Geology: chapters 5, 6, & 7

Tsunami

large harbor waves in Japanese, produced by a sudden vertical displacement of ocean water, can cause a catastrophe up to thousands of kilometers from where they originate.

What triggers a tsunami?

Large earthquakes that cause uplift or subsidence of sea floor, Underwater landslides, Volcano collapse/ Submarine volcanic explosion, asteroids (can produce mega tsunami but low frequency)

Velocity

increase in velocity with an increase in depth

What is the formula for velocity?

𝒗=√𝒈×𝑫

→ g= acceleration due to gravity; 9.8 m/s ^2

→ D= depth of water; average 4267 m

If g= 9.8 m/s ^2 and D=4267 m in the equation 𝒗=√𝒈×𝑫, what is v?

v= 205 m/sec (459 mph)

In the actual observations of a tsunami, what do the speeds normally peak at?

400 to 600 mph

What happens when tsunami waves approach the shore?

Wave will slow downs as approaches shore, but still go fast; height increases

What does a tsunami’s energy depend on?

Tsunami’s energy is dependent on both its waves speed and height

What % of earthquakes vs volcanoes/landslides generate a tsunami?

73% earthquakes, 27% volcanoes/landslides

Where do most tsunamis generated by earthquakes occur?

Most tsunamis generated by earthquakes in subduction zones; Unless there is an underwater landslide, strike slip earthquake WILL NOT cause tsunami

Unless there is an underwater landslide….

…strike slip earthquakes WILL NOT cause tsunami

What are the two mechanisms by which an earthquake can cause a tsunami?

seafloor movement (most common) & triggering a landslide

Seafloor movement occurs….

…when the seafloor sits on a block of Earth’s crust that shifts up or down during a quake.

What can an earthquake of magnitude 7.5 or greater do?

Creates enough displacement of the seafloor and upward/downward movements displace the entire mass of water; Starts a four stage process that leads to landfall of tsunami waves on the shore

How do earthquakes that uplift or downshift the sea floor cause tsunamis?

Rupture uplifts the seafloor → an elongated dome forms on the surface of the water above the fault (parallel) → dome collapses and generates the tsunami waves → waves radiate outward (like a pebble in a pond)

How do tsunamis in the deep ocean behave?

→ Tsunami moves rapidly in deep ocean

→ Can travel faster than 450 mph

→ Spacing of crests is large (60mi) and small amplitude (1-3ft)

→ Boats in open ocean don’t notice the tsunami waves

How do tsunamis close to land behave?

→ loses speed, gains height

→ Depth of ocean decreases , slowing tsunami waves 28 mph (still too fast to outrun)

→ Decreases the spacing between wave crests ( wavelength )

→ More water piles up increasing amplitudes and frequency

How do tsunamis arrive inland?

Often arrive as a quick increase in sea level rather than giant breakings

What is stage 1 of a tsunami?

Initiation: earthquake rupture in the seafloor pushes water upward and starts the tsunami

What is stage 2 of a tsunami?

Split: tsunami moves rapidly in deep ocean reaching speeds of up to 900 km/hr (wave height < 1m)

What is stage 3 of a tsunami?

Amplification: as the tsunami nears land, it slows to about 45 km/hr but is squeezed upward, increasing in height

What is stage 4 of a tsunami?

Runup: tsunami heads inland destroying all in its path (trough of wave may arrive first, exposing seafloor); farthest horizontal and vertical distance of the largest wave.

How does a tsunami return to the ocean?

Once a wave moves to the farthest extent inland, water returns to ocean in a strong, turbulent flow.

What other waves can a tsunami generate?

Tsunami can also generate edge waves which travel back and forth parallel to the shore ( interactions).

How do the runups of tsunamis vary?

Runup from tsunamis varies considerably with the shape of the seafloor immediately offshore and with the type of topography and vegetation landward in the coastal area

Wind-caused waves:

rise up as they near the shoreline roll forward in circles; Isolated waves with shorter wavelength . Come and go quickly.

Tsunami waves:

flow straight, a quick increase in sea level, more powerful; Tremendous momentum. Can last for hours

How do offshore earthquakes affect tsunamis?

Offshore earthquakes can cause tsunamis to go toward land and out to sea; uplifted water dome collapses downward and splits in two waves

Distant tsunami:

Travels out to sea at a high speed and can travel long distances with little loss of energy.

