Download Paleomagnetism: Divergent Boundary

Chapter 17 Notes
Know the definition of each of these vocabulary words and be able to differentiate the boundaries:
Pangaea:
Seafloor Spreading:
Magnetometer:
Theory of Plate Tectonics:
Paleomagnetism:
Divergent Boundary:
Magnetic Reversal:
Convergent Boundary:
Isochron:
Transform Boundary:
Rift Valley:
Subduction
*Early mapmakers noticed the apparent fit of continents on either side of the Atlantic Ocean.
* In late 1800’s Eduard Suess hypothesized that the present southern continents had once been joined as a single
landmass that he called Gondwanaland.
* In early 1900’s Alfred Wegener came up with the idea of continental drift which proposed the Earth’s continents had
once been joined as a single landmass. He called this supercontinent Pangaea which means “all the earth.” He
collected rock, fossil, and climatic data to support his hypothesis. Evidence included coal deposits in Antarctica which
would indicate that Antarctica was once closer to the Equator as coal is formed in warm swampy areas. Additional
support is the discovery of glacial deposits in Africa, India, South America, and Australia which would indicate that they
were once located near the South Pole. Most scientists during this time rejected Wegener’s hypothesis since he could
not explain what was causing the continents to move and what force could move such large masses of rocks such
distances.
During World War II sonar was used to uncover submarines. Vast underwater mountain chains called ocean ridges were
also discovered. They also discovered deep sea trenches which are narrow elongated depressions in the seafloor with
very steep sides. The deepest trench is in the Pacific Ocean which is called the Mariana Trench.
The ages of the rocks on the seafloor vary in ages in different places. The age increases with distance from the oceanic
ridge. The oldest rocks on the seafloor were found to be only 180 million years old which is younger than the
continental rocks. The thickness of the seafloor sediment increases with distance from the oceanic ridge.
Rocks containing iron-bearing minerals provide a record of Earth’s magnetic field. Basalt is rich in iron. As magma cools,
the iron minerals become parallel to the Earth’s magnetic field. When the lava hardens it is a permanent record of the
earth’s magnetic field direction. This has changed numerous times over geological time. This is called magnetic
reversal. When a magnetometer records a higher than normal reading, this would indicate normal polarity (+). When it
shows a lower than normal reading it would indicate reverse polarity (-).
Harry Hess proposed the theory of seafloor spreading which states that new ocean crust in formed at ocean ridges and
destroyed at deep sea trenches. This was the missing link needed by Wegener to complete his model for continental
drift.
There are a dozen or so major plates and several smaller plates. Tectonic plates move in different directions and
different rates over the Earth’s surface. Tectonic plates interact at places called plate boundaries. There are three
types of boundaries, convergent, divergent, and transform boundaries. Most divergent boundaries are found on the
seafloor where they form ocean ridges. The formation of new ocean crust at most divergent boundaries accounts for the
high heat flow, volcanism, and earthquakes associated with these boundaries. Some divergent boundaries form on
continents. When continental crusts separate the stretched crust forms a long narrow depression called a rift valley.
When an oceanic crust and another oceanic crust form a convergent boundary, one of the two plates subducts below
the other plate which creates a deep sea trench. The subducted plate descends into the mantle and melts back into
magma. Some of this is forced back up and creates a volcano. Subduction also occurs when a continental crust
converges with an oceanic crust. The denser oceanic plate subducts and produces a trench and volcanic ash. The last
type is a continental to continental convergence which uplifts to form a mountain range.
At transform boundaries the crust is deformed or fractured along faults to form earthquakes.
Chapter 18 Notes
Know the definition of each of these vocabulary words and be able to label them on a drawing.
Viscosity:
Crater
Pluton
Caldera
Batholith
Shield Volcano
Stock
Composite Volcano
Laccolith
Tephra
Sill
Pyroclastic Flow
Vent
Hot Spot
All volcanoes are fueled by magma deep beneath the Earth’s surface which is a mixture of molten rock, mineral grains,
dissolved gases. Magma forms when temperatures are high enough to melt the rocks involved. Temperature increases
with depth beneath the surface. The factors that affect the formation of magma are temperature, source material,
water, and pressure. Water lowers the melting temperature of a rock.
