earth science 5/20
Terms in this set (349)
Structure of the Earth
The Earth is made up of 3 main layers:
-buoyant (less dense than oceanic crust)
- mostly old
- thin (~7 km)
- dense (sinks under continental crust)
Earth is made of 3 surverys
Plate Tectonics: The Earth's crust is divided into .....major plates which are moved in various directions.
The Earth's crust is divided into 12 major plates
The Earth's crust is divided into 12 major plates and this motion causes them to .......
This plate motion causes them to collide, pull apart, or scrape against each other.
Each type of interaction causes a characteristic set of Earth structures .....
or "tectonic" features.
The word, tectonic, refers to
the deformation of the crust as a consequence of plate interaction.
Plates are made of rigid ...
The lithosphere is made up of the
crust and the upper part of the mantle.
Below the lithosphere (which makes up the tectonic plates) is the ....
"Plates" of lithosphere are moved around by the underlying hot mantle convection cells
Three types of plate boundary
divergent, convergent, transform
-As plates move apart new material is erupted to fill the gap
--- is a linear feature that exists between two tectonic plates that are moving away from each other.
-These areas can form in the middle of continents or on the ocean floor.
-As the plates pull apart, hot molten material can rise up this newly formed pathway to the surface - causing volcanic activity.
Age of Oceanic Crust
Red region is youngest, blue region is oldest. This is because of sea floor spreading. we can do dating chemical composition and determine the youngest and oldest parts and we know this because of sea floor spreading
Why don't we have islands like Iceland where ever we get an Ocean Ridge?
Scientists believe that there is a large mantle plume (an upwelling of hot mantle material) located right underneath where Iceland has formed. This would mean that more material would be erupted in the Iceland area compared with if there was just the divergent boundary without the plume underneath it.
Convergent Boundaries--three styles
There are three styles of convergent plate boundaries
-Continent-oceanic crust collision
-When continental crust pushes against continental crust, both sides of the convergent boundary have the same properties (think back to the description of continental crust: thick and buoyant).
Neither side of the boundary wants to sink beneath the other side, and as a result
------the two plates push against each other and the crust buckles and cracks, pushing up (and down into the mantle) high mountain ranges.
For example, the European Alps and Himalayas formed this way.
Continent-Oceanic Crust Collision dehydration explanation
At a convergent boundary where continental crust pushes against oceanic crust, the oceanic crust which is thinner and more dense than the continental crust, sinks below the continental crust.
The oceanic crust descends into the mantle at a rate of centimetres per year. This oceanic crust is called the "Subducting Slab" (see diagram).
When the subducting slab reaches a depth of around 100 kilometres, it dehydrates and releases water into the overlying mantle wedge
The addition of water into the mantle wedge changes the melting point of the molten material there forming new melt which rises up into the overlying continental crust forming volcanoes.
Oceanic lithosphere subducts underneath the continental lithosphere
Oceanic lithosphere heats and dehydrates as it subsides
The melt rises forming volcanism
Ocean-Ocean Plate Collision
When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone.
The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench.
The worlds deepest parts of the ocean are found along trenches.
E.g. The Mariana Trench is 11 km deep!
When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone.
--At a convergent boundary where continental crust pushes against oceanic crust, the oceanic crust which is thinner and more dense than the continental crust, sinks below the continental crust.
Where plates slide past each other
-The third type of boundary are transform boundaries, along which plates slide past each other.
The San Andreas fault, adjacent to which the US city of San Francisco is built is an example of a transform boundary between the Pacific plate and the North American plate.
Pacific Ring of Fire map explanation
Volcanism is mostly focused at plate margins
--This map shows the margins of the Pacific tectonic plate and surrounding region. The red dots show the location of active volcanism. Notice how the majority of the volcanism is focused in lines along the plate boundaries? For this region this area is known as the "Pacific Ring of Fire".
Volcanoes are formed by:Volcanoes can be formed in three ways:
what are Hotspot Volcanoes?
Hot mantle plumes breaching the surface in the middle of a tectonic plate
A hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time.
The source of this volcanism is a mantle plume of hot mantle material rising up from near the core-mantle boundary through the crust to the surface
A mantle plume may rise at any location in the mantle, and this is why hotspot volcanoes are independent from tectonic plate boundaries.
spot of magma that is underneath the earth's crust and when the magma rises it pretty much creates a volcano
The Hawaiian island chain are an example of hotspot volcanoes (see right photograph).
The tectonic plate moves over a fixed hotspot forming a chain of volcanoes. Explain ages of volcanoes
-The volcanoes get younger from one end to the other
-Hotspot's commonly form volcanic island chains (like the Hawaiian islands). These result from the slow movement of a tectonic plate over a FIXED hotspot.
Persistent volcanic activity at a hotspot will create new islands as the plate moves the position of the "old" volcanic island from over the hotspot.
Therefore at one end of the island chain you see the youngest, most active volcanic islands (directly over the hotspot) and along the island chain the extinct volcanoes become older and more eroded (see diagram).
This way geologists can use hotspot volcano chains to track the movement of the tectonic plate through time.
As with volcanoes, earthquakes are not randomly distributed over the globe
At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur.
---The black dots on this map of the world depict where earthquake activity is occurring. You can see that, as with volcanoes, the earthquakes are NOT randomly distributed around the globe. Instead they occur in linear patterns associated with plate boundaries.
(Note this diagram of earthquake distribution closely resembles the Pacific Ring of Fire distribution of volcanism).
Where do earthquakes form?
