Chapter 2 earthquakes and volcanoes

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.