What is an earthquake? An earthquake is a sudden shaking of the ground caused by breaking rock. They generate seismic waves which can be recorded on a.

What is an earthquake?

• An earthquake is a sudden shaking of the ground caused by breaking rock. They generate seismic waves which can be recorded on a sensitive instrument called a seismograph.

• The record of ground shaking recorded by the seismograph is called a seismogram.

What is a Seismograph?

• It’s a precision instrument that measures seismic waves and arrival times. There are several different types and styles.

Remember lithospheric plate movement?

• Let’s review how the boundaries react to each other.

• Divergent boundaries: This is the most common kind of motion along the mid-ocean ridges. This is a system of undersea mountain ranges that extends beneath the world's oceans.

• At divergent boundaries we would find normal faults

• Convergent boundaries: This is the kind of motion at subduction zones. This motion happens where dense oceanic plates collide, and slide beneath continental plates or less dense oceanic plates.

•At convergent boundaries we would find reverse faults.

• Transform boundaries: A good example of this type of motion is the San Andreas Fault which runs through California. The rock slides past each other at these boundaries. The faults at these boundaries are called strike-slip faults.

The movement between plates and along faults is not smooth. They move in jerks, giving rise to earthquakes. The locations of earthquakes throughout the world delineate the major tectonic boundaries.

Under Stress                                                 

Earthquakes occur on faults. Because faults have friction, they resist the forces trying to move the pieces apart. As the forces build, the fault remains locked and the blocks get deformed because of the increasing stress. Eventually the stresses get so high that the fault breaks. This releases the built up stress and allows the sides of the fault to slide past one another. This is what we call an earthquake. The blocks return to their locked state until the stresses build up enough to cause another earthquake.

Types of Stress

• Shear - a sliding or twisting force

• Tension- a pulling force

• Compression- a pushing force

Now that we know how earthquakes start, what exactly is happening?

• Seismic wavesEarthquakes create seismic waves which shake the ground as they pass. They sometimes cause buildings to topple. Earthquakes create waves just like waves of water moving across the ocean and waves of air moving across a field of wheat. Consider what happens when a drop of rain hits a pond of water. The drop disturbs the flat surface of the water and creates waves that travel outward in all directions from the disturbance. These waves travel on the surface of the pond, along the interface between the water and the air.

The Slinky and The Rope

Earthquakes generate several kinds of seismic waves including P, for "Primary" and S, for "Secondary" waves.

P Waves                                                                         

The P waves move in a compress ional motion similar to the motion of a slinky, while the S waves move in a shear motion perpendicular to the direction the wave is traveling.

S Waves                                                                         

• Surface Waves

Surface wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the surface wave, which can be much larger than the other waves.

A seismogram is a record of the ground shaking recorded by a seismograph.

The P waves travel fastest through the Earth so they arrive at a seismograph first, followed by the S waves and lastly by

the surface waves.

Locating the Shaking • Seismologists locate earthquakes by measuring the time between the P

and S waves in a seismogram. • After a seismogram "feels" an earthquake, scientists compare the time

difference of these waves to figure out how far away the earthquake is. • It takes at least three seismograms to locate exactly where the earthquake

is. • One seismograph can only tell how far away it is from that seismograph.

The earthquake could be located anywhere on a circle of radius equal to this distance and centered on the observation station. By measuring the S-P times at 3 or more stations these circles can be drawn around each station and where they meet indicates the earthquake location.

• Once they find where it is, this is what we call the epicenter•

Directly below the epicenter (but at different depths) is the focus of the earthquake.

Measuring A Quake

• There are many ways to measure the size of an earthquake. Some depend on the amount of damage caused by the earthquake while others depend on the amount of seismic energy emitted by the earthquake. Some are a combination of both. There are three popular earthquake scales.

The Mercalli Intensity Scale assigns an intensity or rating to measure the effects of an earthquake at a particular location.

This is because the amount of damage caused by an earthquake at a particular location depends on the geology of the location.

The population density and the methods used to construct buildings near the location are also important in the Mercalli scale.

Earthquake intensities are rated with Roman numerals ranging from I (not felt) to XII (buildings nearly destroyed).

Mercalli scale

The Richter magnitude scale was originally developed by Charles Richter and Beno Gutenberg to make more quantitative measures of the relative sizes of earthquakes in southern California. Today, modified versions of the scale are used to measure earthquakes throughout the world. The Richter magnitude is related to the maximum amplitude of the S wave measured from the seismogram. Because there is a great range in the sizes of different earthquakes, the Richter scale uses logarithms. Thus, a magnitude 7 (M 7) earthquake is 10 times as large as a magnitude 6 earthquake, and releases over 30 times more energy.