We know there are three types of plate boundaries: Divergent, Convergent and Transform. Movement and slipping along each of these types of boundaries can form an earthquake.
Depending on the type of movement, the earthquakes occur in either a shallow or deep level in the crust.
The majority of tectonic earthquakes originate at depths not exceeding tens of kilometers.
In subduction zones, where old and cold oceanic crust descends beneath another tectonic plate, "Deep Focus Earthquakes" may occur at much greater depths (up to seven hundred kilometers!).
These earthquakes occur at a depth at which the subducted crust should no longer be brittle, due to the high temperature and pressure. A possible mechanism for the generation of deep focus earthquakes is faulting.
Earthquakes may also occur in volcanic regions and are caused there both by tectonic faults and by the movement of magma (hot molten rock) within the volcano. Such earthquakes can be an early warning of volcanic eruptions.
EARTH'S DYNAMIC SURFACE
CHARACTERIZED BY CONTINUOUS CHANGE
EARTHQUAKES SEEM TO HAPPEN IN SPECIFIC LOCATIONS
WHAT CAUSES THIS CONSTANTLY CHANGING SURFACE?
EARTHQUAKES AND FAULTS
An earthquake is caused by the sudden release of energy stored in rocks along a fault
SAN ANDREAS FAULT
WEST SIDE MOVES NORTH
NORTH AMERICAN PLATE
EAST SIDE MOVES SOUTH
-When earthquakes occur, waves of energy SEISMIC WAVES travel outward from the earthquake focus
--3 types of seismic waves are produced AT THE SAME TIME but each behaves differently within earth.
Primary waves or compression waves vibrate parallel to the direction of movement. (slinky)
Travel faster than any other wave (6-8 km./s)
Travel through solids, liquids, and gases
Shear wave or secondary waves vibrate back and forth perpendicular to the direction the wave is moving
Slower than P waves (4-5 kms./s)
TRAVEL THROUGH SOLIDS ONLY
SURFACE OR LONG WAVES
Vibrations travel along earth's surface in a circular motion at relatively slow speeds (2 kms/s) like waves in a pond
Do more damage
because they produce
more ground movement
VELOCITY depends on...... for seismic waves
Velocity depends on the material they are passing through
Increase density and pressure - greater the velocity
Waves are refracted or bent as waves pass through material with different densities
EARTHQUAKE SHADOW ZONES
At an angle of 103° (distance 11,000 kms) from the epicenter both P and S waves disappear
P waves can again be detected at l42° (16,000 kms.)
NO S WAVES EVER APPEAR AGAIN
THIS BAND OF 39° IN WHICH NO WAVE ARE OBSERVED IS CALLED THE EARTHQUAKE'S SHADOW ZONE
-However, scientists noticed something unusual. When the angle around the earth's circumference from the epicenter is more than 102 (This is a distance of about 11,000 kms.) both the P and S waves disappear. Then the P waves can again be detected if the station is more than l43 (l6,000 kms.). NO S WAVE ARE EVER OBSERVED. THE RESULT IS A BAND OF ABOUT 41 OR 4500 KMS. wide in which no waves are observed. This region is called the earthquake's shadow zone. Location of the shadow zone will depend on the focus of the EQ. Each EQ produces its own shadow zone.
EARTHQUAKE SHADOW ZONES
P WAVES DISAPPEAR FROM 105º -140º
S WAVES DISAPPEAR FROM 105º NEVER TO APPEAR AGAIN
ANALYSIS OF SEISMIC WAVES
HAVE RESULTED IN THE INFERENCE ABOUT EARTH'S INTERIOR
-S WAVES CANNOT PASS THROUGH THE LIQUID OUTER CORE
LOCATING THE EPICENTER OF AN EARTHQUAKE
The difference in travel time between P and S waves can be used to determine the DISTANCE from a station to the epicenter
The farther a station is from the epicenter, the GREATER the time interval between the arrival of P and S waves
TO DETERMINE THE EXACT LOCATION OF AN EPICENTER, ITS DISTANCE FROM 3 STATIONS MUST BE DETERMINED AND 3 CIRCLES DRAWN
P WAVES TRAVEL FASTER THAN S WAVES
AS DISTANCE FROM EPICENTER INCREASES -THE GREATER THE TIME INTERVAL BETWEEN P AND S WAVES
LOCATING EARTHQUAKE EPICENTER
MUST HAVE DISTANCE FROM 3 SEISMOGRAPH STATIONS TO DETERMINE EPICENTER
Origin of the Earth--first event
1.)Meteors and Asteroids bombarded the Earth
- increased mass
- therefore increased gravity
origin of the earth--second event
Gravitational energy turned into heat.
origin of the earth--third event
3.) Radioactive element are unstable
Over time became more stable, released he
-Density Stratified planet
Earth's Interior core contents: what is it made out of
Iron and Nickel
Earth's Interior :Mantle
Less dense than core
Iron and Magnesium silicates
Upper mantle is partially molten
Earth's Interior: crust characteristics
Very thin and rigid
Continental - granite
Density = 2.8 g/cm3
Oceanic - basalt
Density = 3.0 g/cm3
composed of two parts. The core is very dense and composed mostly of Iron and Nickel.
partially molten mantle acts as a lubricant. This enables ....
partially molten mantle acts as a lubricant. This enables the plates to move freely along the surface of the Earth
Thin crust + uppermost mantle is rigid. This forms the "lithospheric plates".
Mantle is composed of
Mantle is composed of Iron and Magnesium silicates (Si + O)
It is mostly solid, but the uppermost part of the mantle is partially molten (slush - where you do have liquid and solid mixed).
