GEOL3074: Geology for Engineers - Midterm 2

What is the structure that makes up the Earth's interior?

Inner Core, Outer Core, Mantle, and Crust

What makes up the Inner Earth Core?

Solid Iron

What makes up the Outer Earth Core?

Liquid iron

What makes up the Mantle?

Solid iron-magnesium silicate

What makes up the Crust?

Granite, Feldspars, and Quartz

What are some geophysical tools that are used to probe into the Earth's interior?

Seismic Waves, Magnetic Field, Gravity Field, and Heat Flow

What do seismic waves, reflection, and refraction tell us about the Earth's interior?

- Seismic waves will pass through the entire Earth.

- Seismic reflection is the return of some of the waves to the surface after bouncing off a rock layer boundary.

-Seismic refraction is the bending of seismic waves as they pass from one material to another having different seismic wave velocities.

What are the types of seismic waves and what are the differences between them?

Primary waves (P-waves) and Secondary waves (S-waves).

Primary waves have a "push and pull" movement, travel the fastest, and travel through solids and liquids.

Secondary Waves have a side-to-side movement, travel slower, and only travel through solids.

What factors can affect seismic wave velocity?

1. Density

2. Temperature

3. Bulk and Shear Moduli

4. Liquid vs Solid

5. Angle of Incidence

6. Vertical arrangement of layers

7. Distance

What factor causes wave velocities to increase with greater depth in Earth?

Bulk and Shear Moduli.

What factors cause slower wave velocities with greater depth in Earth's interior?

Density and Temperature.

How does the density affect seismic wave velocity?

Higher density = slower speed.

How does the temperature affect seismic wave velocity?

Warmer temperature = slower speed.

How does the bulk and shear moduli affect seismic wave velocity?

Larger moduli = faster speed.

How does liquid versus solid affect seismic wave velocity?

- Solid = faster speed; p- and s-waves

- Liquid = slower speed, no s-waves

How does the angle of incidence affect seismic wave velocity?

Controls how much energy is reflected and how is absorbed.

How does the vertical arrangement of layers affect seismic wave velocity?

Controls resultant travel vectors.

How does the distance affect seismic wave velocity?

Farther distance = greater attenuation of seismic energy.

What do shallow high frequency seismic waves allow us to see?

Reflectors and refractors at depth.

What is the basic data input for seismological analysis?

Travel times for different types of waves to multiple points across the Earth's surface.

Why are seismic velocities important?

They provide a lot of information about the density and phase structure of the Earth's interior.

What are differences between the p-wave and s-wave shadow zones.

- P-waves refract when they enter the core boundary while s-waves do not passthrough the core at all.

- S-wave shadow zone is larger than the p-wave shadow zone.

- Direct s-waves are not recorded in the entire region more than 103 degrees from the epicenter while direct p-waves are not recorded between 103 degrees and 142 degrees from the epicenter.

What is a seismic derivative tomographic cross-section illustrate?

The models are based on local variation in seismic wave velocities as a function of temperature.

- Red means warm (seismically slow) areas that are in turn mantle upwelling.

- Blue means cold (seismically fast) areas that are in turn mantle downwelling (aka. slab sinking)

How is the Earth's interior separated?

The Earth is "density segregated".

Why have scientists concluded that the Earth's core is primarily composed of iron?

1. Iron is an abundant element in the Solar system.

2. The core has the right density for iron.

3. Liquid iron in the Outer Core which is Earth's magnetic field.

What is a characteristic of Earth's outer core?

It contributes to Earth's magnetic field.

What a characteristic of Earth's rotation?

The rotation axes of the core and full Earth are not aligned.

What is a magnetic field and which direction do the magnetic poles currently point?

It is a region of magnetic force surrounding the Earth that currently has North - South magnetic poles.

What is a magnetic field reversal and how long does it take to complete?

It is when the poles of the Earth's magnetic field switch. The reversal process requires 1,000 to 10,000 years to complete.

What is the importance of Earth's magnetic field?

The Earth's magnetic field forms a shield protecting the atmosphere from erosion by the solar wind.

What is the 600-km discontinuity?

It represents a phase change associated with increasing pressure as you go deeper into the Earth.

How many convection layers make up the mantle?

One.

What are characteristics of the asthenosphere?

- Low-velocity zone

- Rocks are close to melting point

- Rocks may have little strength and be capable of flowing.

What are characteristics of the lithosphere?

- Crust and Upper mantle together form the lithosphere

- Rigid layer of crust and mantle overlying partially molten asthenosphere

Is the Asthenosphere above or below the Lithosphere?

The Asthenosphere is below the Lithosphere.

What are the types of crust?

Oceanic and Continental Crust.

What are differences between oceanic and continental crust?

Oceanic crust is denser than continental crust.

How is oceanic crust generated?

Oceanic crust is generated as mid-ocean ridges where mantle magma wells up along the ridge axis, cools and crystalizes to form new oceanic crust/lithosphere.

What is the correlation between transform faults and mid-ocean ridges?

Transform faults link segments of spreading centers.