Moment Magnitude Scale

• Most current system used•Uses the Magnitude system•Can be used for all earthquakes near and far•Combines several aspects of both Mercalli and Richter scale.

Magnitude 1 Magnitude 2

Recorded difference

Intensity damages

Jump to Tsunamis

What are Volcanoes?

• A volcano is an opening in the Earth that erupts gasses, ash, and lava.

• Can you name some?• Kilauea (kee low AY ah)- most active in

world, located in Hawaii• Hekla, located in Iceland• Mount St. Helens in Washington State• Soufriere Hills in Montserrat; one of the

largest recent eruptions

Where do they form??

Effects of Eruptions

• What effects can erupting volcanoes have?• Lava flow destroys everything in path• Pyroclastic flow (extremely hot cloud of volcanic

ash and debris) can start fires with anything it contacts.

• Falling volcanic ash can collapse buildings, block roads, and cause major respiratory problems that can lead to death.

• Can cause acid rain

How do volcanoes form?

• Magma forced upward from pressure of surrounding rock. The magma is less dense than the rock that surrounds it.

• The magma exits through openings in the earth called vents.

• Often these vents are found at divergent boundaries.• At subduction zones of convergent boundaries, vents

are created for the volcano to form.• Hot spots (like the Hawaiian Islands) also give birth to

volcanoes. The area is especially hot and close to the magma. Eventually the magma makes it to the surface.

Do all volcanoes erupt violently?

• NO!

• Some volcanoes erupt explosively, and some erupt quietly.

• The type of eruption depends on the amount of trapped gasses, water vapor, and silica (the elements silicon and oxygen) in the magma.

What type of boundary do you think will induce large amounts of water vapor?

• Convergent boundaries (at subduction zones)

WHY??

Magma composition

• Two major types: Silica poor, and silica rich.

• Silica poor result in quiet eruptions: we call this basaltic magma. This pours out of the vent and runs down the volcano.

• Silica rich result in explosive eruptions: we call this granitic magma. This is thicker, and traps a lot of gas causing pressure to build. This results in an explosion of magma.

• There is a third which is in between the two called andesitic (named after the Andes)

Types of volcanoes

There are three major types of volcanoes: Shield, Cinder Cone, and Composite.

Shield volcanoesMauna Loa, Hawaii, is an excellent example of a shield volcano

Shield Volcanoes

• This type of volcano can be hundreds of miles across and tens of thousands of feet high. The individual islands of the state of Hawaii are simply large shield volcanoes.

Shield volcanoes have low slopes and consist almost entirely of hardened lava. Lava flows from one or more vents without erupting violently. They almost always have large craters at their summits.

Shield volcano in cross section

Cinder Cone Volcanoes

Cinder Cone volcanoes

• These volcanoes consist almost entirely of loose, grainy cinders, and almost no lava. This means the lava is thrown from the volcano and solidifies in the air, and is combined with bits of rock. This composition is called tephra.

• Cinder cones are small volcanoes, usually only about a mile across and up to about a thousand feet high. They have very steep sides and usually have a small crater on top.

Composite Volcanoes

Composite Volcanoes

• Many are located in populated areas and have well-known names: Vesuvius, Krakatoa, Fujiyama, and Mount St. Helens. These volcanoes are typically tens of miles across and ten thousand or more feet in height. As illustrated in the previous figure, they have moderately steep sides and sometimes have small craters in their summits. Volcanologists call these "strato-" or composite volcanoes because they consist of alternating layers of solid lava flows mixed with layers of sand- or gravel-like volcanic rock called tephra cinders or volcanic ash.

Two more terms…- Volcanic neck - Caldera

Volcanic Neck: A massive pillar of rock more resistant to erosion than the lavas and pyroclastic rocks of a volcanic cone.

Caldera: The Spanish word for cauldron, a basin-shaped volcanic depression; by definition, at least a mile in diameter. Such large depressions are typically formed by the subsidence of volcanoes. Crater Lake occupies the best-known caldera in the Cascades.

Kaguyak volcano, Alaska. This lake-filled caldera formed atop a former stratovolcano (note remnant of upper part of older cone at right center). The caldera formed about 1,100 years ago and is 2.5 km in diameter. The prominent peninsula and small island consists of lava domes erupted after the caldera formed.

Kinds of lava flows…

• Pahoehoe (pa HOY hoy) - Pahoehoe lava is characterized by a smooth, billowy, or ropy surface. A ropy surface develops when a thin skin of cooler lava at the surface of the flow is pushed into folds by the faster moving, fluid lava just below the surface.

• Aa (AH ah) – These are flows that have a rough rubbly surface composed of broken lava blocks called clinkers.

• Pillow lava - Lava that forms from an underwater eruption and is characterized by pillow-shaped masses.

Pahoehoe (pa HOY hoy) Lava

Aa lava flow

Pillow lava