Crust -This is the outermost layer. It is very thin and rigid. Rigid means that it is a solid.
the inner core is solid. This is because
The inner core is solid. This is because there is so much pressure exerted that it can only exist in the solid form.
the outer core is liquid because
The outer core is liquid iron and nickel it is very dense but because it is in its liquid form it is less dense than solid and forms the outer core.
Evidence of Internal Structure: Density
calculate density of Earth
Speculate on probable compositions
Evidence of Internal Structure: Meteorites
Use composition and age to determine composition and age of Earth
Evidence of Internal Structure: Seismic waves
Travel times and direction give indication of internal structure of Earth
Meteorites are composed of
Meteorites are composed of Iron and Nickel.
It is hypothesized the Earth's interior is composed of .....
It is hypothesized the Earth's interior is composed of the same elements. P-waves travel 4x faster than S-waves and also travel through different material. These also give us some clue on the properties of the Earth's interior
evidence of earths internal structure:: If you calculate the Earth's surface using granite, it comes to 2.8 gm/cm3. What is this telling us?
By calculation, the Earth's density is 5.8 gm/cm3. In the Sierras, composed mostly of granite, represents the thin part of Earth's interior, i.e. skin of an apple. There are heavier rocks in the Earth. We know the age of the earth is 4.6 Ga. People who study meteorites determined their age to be about 4.5 Ga - same as Earth.
Seismic Waves Through Earth: scientists use waves
Scientists use waves generated by earthquakes to determine the Earth's interior. that the outer core of the earth is liquid.
Earthquakes generate P-waves and S-waves within the earth.
on the opposite side of the earth from the earthquake epicenter because the outer core reflects S-waves, and bends P-waves. S-waves are reflected because they cannot travel through liquids, and they cast a larger shadow than the bent P-waves.
Geologists and seismologists determined the size of the outer core by using
the 154-degree arc of the S-wave shadow and measurements taken on the surface of the earth.
-Consists of continental, oceanic and upper part of mantle
-Continents composed of granite-type rock, quartz and feldspar minerals, density+2.8g/cm3
-Oceanic crust formed of basalt; basalt rich in iron/magnesium minerals, density+3.0 g/cm3
-Lithosphere is rigid layer of crust and mantle overlying partially-molten asthenosphere
--Why do continents sit higher than ocean basins?
It is their composition and density. Continent = Quartz (SiO2) and Feldspars (K, Ca, Na, Al, SiO2). These are light elements. Ocean = Basalt (Fe and Mg). These are heavier elements. Asthenosphere is partially molten. It forms a lubricant to allow plates to move.
Continental Drift researchers noted....
--Researchers noted geographic fit of continents
e.g. Africa and S. America
Atlantic formed by separation of Africa from S. America
Seuss, 1885, proposed
super continent by studying fossils, rocks, mountains
one big continent--pangea
Wegener and Taylor, early 1900's, proposed
continental drift and Pangaea
Evidence supporting the idea that the continents had drifted
Geographic fit of continents
Continental Drift Fossils
Similar distribution of fossils such as the Mesosaurus found in africa and america
--Studies were done on land animals that could not swim. Once continents split, evolution occurs. Organisms begin to differentiate. Climate is different, food sources are different, predators are different.
Continental Drift Glaciation
Glaciers typically occur near the poles. We are in a warm period, but in the past it was cool and ice caps grew. Deposits related to glaciers were deposited. You also find glacial deposits in Africa near the equator.
How to you get glacial deposits near the equator?
because of continental drift, glaciers were in places where we could not normally have them
Continental Drift Model Problems: Alfred Wegener
-Presented research to professionals
-Did not provide a plausible mechanism to explain how continents drifted
---Wegener was a scientist. Scientists go to scientific meetings to present their research. When Wegener presented his research to other physicists, they did not agree. They wanted to know how do continents drift? There was no driving mechanism to support continental drift. Wegener could not answer "how" so his Continental Drift Model was not accepted.
-Continental drift reexamined in 1960's with new information
--New theory developed - Seafloor spreading
+++Theory combining continental drift and seafloor spreading termed "Plate Tectonics
--------New sea floor created at the mid-ocean ridge and destroyed in deep ocean trenches
---------The idea of seafloor spreading is that new crust is being formed at spreading centers and old crust is being destroyed in deep trenches. \
For example, Mid-Atlantic ridge is a chain of underwater mountain chains where new crust is being formed
the theory that combined seafloor spreading and continental drift
---Supporting evidence for seafloor spreading
= World seismicity
= Age of seafloor
= Heat flow
Evidence for Seafloor Spreading World Seismicity
we can look at seismic activity throughout the world and the areas of large seismic activity happen on the plate boundaries
volcanoes form via subduction
Via subduction. The subducting slab dehydrates to form new melt that will rise through the crust to be erupted at the surface.
volcanoes form via rifting
Via rifting. When two plates pull apart magma rises, producing volcanic eruptions at the surface.
volcanoes form via hotspots: where do they form?
At "Hotspots"....hotspot do not necessarily occur along a plate boundary. So hotspot volcanoes can form in the middle of tectonic plates
The oceanic crust descends into the mantle at a rate of centimetres per year. This oceanic crust is called the subducting slab
--When the subducting slab reaches a depth of around 100 kilometres, it dehydrates and releases water into the overlying mantle wedge
seismic, gravity, magnetics, electrical, geodesy
get data about land, air, sea and satellite
- fieldwork, boreholes, mines
(an upwelling of hot mantle material)
how is Subduction a way of recycling the oceanic crust?
