Final Geology

How do we know the age of the Earth?

Age dating of meteorites

What is relative age dating?

Based on Geologic Principles

How is it different from absolute age dating?

Relative Age is based off geologic principles, whereas absolute is based off of isotopes.

What are the 5 geologic principles used in relative age dating?

Superposition, Original Horizontality, Cross-Cutting, Inclusions, Faunal (Fossil)

Principle of Superposition

In a sequence of rock layers, the oldest is on the bottom and the youngest at the top. (younger as you move up). For sedimentary rocks, it can include a lava flow or ashfall deposits.

Principle of Original Horizontality

Rock layers are originally horizontal. For sed rocks only but can include lava flows and ashfall deposits.

Principle of Cross-cutting Relationships 

Cross cutting = igenous intrusion (dike) or a fault that cuts across rocks. These are intrusions into a rock. The intrusion that does the cutting is YOUNGER than what it cuts.

Intrusions are ALWAYS YOUNGER than what they cut

Principle of Inclusions 

An inclusion is something found WITHIN a rock. States that all inclusions are OLDER than the rock they are found in.

Principle of Faunal (fossil) Succession

A good index fossil has the following characteristics: 1) Easily recognizable in the field 2) Has preserveable hard parts (bone, shell. exoskeleton) 3. Short span of time on earth from evolution to extinction.  4) geographically widespread (animal or plant from many areas around the world). 

What is an index fossil?

 recognizable fossils that indicate the relative age of a rock.

What criteria make it possible for a fossil to be considered an
index fossil?

A good index fossil has the following characteristics: 1) Easily recognizable in the field 2) Has preserveable hard parts (bone, shell. exoskeleton) 3. Short span of time on earth from evolution to extinction.  4) geographically widespread (animal or plant from many areas around the world). 

What are the 3 unconformities?

disconformity, angular unconformity, and nonconformity

disconformity

caused by either erosion or a period of non-deposition, and is identified by rock layers that are parallel to the unconformity surface. 

angular unconformity

 caused by erosion, where the layers below the unconformity surface are at an angle (not parallel to the unc surface). 

nonconformity

 where the rocks below the unconformity surface are igneous or metamorphic (non-sedimentary rocks). This is from erosion.

What is an isotope?

atoms of the same element that have different numbers of neutrons

What is the definition of a half-life?

the amount of time it takes for one-half of a parent to decay to daughter.

What do we call the unstable (radioactive) isotope? What does it turn into?

radioisotope

transforms into a more stable form through a process called radioactive decay

What information do we need in order to determine the absolute age of a rock?

The amount of the parent radioactive isotope currently in the rock.

The amount of the daughter isotope (the product of the decay).

The half-life of the parent isotope (the time it takes for half of it to decay).

What is the only assumption we make when determining the absolute age of something?

rate of radioactive decay has remained constant over time.

Know the stress/strain graph and all of the terms associated with it. W

Compressive, Extensional, Shear.

Elastic deformation (rock returns to original shape)

Plastic deformation- permeanet.

Failure point (rock breaks) - earthquale occurs

What is the definition of strain?

deformation or change in shape/size of a material in response to applied stress.

How does the velocity of any seismic wave change with changes in density?

Seismic wave velocity increases in denser, more rigid materials, and decreases in less dense or softer materials.

What is a seismic discontinuity,

a boundary in the Earth where seismic wave speed changes suddenly.

what regions of the earth does the Moho separate?

separates the crust from the mantle.

Know the difference between the focus (hypocenter) and epicenter of an earthquake

Focus- point inside the Earth where the EQ starts.

Epicenter- point on Earth’s surface directly above the focus

Know how seismic waves travel through the earth (ie. shear motion vs
extension/compression)

P-waves (Primary waves): Compressional waves (push-pull motion).

S-waves (Secondary waves): Shear waves (side-to-side motion).

Surface waves: Travel along Earth’s surface with rolling or sideways motion.

Which seismic waves are the fastest? The slowest?

Fastest: P-waves

Slowest: Surface waves

Which are called body waves?

P-waves and S-waves

What does the time lag between P and S arrival times allow us to determine?

distance from a seismograph to the earthquake epicenter.

What are the two ways we can measure the size of earthquakes

Magnitude (e.g., Richter or Moment Magnitude)

Intensity (Modified Mercalli Intensity Scale – based on observed damage)

What do we need to know in order to find the Richter Magnitude (Moment Magnitude) of
an earthquake?

Amplitude of the seismic waves

Distance to the epicenter

Seismic energy released (for Moment Magnitude)

What materials slow down the seismic waves, and in the case of S-waves, stop them?

Unconsolidated sediments and soft soils slow all seismic waves.

Liquids (like the Earth's outer core) completely stop S-waves, which can't travel through them.

Why are surface waves so destructive? (know three reasons)

They have larger amplitudes.

They travel along the surface, affecting structures.

They last longer than body waves.

Know how to read a seismogram

A seismogram shows the arrival times of:

P-waves (first, smallest squiggles)

S-waves (arrive second, larger motion)

Surface waves (last, most destructive motion)

Know how to read a P and S Travel Time curve graph. What does this graph tell us?

The graph shows how long P and S waves take to reach a location at different distances from the epicenter.

It’s used to calculate the distance to the earthquake using the S–P arrival time difference.

How do we find the epicenter of an earthquake?

Using triangulation:

Measure S–P time at three seismograph stations,

Calculate distances to the epicenter,

Draw circles from each station—where they intersect is the epicenter.

What type of geologic material is safe to build on (in earthquake prone regions)?

Bedrock is safest—stable and resists shaking.

Avoid soft sediments, water-saturated soils, or landfills.