Local tsunami:

Travels toward land very quickly, it can arrive quickly following an earthquake, giving coastal people little warning time

What’s the wave height of each local and distant tsunami in reference to dome height?

Each (distant and local) tsunami has a wave height about one half of that of the original dome of water

What happened in the 1958 Lituya Bay, Alaska tsunami?

→ Rock avalanche from mountains

→ M7.7 earthquake -> landslide

→ 30.5 million m3 of rock fell from a cliff into ocean, instantly displacing a huge volume of water.

→ The splash of bay water surged to 524 m (1720 ft.) above normal water level.

Why is the 2004 Indonesian tsunami important?

→ One of the world’s largest earthquakes triggered tsunami in past four decades

→ Mega-thrust event: the large amount of displacement along the thrust faults in the subduction zone

→ Most deadly tsunami in recorded history: 230,000 people in 14 countries

→ U.S. scientists recognized the risk. No warning system in Indian Ocean

What was the main reason for so many deaths in the 2004 Indonesian tsunami?

→ Lack of Education was a major reason for so many deaths

→ Many did not know tsunami or how to recognize a tsunami. Many went closer to watch

→ Few knew what to do: tourists and residents

What happens when a tsunami nears land?

The height of the waves increases due to a decrease in both water depth and tsunami velocity.

What describes the tsunami in Indonesia in 2004?

M 9 earthquake triggered a 10 m tsunami, which was the deadliest tsunami ever, killing more than 200,000 people in several countries.

Why do sailors rarely notice a tsunami passing in the open ocean?

Tsunamis are too small in amplitude in the open ocean, and the distance between crests is too large to notice its passing.

Why don't earthquakes on transform faults trigger tsunamis?

The earthquakes don't vertically displace the water in the ocean.

Which of the following regions has the LOWEST risk from tsunamis: Japan, Pacific Northwest, Northeastern Indian ocean coast, or Gulf of Mexico?

Gulf of Mexico

What is the difference between a distant tsunami and local tsunami?

Distant tsunamis move farther from their source before they hit land.

Where do volcanoes and eruptions happen?

Convergent Plate Boundaries (subduction); Divergent Plate Boundaries; Hot Spots

Which two plate boundaries do NOT normally form volcanoes.

Transform plate boundaries and continent-continent collision

How many “active” volcanoes are there?

There are approximately 1500 “active” volcanoes (Erupted in the last 10,000 years)

Where are about 2/3 of all active volcanoes located?

About 2/3 of these are located in the Pacific “Ring of Fire”

Where does most magma come from?

Most magma comes from the asthenosphere by partial melting

What is the asthenosphere?

The asthenosphere is a weak layer in the mantle and able to flow because it is close to its melting temperature

What is magma composed of?

Magma is composed of melted silicate materials and dissolved gases

How many types of magma are there?

Three types of magma based on silica content (light colored minerals) relative to FeO + MgO (dark color minerals)

What are the three types of magma from low to high?

Basaltic, andesitic, and rhyolitic

How much silica is in basaltic magma?

Basaltic magma (low silica content, 45-55%, mafic) (The most common magma type)

How much silica is in andesitic magma?

Andesitic magma (middle silica content, 55-65%, intermediate)

How much silica is in rhyolitic magma?

Rhyolitic magma (high silica content, >65%, felsic)

What happens when the three types of magma erupt onto earth’s surface?

When these magmas erupt onto Earth’s surface, each lava type solidifies to produce a different volcanic rock—basalt, andesite, and rhyolite

What % of all magmas are basaltic/mafic?

75%- 80% of all magmas are basaltic/mafic

Where does basaltic magma form?

most of these happen underneath the oceans during the creation of new oceanic lithosphere (divergent boundary/mid ocean ridge)

What % of all magmas are andesitic/intermediate?

10% of magmas are andesitic/intermediate

What % of all magmas are rhyolitic/felsic?

10% of magmas are rhyolitic/felsic

What are effusive eruptions?

Effusive eruptions – mostly lava flows of low-viscosity basaltic lavas

What are explosive (pyroclastic) eruptions?

Explosive (pyroclastic) eruptions – produce clouds of ash and pyroclastic debris along with some lava; usually caused by the build up of gas pressure

What happens when water is introduced into the magma chamber?