Eventually the subducting slab sinks down into the mantle to be recycled. It is for this reason that the oceanic crust is much younger than the continental crust which is not recycled.
Volcanism is mostly focused at
"Deep Focus Earthquakes"
In subduction zones, where old and cold oceanic crust descends beneath another tectonic plate, "Deep Focus Earthquakes" may occur at much greater depths (up to seven hundred kilometers!).
is a crack in a mass of rock along which there has been movement of rock layers on either side of the crack
- where EQ starts below surface
location directly above on land or water
A volcano is a mountain that forms when magma reaches the surface.
Volcanoes can result from several different geological processes and can take a variety of forms.
How do volcanoes form?
Under certain conditions, small amounts of mantle rock can melt, forming liquid magma. The magma rises upward through the crust, erupting at the surface as a volcano.
formation of a volcano
The process that leads to a volcanic eruption begins deep inside Earth.
Magma rises because it is less dense than the solid rock around and above it.
How a Volcano Erupts
-Magma is under pressure and contains dissolved gases, including carbon dioxide and water vapor.
-Lower pressure near the surface allows the gases in magma to expand rapidly.
--An eruption occurs when the gases bubble out through a crack in the crust, propelling magma to the surface.
Structure of a Volcano: magma chamber
Before an eruption, magma often collects in a pocket called a magma chamber
Structure of a Volcano: pipe
Magma slowly accumulates in the magma chamber until enough pressure builds up to start an eruption.
Then, magma rises to the surface in a narrow, vertical channel called a pipe.
Structure of a Volcano: vent
An opening in the ground where magma escapes to the surface
----Often there is one central vent at the top of a volcano. Sometimes there are other vents that open along a volcano's side.
structure of a volcano: crater
At the top of the central vent in most volcanoes is a bowl-shaped pit
structure of a volcano: caldera
After an eruption, a volcano's magma chamber and main vent may empty of magma, creating a hollow shell.
If this shell collapses inward, it creates a huge depression, called a caldera, at the top of the volcano.
Why are some volcanic eruptions quiet and others explosive?
Volcanoes erupt explosively or quietly, depending on the characteristics of the magma.
Magma can vary in viscosity, the resistance to flow.
Magma with high viscosity is thick and resists flowing. Magma with low viscosity is thin and flows easily.
3 main factors that determine viscosity of magma
temperature, water content, and silica content.
3 main factors that determine viscosity of magma: temp
Higher temperatures lower the viscosity of magma, so it flows more easily.
3 main factors that determine viscosity of magma: water content
Water in magma helps it flow more easily
3 main factors that determine viscosity of magma: silica content
Magma that is high in silica has high viscosity.
Volcanoes that have very hot, low-silica magma generally erupt quietly.
In a quiet eruption, lava erupts in a stream of low-viscosity lava, called a lava flow.
Lava flows from a quiet eruption can travel for great distances.
In a quiet eruption, lava erupts in a stream of low-viscosity lava,
quiet eruptions produce 2 diff kinds of lava:
Mt. Kilauea in Hawaii erupts...
quietly, producing low-viscosity lava flows.
Explosive Eruptions: what magma produced explosive eruptions?
High-silica magma produces explosive eruptions.
Thick magma can clog a volcanic pipe, causing enormous pressure to build up.
When the volcano finally explodes, lava and hot gases are hurled outward.
Where are volcanoes found?
Most volcanoes occur along plate boundaries or at hot spots in the crust
Except for hotspot volcanoes, most of the world's volcanoes form near
magma forms intrusive igneous rock
Sometimes magma does not reach the surface, but cools and hardens in the crust.
This magma forms intrusive igneous rock that may eventually be forced upward and exposed at Earth's surface.
What landforms are formed from lava and magma?
Igneous features formed by magma include batholiths, sills, dikes, and volcanic necks
Lava plateaus are features formed of
extrusive igneous rock.
Youngest sea floor is at
Oldest sea floor away from
Why is the ocean so young?
Age of Earth is 4.5 Ga. Oldest oceanic crust is only 180 Ma.
Old oceanic plate is being subjected or destroyed.
because of the rift valley sea-floor spreading and divergent boundaries. New sea-floor is being created in a divergent boundary
evidence of seafloor spreading: Paleomagnetism
Earth has a magnetic field - Probably caused by rotation of solid inner core in liquid outer core (both mostly Fe)
When rocks cool at the Earth's surface, they record Earth's magnetic field (normal or reverse polarity)
when you have seafloor spreading you have a divergent plate boundary and in this boundary magma is going to come up as lava and then it is going to cool and form new crust. Paleomagnetism describes how the atoms are going to orient themselves in the crust as it cools. Therefore you are going to see this striping that is going to represent how the earth's magnetic field is going to reverse periodically. Atoms are going to be arranged in a specific pattern corrdinating to the reverse of the magnetic field
Earth has a magnetic field -
Probably caused by rotation of solid inner core in liquid outer core (both mostly Fe)
Today, Earth's magnetic field points to the North. Why?
They believe that the magnetic field is caused by the liquid outer core moving around the solid inner core by Earth's spin. The iron (Fe) content in the core produces a magnetic field. Magnetic field is important to us in that it prevents harmful radiation of the Sun to reach us. Every 700,000 years or so, the Earth's magnetic field reverses. These reversals are recorded in molten magma. As the magma cools, its iron-rich minerals tend to line up with Earth's magnetic north.
paleomagnetic studies indicate....
alternating stripes of normal and reverse polarity at the mid-ocean ridge.