What are some of the building restrictions or features needed for buildings and homes in

earthquake prone regions?

Flexible materials and foundations

Shock absorbers or base isolators

Low center of gravity

Cross-bracing and reinforced frames

How do anticlines and synclines affect seismic waves, and which one should we avoid
when planning building locations?

Seismic waves amplify in synclines (trough-shaped folds).

Avoid building in synclines due to higher shaking risk.

Anticlines (arch-shaped folds) are generally more stable.

What are seismic gaps, and why are they dangerous?

sections of faults that haven’t had recent earthquakes, unlike surrounding segments. They are dangerous because they may be building up stress and are likely locations for future large earthquakes.

Know the definition of the theory of plate tectonics

Earth’s outer shell (the lithosphere) is divided into several large, rigid plates that move over the semi-fluid asthenosphere. Their movement causes earthquakes, volcanoes, mountain building, and seafloor spreading.

Who proposed the continental drift hypothesis?

Alfred Wegener

What evidence is there to support
continental drift? Why was this hypothesis rejected?

evidence: Fit of continents like puzzle pieces (e.g., South America and Africa), Matching fossils on different continent, Similar rock formations and mountain ranges, Evidence of glaciation in now-tropical areas.

Rejected because: couldn’t explain how the continents moved

Why did the “land bridge” hypothesis fail? Which observation, made by Wegener, was
this land bridge hypothesis good for?

It failed because no evidence of ancient land bridges was ever found. It was used to explain identical fossils found on different continents before continental drift was accepted.

What is the modern day evidence that supports the theory of plate tectonics?

• Seafloor spreading

• Magnetic striping on the ocean floor

• GPS measurements of plate movement

• Earthquake and volcano locations around plate boundaries

• Increasing age of ocean crust away from mid-ocean ridges

Who was Harry Hess (what was his hypothesis and what observations did he make to
create his hypothesis)?

Harry Hess was a geologist who proposed the idea of seafloor spreading. He observed mid-ocean ridges and noted that new ocean crust was forming there, pushing older crust away from the ridge.

What two hypotheses merged into the present day theory of plate tectonics?

Continental drift (Wegener) and Seafloor spreading (Hess)

What changes do we see in the ocean floor, as we move away from the mid-ocean ridge?

Ocean crust becomes older, Sediment layer becomes thicker, Crust becomes cooler and denser

Where are the majority of earthquakes and volcanoes located?

Along plate boundaries, especially around the Ring of Fire in the Pacific Ocean.

What is the relationship between plate boundary type and earthquake focus depth?

Divergent and transform boundaries: mostly shallow earthquakes. Convergent boundaries: can have shallow to very deep earthquakes (due to subduction)

What are the three ways that magma is generated? How are these three melt-generating
processes related to plate tectonics? What plate boundaries produce magma?

Decompression melting – at divergent boundaries and hotspots

Flux melting (addition of water) – at convergent boundaries (subduction zones)

Heat transfer – at continental hotspots or from intruding magma

All three are linked to plate boundaries where mantle material changes in pressure, temperature, or composition.

What is the Wadati-Benioff zone?

A zone of earthquakes that occurs along the slab of subducting oceanic plate, extending into the mantle. It shows the angle and depth of subduction.

What are the 5 types of plate boundaries? Be able to recognize them in a diagram

Divergent – plates move apart (e.g., mid-ocean ridge)

Convergent – plates move together (e.g., subduction zones or mountains)

Transform – plates slide past (e.g., San Andreas Fault)

Ocean-ocean convergence – volcanic island arcs

Continent-continent convergence – large mountain chains, no volcanoes

Which crust type will never subduct, and why?

it is thicker and less dense, so it floats above the mantle.

What is subduction?

The process where one tectonic plate (usually oceanic) sinks beneath another and is recycled into the mantle.

Why is the oldest ocean floor only 180 million years old, when continental crust can be 4
billion years old?

Oceanic crust is constantly recycled at subduction zones, while continental crust is too buoyant to subduct and therefore survives much longer

What is the driving mechanism for plate tectonics?

Mantle convection, along with slab pull and ridge push.

What is the Moho Discontinuity? It occurs between what two layers of the Earth?

The Moho is a boundary between the crust and the mantle. Seismic waves speed up here due to the change in material.

Rock melts when

Pressure decreases, water is added, and temp increases

Be able to relate surface features with the appropriate plate boundary:

Mountain chains with no active volcanoes = Continent-continent convergence

Coastal mountains with active volcanoes = Ocean-continent convergence

Mid-ocean ridges = Divergent boundaries

Volcanic island arcs = Ocean-ocean convergence

Know what the lithosphere is composed of. Where is the lithosphere located?

Crust + uppermost mantle; rigid and brittle; earthquakes originate here

Where is the asthenosphere? Know which is brittle, which behaves plastically, and which one can have earthquakes originate in it.

Below lithosphere; soft and plastic; allows plates to move but does not generate earthquakes

What region of the earth is the only place where earthquakes can be generated?

The lithosphere, because it is brittle and can break under stress.

Know the definition of convection

Convection: heat-driven movement of material (hot rises, cool sinks), which moves pla

review again the regions of the earth (core, mantle, crust); know the differences between oceanic and continental crust.

Core: inner (solid), outer (liquid)

Mantle: solid but flows over time

Crust:

Oceanic: thin, dense, young

Continental: thick, less dense, old

Be able to determine which way a lithospheric plate is moving, based on the ages of volcanic island chains (like the Hawaiian Islands)

The plate is moving in the direction from the youngest to the oldest island. Hotspots stay stationary while the plate moves over them, forming a chain