Overview of Plate Techtonics
The earth's crust and upper part of the mantle is the earths lithosphere, which retains "elastic" properties. When forces are applied to this layer, rocks are able to store energy and change shape. When rocks can no longer store energy, they will break an
The Continental Crust
is of lowest density and will have the LEAST amount of Fe and Mg. The top two elements composing the rocks are silicon and oxygen (silicate minerals) in the crust. Continental crust (mostly silicon and oxygen) is lower density and thicker than oceanic cru
Oceanic Crust
has a HIGHER percentage of Fe and Mg than the continental (it has a good amount of Fe and we will see why this is an important layer) and so it is denser (Fe and Mg are denser elements compared to O and Si)
The Mantle
is composed of minerals with the HIGHEST percentage of Fe and Mg. This is a very thick, dense layer of rocks situated below the crust.
The Core
can be divided into the inner and outer core. The inner core is SOLID Fe and Mg, whereas the outer core is LIQUID Fe and Mg. How do we know this? No one has made it through the crust to the mantle! (Keep this in mind when we study earthquakes)
The Asthenosphere
is the part of the mantle just below the lithosphere. Rocks here are under extreme pressures and temperatures rendering them to behave like silly putty (PLASTIC). Apply forces to these rocks and they will change shape and not rebound.
Pangaea
The super continent that Wegner said was assembled roughly 200-250 million years ago, and have been splitting in their current geographical positions ever since.
Evidence to Support Continental Drift
1. The continents fit like a puzzle.
2. Mesosaurus fossils are found on two different continents.
3. Glossopteris fossils are found on several continents
4.Global Coal Deposits
5. Glacial Deposits.
6. Mountain Ranges
The reptile fossil Mesosaurus cannot s
Sea Floor Spreading
-The black lines on the map are chains of volcanoes beneath the earths ocean called mid-ocean ridges. There are many features (guyots, seamounts, extinct volcanoes, etc) on the sea floor but initially in was thought to be flat and barren.
-We began to lea
subduction zone
New rock forms at the ridge but OLD rock is recycled in a subduction zone. So since the last Pangaea (~250 million years ago) when the continents began to split, the seafloor before Pangaea has mostly been recycled. Most of the seafloor is younger than Pa
Paleomagnetism
-The earth has magnetic properties. At one pole it has a north magnetism and at the other a south magnetism (like two opposite ends of a bar magnet)
-Iron crystals in igneous rocks will point to the direction of the earth's magnetic north (wherever it is)
Interactions and Geologic features at Plate boundaries
-Okay so continents come together (converge), spread apart (diverge) and slide past one another
(transform).
-Divergent Boundaries can occur along mid ocean ridges by sea floor spreading as we already discussed. Remember there is a convection cell just be
Volcanism
Refers to the rise of magma which makes its way to the earth's surface as LAVA cooling above the earth's surface.
Magma
a mixture of liquid rock, crystals, and dissolved gases beneath the earth's surface (within the crust).
Volcanoes
are conical or dome shaped landforms built by the emission of magma and its contained gases from a constricted vent onto the earths surface. Magma rises in a narrow, pipe-like conduit from a magma reservoir beneath and flows at the earths surface as LAVA.
Volcanic Properties
The violence and explosiveness of a volcanic eruption depends on the amount of time since the last eruption, and the magma's viscosity and gas content. The more viscous (thick) and gore gaseous the magma, the more explosive the eruption. The
Volcanic Expl
Main Types of Volcanic Features on Earth
-Shield Volcanoes
-Stratovolvanoes and calderas
-lava domes
-cinder cones
The main hazards associated with Volcanic Eruptions
Pyroclastic flows: hot, fluid mixtures of rock particles and gas that travel at great speed down the flanks of a volcano; have caused thousands of fatalities.
Lahars- fast-moving mud flows caused by mixing volcanic ash with water (from rain or from erupti
Benefits to Volcanoes
Geothermal Energy: magma close to the earths surface is used to make steam for electrical generation
Construction: volcanic rock is used for some building materials
Produces
NEW LAND
at the earth's surface. (otherwise weathering and erosion would render c
The main precursors that
signal an impending eruption
increased seismic activity ("earthquake swarms") caused by magma rising below the volcano
- tilting and swelling of the volcano's sides
- increased gas emissions
Locations of Volcanic Activity and Volcanic Hazards
1. Subduction zones at convergent plate boundaries, where the plate is sinking into the mantle melts and creates magma.
Example: Volcanoes lining the trenches of the Pacific Ocean, forming the Pacific "Ring of Fire".