--The record of these alternating reverse and normal polarity helps support the idea of seafloor spreading.
seafloor spreading: heat flow
Heat flow provided the mechanism to move the lithospheric plates.
we can look at a heat map at regions where there is a lot of heat or a little heat and see the focused plate margins
Seafloor Spreading: Convection Currents
In 1960, proposed as driving force to move continents
in heat flow you can look at a heat map of the earth and places of hot areas are near the plate boundaries and convection currents have hot stuff rising, cool stuff sinking, and that is going to cause a flow that is going to be one of the proposed mechanisms from plate tectonic motion
Magma was found were new seafloor was being made.
a plate is
=the cooled surface layer of a convection current in upper mantle
theory of plate tectonics: john tuzo wilson
John Tuzo Wilson combined ideas of continental drift and seafloor spreading into "Plate Tectonics"
Principles of plate tectonics: earths outermost layer is composed of
Earth's outermost layer composed of thin rigid plates moving horizontally
Plates interact with each other along their...
along their edges (plate boundaries)
Plate boundaries have high degree of tectonic activity examples
What are plate tectonics and continental drift?
The theory of plate tectonics explains the formation and movement of Earth's plates.
Wegener hypothesized that......
Wegener hypothesized that the continents were once joined in a single supercontinent, which then broke into pieces that moved apart.
is the theory that pieces of Earth's lithosphere, called plates, move about slowly on top of the asthenosphere
According to Wegener's hypothesis, the continents move slowly across Earth's surface
In 1912, Alfred Wegener proposed a hypothesis of continental drift to explain these puzzling observations.
Wegener called the ancient supercontinent Pangaea.
Continental drift explains why
the continents seem to fit together. It also explains why the fossils from a single region appear across the globe.
Wegener was unable to explain
Wegener was unable to explain how the continents could plow through the solid rock of the sea floor or what force could move entire continents.
As a result, most geologists rejected continental drift.
What are the roles of sea-floor spreading and subduction in plate tectonics?
Sea-floor spreading creates new oceanic crust at mid-ocean ridges. Subduction destroys old oceanic crust at subduction zones.
a chain of underwater mountains which they called the mid-ocean ridge.
It forms the world's longest mountain chain.
Formation of Oceanic Crust
The mid-ocean ridge is a huge crack where magma pushes upward.
The parts of the ocean floor on both sides of the central valley are moving apart.
Magma from the mantle wells up and solidifies to form new oceanic crust.
Sea-floor spreading is the process
by which new oceanic crust is created at mid-ocean ridges as older crust moves away.
During sea-floor spreading, oceanic crust forms at the mid-ocean ridge-> the crust...
This crust gradually moves toward a subduction zone, where old crust sinks beneath a trench.
As a plate sinks through a subduction zone, it bends, forming a depression in the ocean floor
Subduction occurs because,
as an oceanic plate moves away from the mid-ocean ridge, it gradually cools and becomes more dense.
During subduction, the force of gravity....
slowly pulls the dense edges of oceanic plates into the mantle, destroying old ocean floor.
Evidence for Sea-floor Spreading
Scientists discovered patterns of parallel magnetic "stripes" that were identical on the two sides of the mid-ocean ridge.
Earth's magnetic field has reversed itself many times.
The magnetic field causes rock crystals to line up in a certain way before the rock solidifies.
Stripes show that new ocean floor was added to both sides of the mid-ocean ridge.
that new ocean floor was added to both sides of the mid-ocean ridge.
Geologists used radioactive dating to determine
the ages of rock samples from the ocean floor.
Geologists used radioactive dating and they found
They found that rocks nearer the mid-ocean ridge were younger, and rocks farther from the ridge were older.
Why do tectonic plates move?
Plate motions are the visible part of the process of mantle convection.
form in the mantle as hot rock rises, cools and spreads out, and then sinks back into the mantle at subduction zones.
These sinking slabs of dense lithosphere and heat from within Earth drive the circulation of convection currents in the mantle.
hot stuff rising cool stuff sinking
The Theory of Plate Tectonics
Heat flows from Earth's hot interior toward the cooler surface mainly through large convection currents in the mantle.
Plates are the uppermost part of a global convection system.
The heat that drives convection in the mantle comes from two sources.
Earth was very hot when it first formed, and some of the heat moving upward in convection currents is due to the gradual cooling of its interior.
A second source of heat is the result of the decay of radioactive isotopes that are distributed throughout the mantle and crust.
The plates move very slowly, about
0.1 to 10 centimeters per year
When plates move apart, magma ...
rises to fill the gap and form new rock at the edge of each plate.
What type of plate boundary causes mountain chains, such as the Himalayas, to form?
What causes Earth's plates to move?
convection currents in Earth's mantle
Deformation and . Mountain Building: 5 points
Plate Tectonics and Stress
Origin of Mountains
Tectonic Stresses ->
Large Scale Strain of the Crust i.e., Geologic Structures
Tectonics and Structural Geology:
(heat driving convection)
Strains (deforms) the Mantle and Crust
Bends Rocks, i.e.,
ductile strain (Folds)
Breaks Rock, i.e.,
brittle strain (Joints) and
Moves large blocks along
Geologic Structures: Folds)
Bends Rocks, i.e.,
(compressive stresses may cause ductile strain)
Breaks Rock, i.e.,
Geologic Structures: Faults
Moves large blocks along
Faults (Any type of stress may cause brittle strain. The type of fault depends on the type of stress)
stresses at plate boundaries: divergent
stresses at plate boundaries: convergent
stresses at plate boundaries: transform
strikes and dips are used to
identify geologic structures
Strike and Dip
Define and map the orientation of planar features
planar features examples
Bedding planes (sedimentary rocks)
The intersection of a horizontal plane (the water surface) and an inclined plane (the surface of any of the rock layers) forms a line
(The line of intersection between the plane and a horizontal surface)
is their maximum angular deviation from horizontal. Notice the strike and dip symbol with 50 adjacent to it, indicating the angle of dip.
