chapter 2 earthquakes and volcanoes
TRANSCRIPT
Chapter 2
Earthquakes and Volcanoes
Earthquakes
• Earthquake- The shaking and trembling that results from the sudden movement of part of the Earth’s crust.
– The ground shakes and trembles
– During severe earthquakes the ground can rise and fall like waves in the ocean.
• When rocks in the Earth’s crust break, earthquake waves travel through the Earth in all directions.
• This is like dropping a rock in a puddle, the waves move outward from the point of origin.
• The motion of the ground causes buildings, trees, and telephone polls to sway or fall.
• Scientists estimate that more than one million earthquakes occur each year.
– Most of the earthquakes are so small that the surface of the Earth barely moves.
– Several thousand earthquakes a year effect the surface of the Earth in a significant manner.
– Several hundred earthquakes make major change in the Earth’s surface features.
– Only about 20 earthquakes a year cause severe changes in the Earth’s surface.
• The most common cause of Earthquakes is faulting.
– Fault- is the break in the Earth’s crust
• During faulting, parts of the Earth’s crust are pushed together, pulled apart, or slide by past one another.
• When this happens energy is released
• The rocks move causing nearby rocks to move until all of the energy is used up.
• Earthquakes can occur on the floor of the ocean.
– Tsunamis- a giant sea wave produced by an earthquake.
• Tsunamis can travel at speeds of 700 to 800 kilometers per hour.
• As they approach the coast they can reach heights of greater that 20 meters.
• Tsunamis can cause great damage.
• Japan is common for tsunamis.
Fractured road in Japan
Earthquake in Marina District, San Francisco
Seismic Waves
• Focus- the point beneath the Earth’s surface where the rocks break and move.
• This is the underground point of origin of an earthquake.
• Epicenter- the point on the Earth’s surface directly above the focus of an earthquake.
• Earthquake waves reach the epicenter first.
• During an earthquake, the most violent shake is found at the epicenter.
• Seismic waves- this an earthquake wave
• Three main types
• Primary waves- seismic waves that travel the fastest.– Also known as P waves
– Travel through solids, liquids, and gases.
– The more dense the material the faster P waves travel.
– P waves are push-pull waves (they push rocks particles into the particles ahead of them, thus compressing the particles. The rock particles then bounce back).
• Secondary waves- seismic waves that do not travel through the Earth as fast as P waves.– S waves travel through solids but not through
gases or liquids.
– They arrive at a given point after P waves do.
– S waves speed up when they pass through denser material.
– S waves cause rock particles to move from side to side. The rock particles move at right angles to the direction of the waves.
• Surface waves- the slowest moving seismic waves or L waves.
– L waves arrive at a given point after primary and secondary waves do.
– They originate on the Earth’s surface at the epicenter.
– The Earth’s surface moves up and down which each wave that passes (like waves in the ocean).
– L waves cause most of the damage during the earthquake because they bend and twist the Earth’s surface.
Seismograph
• Seismograph- is an instrument that detects and measures seismic waves.– The first practical seismograph was developed by
John Milne in 1893
– Seismograph is made up of a weight attached to a spring or wire. • The weight is not directly connected to the Earth, so
the weight will remain still even with the Earth moving.
• The pen is attached to the weight and records any movement of the Earth on a sheet of paper wound around constantly rotating drum.
• Seismologists- scientist who study earthquakes, can determine the strength of an earthquake by studying the height of the wavy lines recorded on the paper.
• Seismogram- seismograph’s record of waves
– The higher the wavy lines on a seismogram are, the stronger the earthquake is.
• Richter scale- the scale used to measure the strength of earthquakes.
– Created by Charles Richter in 1935.
– It gives scientists a way to determine earthquake strength.
– Any number above 6 indicates a very destructive earthquake.
Predicting Earthquakes
• This will help save lives in the area that will be affected by the earthquakes
• Sometimes a slight tilt of the Earth’s surface.
• The land near a fault may rise or sink slightly.
• The water levels in wells often goes up or down.
Formation of Volcanoes
• Magma- molten rock beneath the Earth’s surface.
– The molten rock is found in pockets called magma chambers.
– Magma is constantly moving
• Magma can either work its way towards the earth’s surface through the cracks in solid rock or by melting the solid rock.
• Lava- is what magma is called when it reaches the Earth’s surface.– A lava flow is so hot that it incinerates every
burnable thing in its path.