How do mantle plumes form and what is it called when it break's the earth's surface?

They form at the base of the mantle when excess heat at the core-mantle boundary generates a "blob" that rises rapidly through the mantle. It is called a "hot spot" where it breaks the Earth's surface.

What is the best-known example of a mantle plume/hot spot system on Earth today?

The Hawaiian Islands.

What is a characteristic of the Hawaiian Islands?

They are a chain of islands that were formed in succession as the Pacific Plate moves over a stationary mantle hotspot.

What does the bend in the Hawaiin Island-Emperor Seamount chain represent?

The bend represents the reorganization of global plate motions as India collided with Asia.

How are atolls and seamounts formed?

Volcanic islands sink as oceanic crust cools and subsides; eventually cones are eroded/buried by younger debris, producing a fringing reef and then an atoll and lagoon.

Where are the hotspots concentrated globally?

Above the South Pacific Superswell and the African Superswell.

How many tectonic plates are currently on the Earth?

12 major plates.

What is the driving force for plate motion?

Mantle convection

What is the purpose of mantle convection?

It is to accelerate transfer of heat from Earth's core to its surface in order to achieve thermal equilibrium.

What are some pieces of evidence for Continental Drift?

1. Geographic fit of continents (ex. Pangea)

2. Fossils

3. Mountains (ex. Appalachians)

4. Glaciations

What are some pieces of evidence for Seafloor Spreading??

1. World seismicity

2. Volcanism

3. Age of seafloor

4. Paleomagnetism

5. Heat flow

How does the age of seafloor serve as evidence for seafloor spreading?

Youngest crust is at mid-ocean ridges and oldest crust is far away from mid-ocean ridges.

How does heat flow serve as evidence for seafloor spreading?

High heat flow at mid-oceanic ridges indicates thin crust and shallow depth of mantle. These features are a product of the young age of oceanic crust at ridges.

How does paleomagnetism serve as evidence for seafloor spreading?

As oceanic crust form along a mid-ocean ridge, the iron bearing minerals in the crust lock in the ambient magnetic field. As the magnetic field flips back and forth through time, magnetic stripes are produced in the oceanic crust.

What are the types of tectonic plate boundaries and what are the differences?

1. Divergent Margins - spreading centers where new oceanic crust is formed.

2. Convergent Margins - subduction zones where old oceanic crust is destroyed.

3. Transform Margins - where two tectonic plates slide past one another as a major fault.

What is important about Iceland?

Iceland has a divergent plate boundary running through its middle. It is one of the few places globally where a mid-ocean ridge is visible above sea level.

What is the difference between mid-ocean ridges and continental rift valleys?

They are divergent plate boundaries; one is in oceans and the other is on continents.

How are new ocean basins formed?

When rifting with a continent is tripped by uplift, but as spreading continues the continental crust will thing and split apart, resulting in formation of a new ocean basin.

What are the different convergent plate boundaries and are the differences between each?

1. Ocean-Continent: the plate with oceanic crust is always subducted and generates a continental volcanic arc.

2. Ocean-Ocean: generates an oceanic island volcanic arc.

3. Continent-Continent: generates an orogenic zone (mountain range) and causes thickening of continental crust in collision zone.

Where are subduction zones formed?

Formed where an oceanic plate collides with a continental plate. The ocean plate, composed on denser rock, always sinks below the continental plate.

What is hydrous melting?

Where water causes the melting point of the rock to decrease, eventually melting the rock into magma in subduction zones.

What are features of crustal production?

Volcanic arcs/Subduction zones.

What does an accretionary complex represent?

It represents mostly material scraped off the surface of the descending oceanic plate.

What is a subduction zone characterized by?

An accretionary complex and a fore-arc basin.

What does a fore-arc basin represent?

It represents mostly eroded material from the volcanic arc or adjacent continent.

What is an example of a continent-continent orogenic collision?

The Himalayas. The Indian plate has been thrust under Eurasia, resulting in uplift of the Tibetan Plateau.

What are the Alps, Hellenides, and Taurides a product of?

The collision between Europe and Africa.

What are characteristics of transform plate boundaries?

Plates slide past one another, marked by strike-slip faults which can either be right-lateral or left-lateral depending on the offset, and crust is neither created nor destroyed.

What is an example of a transform plate boundary?

San Andreas Fault in California.

Where did Madagascar come from and how did it get to where it currently is located?

Madagascar broke away from East Africa. Its southward movement was driven by a spreading ridge, with a motion bounded by a transform fault.

What is the connection between India and Madagascar?

Madagascar was attached to India around 80 million years ago when it broke away from East Africa, but the India started moving rapidly northward, leaving Madagascar behind.

What is tectonic escape and how does it relate to the India-Eurasia collision?

Tectonic escape is a process where a piece of the Earth's crust moves away from a collision zone between two tectonic plates. In the case of India colliding with Asia, there was a piece of crust called the Indochina block that was getting squished and deformed by the collision. But instead of being crushed, the Indochina block was able to move sideways along a weak area underneath it and "escape" the stress caused by the collision. This movement helped to shape the geography and rocks in Southeast Asia.