There are three types of magma: basaltic which rises rapidly to the surface and has low viscosity and higher
temperatures. It creates a mild eruption. Andesitic magma is found along the continental margins where oceanic crust
is subducted. It has 60% silica which is a medium viscosity and medium eruptions. Rhyolitic magma has high viscosity
which slows the flow. Along with the large trapped volume of gases, it is a very explosive magma.
The hotter the magma, the lower the viscosity, and the faster the lava flow. The amount of silica increases the viscosity.
Magma because it is molten is less dense than the surrounding rocks enabling it to move upward and intrude the
overlying crust. These intrusive igneous rock bodies are called plutons.
Shield volcanoes have gently slopes with basaltic magma. They are not extremely explosive.
Cinder-Cone Volcanoes form when material that is ejected high into the air falls back to Earth and piles up around the
vent. They have steep sides and are generally small.
Composite volcanoes are much larger than cinder-cone volcanoes with steep side and very explosive.
Rock fragments thrown into the air during a volcanic eruption are call tephra. They are classified by size. The smallest
tephra is dust. Some of the larger tephra which are angular are called volcanic blocks and when they are rounded they
are called volcanic bombs.
The Hawaiian islands are samples of volcanoes created from a hot spot.
Chapter 19 Notes
Know the definition of these words:
Stress
Seismometer
Strain
Seismogram
Fault
magnitude
Primary Wave
Richter Scale
Secondary Wave
moment magnitude scale
Surface Wave
modified Mercalli Scale
Focus
tsunami
Epicenter
seismic gap
There are three types of stress, compression, tension, and shear.
Compression is when an item is pushed together making the volume decrease. A compression earthquake would be a
reverse fault. Tension is when an item is pulled apart making the volume increase. A tension earthquake would be a
normal fault. Shear causes a material to twist or deform. A strike-slip earthquake is an example.
When stress exceeds a certain value, a material undergoes ductile deformation. Unlike elastic strain, this type produces
permanent deformation which means that the material stays deformed even if the stress is reduced to zero. When
stress exceeds the strength of a material, the material breaks or fails.
Be able to identify, reverse, normal and strike-slip faults.
P-Waves and S-waves pass through the earth’s interior and surface waves only travel along the earth’s surface. P-waves
can travel through solid, liquid, or gas but get bent as they enter the outer core and again when they enter the inner
core. S-waves can only travel through solids so the disappearance of the s-waves has allowed sceismologists to reason
that Earth’s outer core must be liquid.
Most earthquakes are caused by movements along the faults.
The Richter scale is based on the size of the largest seismic wave generated by the quake. Each successive number in the
scale represents an increase in seismic wave size or amplitude of a factor of 10. The difference in the amount of energy
released by earthquakes is even greater than the differences between the amplitudes of their waves. Each increase in
magnitude corresponds to about a 32 fold increase in seismic energy.
Moment magnitude values are estimated from the size of several types of seismic waves produced by an earthquake.
Richter scale values are based on the largest seismic waves generated by a quake. The Mercalli scale rates the type of
damage using roman numerals from 1 to 12.
Earthquake intensity depends primarily on the amplitude of the surface waves generated. Surface waves like body
waves gradually decrease in size with increasing distance from the focus.
The depth of the focus also has to do with the intensity of an earthquake. A deep focus produces weaker vibrations
than a shallow focus.
It takes 3 or more seismic stations to determine the epicenter of an earthquake. The distance to an earthquake’s
epicenter is determined by the distance between the p and s waves.
Soil liquidification can cause landslides which can cause buildings to sink or fall over.
Tsunamis are generated by the vertical displacement of the seafloor that results from a quake.
Earthquake recurrence rates, seismic gaps, and strain-accumulation rates are some of the factors considered in
earthquake probability studies.
Be sure to read through Chapters 17-19, look through your notes on these chapters, and review the vocabulary words.
There is no guarantee that I have not missed some important material and it is your responsibility to be familiar with
these chapters. Happy Studying….