2. Rifting and sea floor spreading at
VOLCANIC EXPLOSIVITY
The size and the violence of a volcano's eruption is expressed by the Volcanic Explosivity Index (VEI). Values range from 0-8 based on the following:
-The volume of material (lava and particles) erupted.
-the height of the eruption column
-how long the er
Viscosity
Viscosity is a measure of a fluids resistance to flow. For example, honey has a higher viscosity than water. "thickness" of the fluid. The main factor that determines the viscosity of magma its
silica
(Si02) content. The more silica in the magma the more
Gas Content
The gas content of a magma also relates to its behavior. A magma with low gas content will tend to flow out of a volcano as relatively quiet lava. A magma with high gas content will tend to blow apart violently upon erupting. The higher the gas content, t
Composition of gases in magma
-
mostly water vapor
and some carbon dioxide
-minor amounts of sulfur, chlorine, and flourine gases.
-It turns out that viscosity and gas content of magma both relate to plate tectonic setting
-� Divergent boundaries (mid-ocean ridges) and hot spots both
STEPS TO A VOLCANIC ERUPUTION
Magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rock. As they rise they may encounter a depth (or pressure) where the dissolved gas no longer can be held in solution in the magma, and th
Mafic Magma
is characterized by relatively low silica and low
gas content
Felsic Magma
is characterized by relatively high silica and high gas content. Felsic
magma often erupts with great violence (the eruptions have high VEI).
Shield Volcanoes
Are very large (largest) with gently sloping sides and a convex shape. They form from relatively quiet eruptions of mafic magma. You will find shield volcanoes at oceanic hot spots (Hawaii) and places where mid ocean ridges stick up above sea level (the c
Composite Cones
Or (Stratovolcanoes) are high and steep sided. Unlike shield volcanoes, they form from violent eruptions of
felsic to intermediate magma
. You will find these on almost all
convergent plate boundaries
.
-Silica rich and gas-rich magma.
-Most commonly foun
Caldera
formed when the composite volcano collapses into the empty magma chamber beneath, leaving a large crater behind.
Lava Domes
Form when viscous lava piles up in and around a volcanic vent. Some domes appear to form in the last stages of an eruption, when the final masses of felsic magma squeeze out of a vent and plug it up.
Cinder Cones
Are smaller volcanic cones that occur worldwide. They appear to be largely "one-shot" events where a source of magma erupts once than becomes inactive.
-Cinder cones form as erupted rock particles pile up around a vent, building up a cone shaped hill.
-Th
Earthquakes
are caused by movements on faults--fractures in the earth's lithosphere generally related to movements of tectonic plates. Most occur near plate boundaries.
-P-waves and S-waves travel through the earths interior, P-waves are faster than S-waves, so their
Three main methods for long term forecasting of earthquakes
1. Statistical Methods use the history of past earthquakes in a region
2. Geophysical Methods attempt to identify seismic gaps along faults where strain may be building up.
3. Geological Methods involve studying the long-term geologic history of earthquak
Earthquakes are....
vibrations of the earth caused by the rupture and sudden movement of rocks that have been strained (deformed) beyond their elastic limit.
Dip-Slip Faults
The displacement is vertical
Strike-Slip Faults
the displacement is horizontal
reverse faults
faults that result from horizontal compressional stresses where the hanging wall block has moved up relative to the footwall block.
� Reverse faulting occurs along convergent boundaries.
� There are two types of converging plate boundaries.
1. Subduction
COLLISION BOUNDARIES
� At collision boundaries two plates of continental lithosphere colide result in in fold-thrust mountain
belts.
� Earthquakes occur due to the thrust faulting and range in depth from shallow to about 200 km.
� Example: The Himalayas from the collision of
Waves
As S and P waves bounce around inside the earth, they generate a third set of waves that travel along the surface of the earth. These are called
surface waves
, and they consist of several types.
-One common type of surface wave produces a rolling, undula
Seismograph
-P-waves arrive first, followed by S-waves, and then Surface waves.
-Seismogram - the record of an earthquake as recorded by a seismograph. It is a plot of vibrations versus time. Seismograph - records incoming waves
lag time is proportional to distance t