(Angle that the plane makes with that horizontal plane)
Sipping Bedding Planes
youngest = top
oldest = bottom
sedimentary rocks dip in the direction of the
oldest rocks occur in the center of the fold
youngest rocks occur in the center of the fold
the angle in which something folds at
Fold terminology: Axis
the line through which a fold happens
Fold terminology: Axial plane
the plane through which the fold happens
Folds and faults resulting from compressive stresses
Anticlines (many plunging)
Synclines (many plunging)
we have certain types of compression forces
-similar to an upside-down cereal bowl
-oldest stuff in the middle and youngest stuff on the outside
youngest stuff in the middle, oldest stuff on the outside
-similar to an upright cereal bowl
brittle strain ask croce
When shallow crust is strained rocks tend to exhibit brittle strain
layers of a book
a bunch of layers and a fracture causing it to look like a book
Strike of fault plane parallels the
fault trace and
Direction of Dip of the fault plane indicates the
Hanging wall block
Movement occurring along a discontinuity
Brittle strain and subsequent movement as a result of stress
strike slip 2 types
Structures at Divergent Boundaries
Tensional Stresses cause brittle strain and formation of sets of normal faults
formed by the horsts...
Older Rocks are exposed along the ridges
horsts = hill
Younger rocks lie beneath the
Sediment fills in the
nevada-- as a result of horsts and grabens
Compressive stress causes the hanging wall to move upward relative to the foot wall
At convergent plate boundaries ancient rocks can be thrust over younger rocks
Rising of less dense salt
Stretches overlying crust
Forming normal faults and
structural oil traps how do they form?
the salt rock is much less dense and it is going to push up and as it is pushing up it is going to force the oil upwards
Evidence for Continent Movement
Similarity of fossils and rock formations on distant continents
Actual measurement of continental separation
Earth's Layers (based on Seismology) - MANTLE
Composed of iron-rich silicates
Has an upper layer that is "plastic" or semi-fluid
Has a higher temperature than the crust
"plastic region in the upper part of the mantle under the lithosphere; the plates "ride" on the asthenosphere
Divergent Plate Motion
Tensional forces stretch the lithosphere
New Earth materials are formed between plates where hot mantle material rises into stretched area
Convergent Plate Motion
Causes collision of plates with compressional forces. Convergence leads to subduction of one plate under another
Oceanic crust may go under continental crust generating a trench and causing mountain building
-Continental crust may go under other continental crust with mountain building
Convergent Plate Motion: Ocean crust may go under ocean crust in a trench causing
Transform Fault Boundary
Plates may slide past each other laterally with no subduction
Seismic activity is high along these slip boundaries
Relative Age Dating: Comparative Records of Time:
Nature of rock records
Principles of stratigraphy
Deposition, succession, continuity, and correlation
Relative Age Dating: tools list
Certain fossils were only alive at particular periods of time
Knowing their range of existence helps you date the sedimentary layer
Relative ages determined from fossil assemblages
Biotic changes are a function of extinctions and evolutionary processes
Studying magnetic reversals
Polarity will reverse about every 200,000 years
Knowing if a piece of rock layer has a normal or reverse polarity will help date the layer.
Approach based on intermittent, irregular reversal of the Earth's magnetic field.
Rocks record field at time of formation (cooling)
Magnetic signals preserved in stratigraphic sections show alternating sequence of polarity
Series of polarity shifts:
Polarity intervals are independent of lithology
Vary in duration
Progression of sedimentary rocks from the changes in sea level
Certain areas were covered in water and the sea will deposit sediment differently than the way sediment is deposited above sea level.
Study the chemical composition of the rock. Those changes can be identified with particular time periods.
"beds" or " horizons" and depositional duccession
Sedimentary rock is constantly deposited as "beds" or " horizons" in rock units
This allows a record to be preserved of how sediment was deposited
Beds are often discontinuous
Beds can be eroded or lost
Results in a gap in the temporal record
Known as an "unconformity" or "hiatus"
Results in a gap in the temporal record
Known as an "unconformity" or "hiatus"
(to form deposit arrange)stratified rocks overlying unstratified rocks
Distinct change in rock type, age, orientation, or structure
Laws Governing Stratigraphic Relationships list
Relative ages from sequence of rock deposition
Orientation of beds when deposited
Spatial correlation of individual horizons and rock units
Sequence of events are recorded in rock relationships
The Law of Superposition
Will show you the order of layered units
A rock unit is younger than the one below and older than the one above
Temporal succession of rock units
Deposition is not necessarily continuous, but is sequential
Law of Initial Horizontality
Constraints on Original Orientation:
Sediments are deposited as horizontal beds
Principle ONLY applies to sedimentary rock formed in an aqueous environment
states that when they first are deposited they get deposited horizontally
Law of Initial Horizontality:: Evidence of Deformation:
Non-horizontal sedimentary rocks
Modified by post-depositional events (folding, fractures, faulting)
Law of Lateral Continuity
Sediments form as continuous layers
Individual horizons or layers thin or end only when the environment of deposition changes
Enables correlation of beds with specific characteristics
states that if you take a chunk out of something and if they connect horizontally laterally that means that they are the same age.