If water is introduced into the magma chamber, it will be suddenly transformed into steam

How can you tell which eruption style will happen?

Style of eruption depends on the type of volcano (Hint: effects of viscosity and volatiles content)

Magma composition is…

Composed of three distinct parts:

→ Dissolved gases (H2O, CO2, et al.) – “volatiles”

→ Liquid component – “molten” part

→ Solid component – any crystals/pyroclastic materials

What is magma composition dependent on?

Composition is dependent on:

→ Original parent materials

→ Degree of partial melting of parent material

Viscosity is…

→ Resistance to flow (greater viscosity -> harder to flow)

→ Affected by temperature and composition

What happens to viscosity as magma cools?

As magma cools, viscosity increases

What happens to viscosity as silica content increases?

As silica content increases, viscosity increases

What influences viscosity?

→Mobility of magma under surface and the flow of lava

→Shape of resulting volcano

→Correlated to the volatile content (volcanic eruptions)

Do basaltic (mafic) magmas have a low or high viscosity?

Basaltic (mafic) magmas have low viscosity

Do rhyolitic (felsic) magmas have a low or high viscosity?

• Rhyolitic (felsic) magmas have high viscosity

What does volatile content determine?

Determines how explosive the eruption will be

What will a high concentration of dissolved volatiles do?

High concentration of dissolved volatiles will explode violently

What will volatile-poor magma do?

Volatile-poor magma results in effusive eruptions

How is volatile content and silica content correlated?

Volatile content increases with increasing silica content

Which has more dissolved gas and is more prone to explosive eruptions: andesitic-to-rhyolitic magma or basaltic magma?

Andesitic-to-rhyolitic magma has more dissolved gas (2–5 %) than basaltic magma (< 1 %). More prone to explosive eruptions

What is pyroclastic debris?

Volcanic materials (like ash) that are explosively ejected

What are the characteristics of a stratovolcano?

High-volatile content, andesitic to rhyolitic magma

What are the characteristics of a lava dome?

Highly viscous magma, low to moderate volatile

What are the characteristics of a shield volcano?

Low viscosity, low volatile content

What are the characteristics of a cinder cone?

basaltic eruptions (scoria), intermediate volatile

What are the characteristics of a continental caldera?

Very high volatile contents, rhyolitic magma

How are volcanoes related to plate tectonics?

Volcanoes are intimately related to plate tectonics and most occur along the Ring of Fire surrounding the Pacific Ocean basin

What are some volcano Hot spot activities outside the ring of fire?

Hawaii, Yellowstone, et al.

What are some volcanoes on a portion of the midocean ridge outside the ring of fire?

Iceland, et al.

What are some volcano continental rifts outside the ring of fire?

East Africa

How are transform plate boundaries related to volcanoes?

Generally, transform plate boundaries are not related to volcanoes

Where are some volcanoes in the U.S?

The volcanoes in the US form over the Cascadia subduction zone, Alaska subduction zone, and hot spots over mantle plumes.

Why are so many volcanoes found on the Ring of Fire around the margin of the Pacific Ocean?

Many subduction zones occur there.

Where is the majority of lava extruded?

At divergent plate boundaries

What is effusive eruption?

low-viscosity basaltic lavas flows out of volcanic vent

What is the difference between magma and lava?

Magma is found below the Earth's surface, lava is found on the Earth's surface.

Mt. St. Helens in Washington is an example of which type of volcano?

Stratovolcano

Volcanoes associated with high viscosity magma are usually less dangerous than those with more freely flowing magma.

False

Other than Hawaii, there are about ____ volcanic eruptions per year in the United States.

2 – 3

Where are the majority of the active volcanoes in the United States?

Alaska

If you observe a lava flow that did not move very far from the volcano before it solidified, what can you conclude about that lava?

It had a high viscosity

What is weathering?

Weathering is the physical and chemical breakdown of rocks into smaller pieces.

What is physical (mechanical) weathering?

Physical (or Mechanical) weathering – physical breaking of rocks into smaller pieces.

What are some types of mechanical weathering?

Types of mechanical weathering: Abrasion & Frost wedging & Exfoliation & Biological activity

What is chemical weathering?

Chemical weathering breaks down rock components and internal structures of minerals. Removes unstable minerals

What is the most important agent in chemical weathering?

The most important agent in chemical weathering is water.