Explain how back-arc spreading related to the Japan Sea?

The oceanic crust found in the Pacific Ocean near Japan is so old and cold that it is sinking into the mantle which is causing the retreating of a subduction zone.

The continental plate that Japan sits on is following the retreating trench due to a new spreading center that has appeared behind the subduction zone.

Why is there curvature of volcanic arcs in the North and West of the Pacific?

It is attributed to bending of the subducting Pacific Plate, as it is pushed down beneath the margins of North America and Asia. It is due to the geometry of the subducting plate.

What is a unique characteristic of the shelf found beneath the Bering Sea?

The Bearing Sea is a shallow continental shelf.

What happens when young ocean crust is subducted and what is an example of this effect?

An example is the Andes Mountains.

Oceanic crust is buoyant and does not subduct readily, so it tends to push beneath an overriding continent, producing high mountains and strong compressional forces on the continent margin.

What is asthenosphere flow and what are examples of its effects?

As South American moves rapidly westward into the Pacific region, asthenospheric material builds up - like snow in front of a flow. This triggers lateral flow around South American to both its north and south. This produces elongated microplates with reversed subduction polarity.

Examples are the Caribbean Plate to the North of South America and the Scotia Plate to the South of South America.

What is significant about plate tectonics?

Plate tectonics are essential to renew a planet's surface. Without plate tectonics, a planet's surface becomes stagnant and old.

What does the size of a planet mean for its internal temperature?

Larger planets remain internally hot for longer periods than smaller planets.

What did Early Earth look like and how did it get to its current state?

Originally after the collision, it was entirely molten. This allowed for easy migration of materials within Earth's interior, resulting in segregation.

Describe plate tectonics in the Archean time period.

Since the Earth was much hotter at the time, internal convection was more rapid, and the rate of formation and destruction of tectonic plates was strongly accelerated relative today. As a consequence, the number of tectonic plates was much greater than today.

What is continental crustal production closely tied to?

Supercontinents.

How many supercontinents have formed and broken up in the last 2 billion years and what are they called?

1. Columbia - oldest and speculative

2. Rodinia

3. Godwana

4. Pangea - most recent

How are cratons formed?

They are assembled through multiple collision of volcanic arcs, forming greenstone belts, and deformation and metamorphism of sedimentary deposits.

How are continents formed?

Volcanic arcs collide, forming cratons, then cratons collide together which form a continent.

What is a supercontinent?

All landmasses combined into one.

What is the relationship of supercontinents to mantle thermal structure? What is another name for it?

Another name is "log-jam tectonics" and "plume tectonics".

- Supercontinents form over cold regions of the Earth's mantle, where strong downwelling occurs.

- Continents are dragged into these downwelling zones but cannot be subducted, creating "log-jam tectonics".

-Once a supercontinent forms, the underlying mantle becomes warmer due to a blanketing effect of the thick crust above it.

- The cold downwelling zone slowly turns into a warm upwelling zone, initiating uplift and rifting of the supercontinent ("plume tectonics").

When was Pangea around?

300 - 200 million years ago.

How are Pangea and the current mid-oceanic ridges related?

The current mid-oceanic ridges are where Pangea's "seams" broke up the super continent.

What are the 4 stages of internal heat production and what the characteristics of each?

1. Hadean: no permanent crust (high heat production)

2. Archean: cratonization

3. Proterozoic: continental growth

4. Phanerozoic: "supercontinent cycle" (lower heat production)

Where is the nucleus of the next supercontinent?

Asia.

What is the structure of the continental crust?

1. Surface

2. Upper (brittle crust)

3. Middle (ductile crust)

4. Lower (melt-depleted)

5. Mantle

Which region of the continental crust is where most earthquake epicenters are located?

Most crustal earthquakes have their epicenters in the brittle upper crust.

What is the brittle/ductile crust transition?

It is at 10-15km below where the rock strength decreases due to higher temperature and confining pressures that allow plastic deformation.

What scales measure earthquake intensity?

Richter Scale - logarithmic scale related to earthquake energy.

Mercalli Scale - geometric scale representing earthquake damage.

What is the crustal composition of the upper and lower crust?

Upper Crust: Potassium, Sodium, and Calcium.

Lower Crust: Iron and Magnesium.

What are some effects of the New Madrid earthquakes?

- Localized uplift, forming temporary waterfalls on the Mississippi River

- Causing the river to run backwards (northward)

What was the New Madrid earthquake?

It was a series of 5 quakes, each quake was greater than 8.0 on the Richter scale.

What is liquefaction?

When the water-saturated ground turns to "jelly" from seismically induced shaking.

What can you use to identify paleo-seismic events?

You date sand blows.

What are effects of seismic shaking?

1. Soil liquefaction can significantly increase earthquake damage

2. Seismic shaking can induce upward flow of water of out of water-bearing sediments.

3. Loss of water causes reorientation and settling of sediment grains and compaction of sediment layers.

4. Uneven settling and compaction during seismic events can damage infrastructure.