Cross Cutting Relationships
Younger units cross-cut older units
Erosion surfaces, intrusions, unconformities
Sequential order of deposition determined by correlation of separate, related stratigraphic records
Unconformities may be recognized but uncertainties may persist.
(reference points through which measurements are made in geology) record timing of biotic changes
First appearances and last appearances
Boundaries calibrated by absolute ages
Biostratigraphy: Temporal Records of life:
Recognition of species unique to a particular time interval
relative ages determined from fossil assemblages
biotic changes are a function of extinctions and evolutionary processes
Magnetostratigraphy: Sequence of polarity reversals recognized
Require excellent stratigraphic resolution
Globally uniform series of time dependent reversals
Ages determined by absolute dating.
Magnetostratigraphy:; records and correlations
Record compiled from multiple overlapping sequences
Correlations to stages often based on biostratigraphy
Sequence of Stratigraphy: Principles
Controls on the sedimentation process:
Sequence of Stratigraphy: Principles : Controls on the sedimentation process:
Sediment production and accumulation is controlled by:
Sea level- water depth, accommodation space
Tectonic subsidence- accommodation space
Climate- weathering rates, grain production
Cycles in these variables operate over different time scales
Result from multiple studies of these cycles
A temporal record of changes in sea level
Shallowing upward sequences produce defined patterns of sediments
Rock records indicates shallowing
Spatial Arrangement of Stratigraphic Units
Lateral and vertical relationships in parasequences
Predictable, recognizable sequences develop
Stacking of parasequences:Vertical sequences record sea level change:
Sequential order of characteristic elements:
Sequence boundary, lowstand systems tract, transgressive service, transgressive system tract, maximum flooding surface, highstand system tract, sequence boundary
Depositional Sequence: relevance
a sea-level change
we can look at how different parts of a rock is deposited and based off of that we can determine what types of erosion occurred and can help us determine the sea level at a particular time
Chemostratigraphy or Isotope Stratigraphy
Stratigraphic variations in specific chemical or isotopic characteristics
Choose stable isotopes: C (13C), O (18O), S (34S)
isotopic abundances and looking at the chemical composition of how rocks have changed
the bock of rock that lies above the fault
block of rock that lies below the fault
a force that acts on a rock to change its shape or volume
a fold in rock that bends downward to form a valley
a fold of rock that bend upward to form an arch
a large area of flat land elevated high above sea level
Reverse fault: movement along fault
hanging wall goes up, footwall goes down
normal fault: movement along fault
hanging wall goes down, footwall goes up
transform fault: movement along fault
side against each other
volcanoes at boundaries where two oceanic plates collide create a string of islands
ring of fire
formed by several volcanoes that rim the pacific ocean and is a major volcanic belt
many volcanoes form near...plate boundaries where.....
converging, oceanic crust returns to the mantle
Volcanoes can form along .....plate boundaries on land
types of plates that collided to form the andes mountains on the west coast of south america
continental plate and an oceanic plate
Density stratified planet
speculate on probable composition
meteorites use composition and age to determine composition and age of earth
seismic wave travel times and direction dive indication of internal structure pf earth
1885- super continent
1960 proposed as driving force to move plate tectonics
indicates stripses of normal and reverse polarity at the mid-ocean ridge
how do fault block mountains form
when two normal faults cut though a block of rock and fault m0vements may push up a fault block mountain--created by tectonics and localized stresses in the earths crust
how do folded montains form
formed when 2 tectonic plates that make up the earths crust push together at the border causing extreeme pressure which forces the edges of the plates upwards into a series of folds
law of inclusions
if one rock body contains fragments of another rock body it must be younger than the fragments of rock it contains
or dots are older
the inclusions are older than the rocks which contain them
crack in a rock
1960 proposed seafloor spreading
compressional folds examples
buckling and bending
they are open folds
happens when something is compressed horizontally
direction of dip of fault plane indicates...
the hanging wall block
happens when something is pushed on vertically
right later and left later faults only occur in
passive continental margins
the margins where we are not necessarily having plate tectonic activity
san Andreas fault is what transform
earths magnetism (solid inner core and outer liquid are both mostly made of Fe)
when rocks cool at the surface they record earths magnetic fields
study of chem comp of rock
changes can be identified with time periods
magnetic signals preserved in stratigraphic sections shown an alternative sequence of polarity
series of polarity shifts
polarity intervals are independent of lithology
-vary in duration
constant periods when the earth's magnetic field stays constant over a long period of time
ossilation very rapidly periods of time when the earth's magnetic field switches rapidly
sediment production and accumulation is controlled by
sea level-water depth
climate-weathering rates, grain production
isotopic stratifraphy chemistratigraphy
stratigraphic variations in specific chemical or isotopic characteristics- what is left
chose stable isotopes--radioactive decay
what is left shows age
using the relative abundance of isotopes at a particular period of time because a long time ago carbon 13 had a different abundance than it does not, for example.
direction: move away
plate boundary: rises, volcanoes, earthquakes, creating crust
direction: move away
plate boundary: plates separate or pull apart
transform continental continental
direction: side to side
plate boundary: plates slide past each other, earthquakes, crates/destroy crust
convergent oceanic continental
direction: move close
plate boundary: earthquakes
volcanoes destroys coast
plate boundary: plates come together
mountians form no volcanoes
oceanic is .....dense than continental
inner core and outer core and its affect on polarity
the outer core is going to be convecting because it is going to be made of liquid, and the inner core is going to be rotating because it is solid. This rotation of the inner and outer core is going to create a magnetic field. Because it is convecting and rotating, the magnetic fields can change.