• Volcano- a place in the Earth’s surface through which molten rock and other material reach the surface.– In some places the lava can build up to form cone-
shaped mountain.
– Vent- the opening from which lava erupts
– Volcanoes often have more than one vent.
Volcanic Eruptions
• Not all eruptions are the same
– Some are quiet with lava flowing out the vent.
– Others are very violent with lava and other materials being hurled hundreds of meters into the air.
• Gases from within the Earth’s interior mix with huge quantities of volcanic dust and ash and rise into the air as great dark clouds.
• Many scientist spend their whole career studying volcanoes.
– By studying the material makeup of the lava, scientist can determine the chemical composition of the magma from which lava is formed.
– Helps provide information about the composition of the part of the Earth that remains unseen.
• Four main types of lava
– Dark-colored lava
• Contains a lot of water
• Rich in elements iron and magnesium
• When lava cools, igneous rock such as basalt id formed
– Light in color lava
• Contains little water
• Rich in elements silicon and aluminum
• When cooled forms igneous rock rhyolite (resembles granite)
– Light and dark colored lava
• Has chemical composition similar to that of both the dark-colored type and the light-colored type.
• Different varieties of igneous rock like andesite
– Large amounts of gases
• Steam and carbon dioxide
• Forms rocks with many holes in them– Lava forms as gas bubbles are trapped in the molten rock as it
hardens.
• Pumice and scoria are igneous rocks formed
Difference between light and dark lava
• Dark-colored– Lava is thin and runny
– Tends to flow
– Islands of Hawaii and Iceland were formed by lava flows
• Light-colored– Causes explosive eruptions
– Because rich in silicon, it usually hardens in the vents of the volcano.
– Over time pressure builds up causing a great explosion.
– Mt. Saint Helens
• Volcanic dust- the smallest rock particles blown into the air by a volcanic eruption.– Particles of dust are very fine, less than 0.25 mm in
diameter.
– Like tiny grains of flour
• Volcanic ash- rock particles more than 0.25mm but less than 5mm across that are blown into the air by a volcanic eruption.– About the size of rice grains
– Ash will eventually form small rocks
– Can fall next to the volcano or can be blown all across the world.
• Volcanic bombs- are a few centimeters to several meters in diameter.
– When they are hurled out they are molten, they harden as they travel through the air
• Cinders- a small, rough volcanic bomb no more than several centimeters across.
Types of Volcanoes
• Different types of volcanic eruptions form different types of volcanoes.
• Some are built from quiet flows of thin runny lava.
• Some are built from violent eruptions.
• Some volcanoes are built from both quiet flows and violent eruptions.
• Cinder cones- a volcano made mostly of cinders and other rock particles that have been blown into the air.
– Formed from explosive eruptions
– Material in the cinder cone is loosely arranged, they are not high.
– They have a narrow base and steep sides
– Paricutin in Mexico
• Shield volcanoes- a volcano composed of quiet lava flows
– After several quiet eruptions, a gently sloping, dome shaped mountain is formed.
– The lava flows over a large area.
– Mauna Loa in Hawaiian Islands.
• Composite volcanoes- a volcano built of alternating layers of rock particles and lava.
– During the formation of the volcano
• A violent eruption occurs first, next a quiet eruption, this an alternating pattern
– A large cone-shaped mountain
– Mount Vesuvius Italy
• Crater- a funnel-shaped pit at the top of a volcanic cone whose diameter is less than three times its depth.
• Caldera- a roughly circular, steep-sided pit at the top of a volcanic cone whose diameter is at least three times its depth.
Volcanic Activity
• Scientific labeling of volcanoes
– Active- is a volcano that erupts either continually or periodically.
– Dormant- a volcano that has been known to erupt in modern times but is now inactive.
– Extinct- a volcano not known to have erupted within modern history.
Volcanoes and Earthquake Zones
• Most major earthquakes and volcanic eruptions occur in three zones of the world.
– Ring of Fire- extends nearly all the way around the edge of the Pacific Ocean.
– The goes through New Zealand, the Philippines, Japan, Alaska, and along the western coast of North and South America
• Mediterranean Zone- is located near the Mediterranean Sea. The zone extends across Asia into India.
– Includes Italy, Greece, and Turkey
– This zone has had many violent earthquakes and volcanic eruptions.
• Mid-Atlantic Ridge- this extends through Iceland to the middle of the Atlantic Ocean
– Scientists believe that the volcano and earthquake activity in this area are due to the formation of new parts of the Earth’s crust along the ridge.