to fluoresce- meaning they glow
Phosphorescence: After being exposed to ultraviolet light, some minerals continue to glow- giving off light.
Rocks and the Rock cycle
is a piece of matter that is:
2. Naturally forming
3. Made up of either more than one type of mineral OR mineral-like matter
There are three MAJOR types of rocks:
formed from the cooling and hardening of magma
formed by hardening and cementing layers of sediments
formed when rocks are subjected to INTENSE heat and INTENSE pressure
Because crystals form from elements that begin to solidify in igneous rocks, igneous rocks are considered
of an igneous rock depends on the:
of the crystals
of the crystals
of the crystals
The size of the crystals that form in a rock depend on how fast the magma cools!
rocks have larger crystals- considered
take longer to cool. These are also called
rocks as they have formed inside the earth.
rocks have smaller crystals, considered
cools faster. These are also called
rocks as they have formed on the surface.
Some extrusive rocks cool so fast, they don't form any crystals and are as smooth as glass!
Sedimentary rocks are formed from sediments that are bound together
Made up of the following:
1. Pre-existing rocks
2. Weathered or Eroded rocks
The glue that holds sedimentary rocks together: minerals dissolved in water OR pressure that forces fragments (like clay and silt) together
NORMALLY... the high energy of a moving river causes water to push rocks (From boulders to clay) down stream
When the river meets a low energy body of water (like Lake Champlain,) the sediments settle to the bottom.
Rocks come from dissolved minerals by either:
Examples of chemical sedimentary rocks:
(formed in ancient calm seas or warm lakes)
Chemical sedimentary rock can erode and form large caverns and caves, which form things like stalagmites and stalactites
Organic sediments come from living plants and animals that have died and which contain carbon
Limestone of organic origin
Features of Sedimentary Rocks
Sedimentary rocks are identified through their features:
2. bedding planes
4. ripple marks
5. mud cracks
Features are important because it gives clues to the environment that the rock formed in
Two ways that metamorphic rock happens:
Large areas of rock that undergo heat
and pressure from MOUNTAIN BUILDING
Heat comes from: force of friction
Pressure comes from: force pushing grains closer together
Occurs when hot magma heats heats the surrounding rocks.
THE ROCK CYCLE
Most mountains and mountain belts form along convergent plate boundaries- both old and current
To study mountain building, we study what happens at a
- a boundary between continental crust and oceanic crust
Two kinds of margins:
How mountains form at active margins
a measure of the force applied over a given area
in rocks are applied at converging plate boundaries
Rocks at the earth's surface are rigid and when under stress they can fracture.
Rocks below the surface can behave like molten glass- easily stretched without fracturing
By looking for stresses and folds, geologists can tell how and why certain mountains form
Folds and Joints
All stresses contribute to mountain building- many happening at the same type of boundary
Different rocks will respond differently, however, to the same stress
Stress can cause rock layers to crumple into folds and geologists use terms to describe what they see:
Anticline- an upfold in the rock layers
Syncline- a downfold in the rock layers
Limb- the straight piece of rock between anticlines and synclines
Joints are cracks in the bedrock that are vertical- and typically happen where no movement has occurred.
Fault: a break in the lithosphere along which movement has occurred
Faults are important to mountain building since moving large blocks of rock will allow new layers to be exposed to the surface of the earth
Geologists classify faults into one of four categories:
The surface between two faults is called a FAULT PLANE
Pieces of rock that are above the fault plane is called a foot wall, and pieces of rock that are below a fault plane is called the hanging wall
Faults and Stresses
lead to 4 distinct mountain types:
Horsts & Grabens