boundaries vs faults
fault= how the rock moves
boundary= how the plate moves
Who suggested seafloor spreading?
henry Hess in the 1960's
Heat of the earth. Where does it come form?
when the earth first formed there was a lot of uranium in it due to nuclear fusion, due to heavy elements being formed together, but uranium 235 is a very unstable isotope, thus it is going to decay emitting alpha and beta particles, as it released alpha and beta particles it is going to release a lot of energy causing it to be very hot and that is where the heat of the earth comes from. Uranium 235 will decay into a stable isotope of lead 206
have compression going horizontally the joint sets are going to be vertical
vertical strain= horizontal joint sets
tensional stresses cause brittle strain and formation of sets of normal faults
A rock is a naturally occurring solid mixture of one or more minerals, or organic matter
How are rocks classified
Rocks are classified by how they are formed, their composition, and texture
rocks change over time through....
the rock cycle
Igneous Rock begins as....
Igneous rocks: Magma can form
-When rock is heated
-When pressure is released
-When rock changes composition
Igneous rocks: Magma freezes between
700 degrees c and 1250 degrees c
a mixture of many minerals
Igneous Rock: Felsic
light colored rocks that are rich in elements such as aluminum, potassium, silicon, and sodium
Igneous Rock: Mafic
dark colored rocks that are rich in calcium, iron, and magnesium, poor in silicon
Igneous rock: coarse-grained
takes longer to cool, giving mineral crystals more time to grow
Igneous Rock: Fine-grained
cools quickly with little to no crystals
gabbro= coarse grained
Igneous Rocks: Intrusive igneous rocks
magma pushes into surrounding rock below the Earth's surface
Igneous Rocks: Extrusive rocks
forms when magma erupts onto the Earth's surface (lava), cools quickly with very small or no crystals formed
Igneous Rocks: obsidian
Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form
this rock is mafic, finegrained, and extrusive
Sedimentary Rocks are formed
by erosion, and at or near the earth's surface
sediments are deposited in...
layers with the older ones on the bottom
the layers become compacted and cemented together
sediments are moved
from one place to another
what is not involved in sedimentary rocks?
heat and pressure
layers of rock
the process in which sedimentary rocks are arranged in layers
Sedimentary Rock: Clastic
made of fragments of rock cemented together with calcite or quartz
Breccia is a term most often used for clastic sedimentary rocks that are composed of large angular fragments (over two millimeters in diameter).
The spaces between the large angular fragments can be filled with a matrix of smaller particles or a mineral cement that binds the rock together.
minerals crystallize out of solution to become rock
is a sedimentary rock composed primarily of calcium carbonate (CaCO3) in the form of the mineral calcite. It most commonly forms in clear, warm, shallow marine waters.
It is usually an organic sedimentary rock that forms from the accumulation of shell, coral, algal and fecal debris.
remains of plants and animals
Coal is an organic sedimentary rock that forms from the accumulation and preservation of plant materials, usually in a swamp environment.
Coal is a combustible rock and along with oil and natural gas it is one of the three most important fossil fuels.
to change shape
metamorphic rock changes with
temp, pressure, but remains solid
metamorphic rocks usually take place
deep in the earth
Contact Metamorphism -
heated by nearby magma
Increased temperature changes the composition of the rock, minerals are changed into new minerals
is a fine-grained non-foliated metamorphic rock produced by contact metamorphism
Regional Metamorphism -
pressure builds up in rocks that is deep within the Earth
Large pieces of the Earth's crust collide and the rock is deformed and chemically changed by heat and pressure
contain aligned grains of flat minerals
is foliated metamorphic rock that has a banded appearance and is made up of granular mineral grains.
It typically contains abundant quartz or feldspar minerals.
mineral grains are not arranged in plains or bands
is a non-foliated metamorphic rock that is produced from the metamorphism of limestone.
It is composed primarily of calcium carbonate.
Part(s) of the earth responsible for the earth's magnetic field
outer and inner core
The part of the earth that makes up the plates.
The part of the earth that magma comes from.
The reason why scientists know that the outer core is liquid and the inner core is solid
the refraction of P waves and the stopping of S waves
the density of the oceanic crust
The law that describes the order of sedimentation
the Law of Superposition
The area of lithography that uses index fossils to date a layer of rock
A gap in the stratigraphy record due to erosion
Evidence of the earth's pole reversals is evident in this. Describe how to tell the pole orientation, how it switches, and how we can classify different intervals.
Magnetostratigraphy is the study of the pole reversals based on the orientation of iron atoms in the igneous rock. It switches due to the convection of the outer core and the rotation of the solid inner core. We can classify it as normal or reversed polarity, major intervals, and minor intervals
The type of rock that occurs due to changes in temperature and pressure
The plate boundary that we live on and the plate that Los Angeles is on.
the North American Plate, Pacific Plate
The geographic fit of continents is evidence of this.
This scientist was the first to put seafloor spreading and continental drift together into the theory of plate tectonics
John Tuzo Wilson
This is the proposed mechanism for plate tectonics
These are the pieces of evidence to support seafloor spreading.
Heat flow maps, age of the sea floor, volcanism, seismic activity, paleomagnetism
This is the region where oceanic crust collides with either another plate of oceanic crust or a plate of continental crust.
How does the arrival time of the P v S v L waves relate?
P wave arrive first, S waves second, and L waves last