Earthquakes
•Earthquake – the shaking and trembling that results from the sudden movement of part of the Earth’s crust.
•Scientists estimate that more than one million occur each year. (one every 30 seconds)
•Most common cause is faulting which is a break in the earth’s crust.
San Andreas Fault
•900 km long, from the border of Mexico past San Francisco
•Land to the west is moving north and land to the east is moving south
Ocean Floor Earthquakes
•Seafloor quakes cause tsunamis
•They can travel 700-800 km/hr and reach a height of 20 meters (60 feet), A six story building basically!
•They are not always that large… some can be inches tall
Seismic Waves
•Focus -the point beneath the Earth’s surface where the rocks break and move, the origin of the earthquake.
•Epicenter -directly above the focus on the Earth’s surface
•Earthquake waves are called seismic waves.
•P waves – (primary waves) arrive first and travel through solids, liquids, and gases.
They are push-pull
(longitudinal) waves.
•S waves – (secondary/shear waves)
arrive secondly and travel through solids only. They travel in an S pattern (transversely… up/down).
•Surface waves – (L waves)
arrive last, they are the slowest moving and the most damaging; they travel in a circular motion.
The Earth’s Core
•Most of the information scientists have gathered about the Earth’s interior has come not only from complex
instruments but from earthquakes.
•Seismic waves penetrate the depths of the Earth and return to the surface.
•The speed and the direction of the waves change
P waves and S waves
At a depth of 2900 km P waves slow down and S waves disappear
•P waves do not travel well in liquid and S waves don’t travel at all
•At 5150 km P waves begin to move rapidly again
•By observing the change in speed, scientists know that the Earth has a core, made of two parts
VOCAB TO KNOW…
•Seismograph - Instrument that detects and measures seismic waves
•Seismologists study earthquakes
•Seismogram record of the waves strength; the higher the wave, the stronger the quake
•Richter Scale determines the strength of the quake
Can we predict earthquakes?
•Seismologist have identified some warning signals that help to predict earthquakes:
-change in the speed of P and S waves
-slight change in the tilt of the land
-change in the water level of wells
-behavior of certain animals
The Earth’s Core
•The core is made of 2 separate layers, both consisting of iron and nickel
•The inner core is solid due to the tremendous pressure, it is 5000° C, 1300 km in depth
•Because it is solid iron, it produces a magnetic field
•The outer core begins at 2250 km thick, 2200° to 5000° and is a liquid
MOHO!!
•The layer directly above the outer mantle
•It consist of 80% of the volume and 68% of the mass of the Earth
•Andrija Mohorovičić discovered a boundary between the outermost layer of the crust and the mantle, it is
called the moho
Mantle
•Scientist determined the mantle is made of silicon, oxygen, iron, and magnesium
•Temperature 870° to 2200° C
•The mantle has the property of plasticity, it can flow like a liquid (pudding like)
The Earth’s Crust
•The Earth’s crust is the outermost layer
•The crust is very thin compared to the other layers of the Earth
•Oceanic crust is about 7-8 km thick and is composed of silicon, oxygen, iron, and magnesium… Made mostly
of basalt and is dense
•Continental crust is about 32 km thick and is composed of silicon, oxygen, aluminum, calcium, sodium, and
potassium…Made mostly of granite
•The crust forms the upper part of the lithosphere, which is between 50 – 100 km thick and is broken into plates
•The layer directly underneath is called the asthenosphere which is 130- 160 km thick.
•It is the uppermost edge of the mantle
VOLCANOES
Formation of a Volcano
•Magma – hot liquid rock found in magma chambers under the earth’s surface.
•Lava – once it is outside the earth.
•It comes to the surface through volcanoes
•The opening in a volcano is called a vent
Types of Magma/Lava
•There are 4 types of lava based on their composition, viscosity, silica content, and explosiveness
•Viscosity: Resistance to flow…
•low = runny, high = thick & syrupy
•Basaltic low viscosity
50% silica
•Andesitic intermediate
•Rhyolitic high viscosity
•Dark lava (2 basic forms)
about 60%
about 70%
Pahoehoe- hot, fast moving
Aa- cooler, slower moving
Volcanic Material
•Tephra is the material thrown into the air during eruptions, it consists of rock fragment
•Volcanic dust – very fine, less than .25 mm in diameter (grains of flour)
•Volcanic ash – bigger than .25 mm but less than 5 mm. (rice grains)
•Volcanic bombs – larger particles; few cm to several m in diameter. (can be size of a car)
•Cinders – volcanic bombs the size of golf balls.
•Pyroclastic flows are violent eruptions of gas, ash, and other tephra, thousands of degrees, that can move 200
km/hr
Types of Volcanoes
•Lava erupts through the vent of a volcano, the top around the vent can become bowl shaped and form a crater,
larger craters are called calderas
Cinder-cone Volcano
•It has steep sides and generally is small, has a violent eruption pattern… made of cinders only
Shield Volcano
•It is a mountain with broad, gently sloping sides and a nearly round base, it is formed by gentle eruptions when
layer after layer of lava accumulates
Composite Volcano
•They are larger than cinder-cone but smaller than shield, made of alternating layers of cinders and lava, gentle
eruption first then explosive.
Vocabulary to know
•Plutons: Intrusive Igneous rock that can be exposed at the surface as a result of uplift and erosion.
•Batholith: Largest plutons, common in interior of mountain chains, cut across older rocks
•Laccolith: Mushroom shaped pluton with a round top and flat bottom, small
•Sill: Pluton that forms when magma intrudes parallel to rock layers
•Dike: Pluton that cuts across preexisting rock
What are hot Spots?
•Unusually hot regions of Earth’s mantle were plumes of magma rise to the surface
•Hawaiian Islands formed over a hot spot, Hawaii has just started to move away from the hot spot and an
underwater seamount is beginning to form to the southeast of the Big Island
3 zones of occurrence
•Most plate movement occurs in 3 zones.
•Ring of Fire – zone that circles the Pacific Ocean.
•Mediterranean Zone – (includes Italy, Greece, and Turkey).
•Mid Atlantic Ridge zone – (Iceland and other volcanic islands in the Atlantic Ocean).