GEL 001 MIDTERM 2

Question 1

Weather

Answer 1

Weather consists of the short-term (minutes to months) changes in the atmosphere. Most people think of weather in terms of
temperature, humidity, precipitation, cloudiness, brightness, and wind.
In most places, weather can change from minute-to-minute, hour-to-hour, day-to-day, and season-to-
season.

Question 2

Climate

Answer 2

is the average of weather over time and space. NASA defines climate as the average weather for a particular region and time period, usually taken over 30-years.

Question 3

Climate change

Answer 3

refers to a broad range of global phenomena created predominantly by burning fossil fuels, which add heat-trapping gases to Earth’s atmosphere. These phenomena include increased temperature trends (i.e., ‘global warming’), but also encompass changes such as sea-level rise; ice-mass loss in Greenland, Antarctica, the Arctic and mountain glaciers worldwide; shifts in ecological zones; expansion of deserts, and an increase in extreme weather events.

Question 4

What’s the connection between CO2 and global temperature?

Answer 4

CO2 composes a very small percentage of Earth’s modern atmospheric composition (~0.04%), but
because it traps outgoing heat energy as a greenhouse gas it plays an outsized role in influencing
planetary climate

• we need CO2 (and other greenhouse gases) to make our planet temperate and livable. Without it,
  freshwater would freeze, making life difficult if not impossible.

Question 5

Earth’s average surface temperature

Answer 5

Earth’s average surface temperature is 15°C (59°F), the same as the mean annual temperature of San Francisco. Earth is not only habitable, it’s a relatively pleasant place to live.

Question 6

Without its atmosphere and natural greenhouse effect, Earth’s average surface temperature

Answer 6

Without its atmosphere and natural greenhouse effect, Earth’s average surface temperature would be -18°C (0°F), far below freezing So without an atmosphere with greenhouse gases (H2O, CO2, CH4, several others), Earth would be 33°C (59°F) cooler than the observed value. (15C - -18C = 33°C) Earth would be an icy, desolate world.

Question 7

Solar radiation from the Sun is emitted as “shortwave” energy

Answer 7

Of the 100% of solar radiation that enters our upper atmosphere, ~30% is reflected back to space by clouds, scattered by atmospheric gases, and reflected by the Earth’s surface (from the high albedo of
ice and snow, mainly).
- the remaining 70% of solar radiation is absorbed by the Earth System where it powers the winds, weather patterns, ocean currents, and other processes in the hydrologic cycle, as well as
photosynthesis.

Question 8

Solar radiation energy used

Answer 8

the energy that is absorbed and used by the Earth System eventually ends up as heat, which is reradiated back toward space as “longwave” (infrared) radiation.

Question 9

greenhouse gases absorbed heat energy

Answer 9

in the atmosphere, part of the reradiated heat energy is absorbed by greenhouse gases (such as H2O, CO2, CH4, and many others) and is re-radiated back down toward the surface, warming the planet and
maintaining comfortable temperatures for life.

Question 10

Greenhouse effect

Answer 10

Solar radiation from the Sun creating “”shortwave” & “long wave” energy

Question 11

natural greenhouse effect

Answer 11

in the atmosphere, part of the reradiated heat energy is absorbed by greenhouse gases (such as H2O, CO2, CH4, and many others) and is re-radiated back down toward the surface, warming the planet and maintaining comfortable temperatures for life.

This is the natural greenhouse effect that warms our planet 33°C beyond what it otherwise would be without heat-trapping gases. (Greenhouse gases act somewhat like the panes of glass on a greenhouse.)

Question 12

The greenhouse effect is a natural process that enables life to exist on Earth

Answer 12

But the natural greenhouse effect is being super-charged by the rapid addition of CO2 to the atmosphere due to the combustion of fossil fuels (as well as deforestation). Our planetary thermostat is highly sensitive to changes in CO2 (as you saw in the ice core data where temperature and CO2 change in concert with one another)

Question 13

What are greenhouse gases and where do they come from?

Answer 13

-Water vapor

-CO2

-Methane (CH4)

-Nitrous Oxide (N2O)

Question 14

Water vapor

Answer 14

is continuously recycled over very short time scales through the hydrologic cycle, transferring heat energy within the Earth system. Water vapor is by far the most powerful greenhouse gas in terms of its heat-absorbing capability. But it cycles through the Earth system continuously, without much excess accumulation in the atmosphere.

Question 15

CO2

Answer 15

is continuously recycled through the biosphere, with no net gain or loss. Natural sources of CO2 are volcanism, respiration from animals (like you and me) as well as from fungi and microbes, naturally occurring wildfires, and outgassing from the ocean. Fossil-fuel burning and deforestation are the modern sources of excess atmospheric CO2.

Question 16

Methane

Answer 16

is produced biologically by microbes that live in oxygen-poor environments, such as the black mud of ponds and rice paddies, and the guts of cattle and termites. Massive amounts are released by leaks from natural gas wells and pipelines, as well as from the intentional flaring of natural gas from oil wells. The average atmospheric lifetime for a CH4 molecule is about a decade before it converts to CO2. CH4 is about 25 times more powerful than CO2 as a greenhouse gas.

Question 17

Nitrous oxide

Answer 17

is the natural byproduct of microbes in soils and the ocean, but anthropogenic sources are primarily from the use of nitrogen fertilizer for agriculture to replenish soil after woodlands and rainforests are cut. These human sources have increased the flow of N2O into the atmosphere by 40–50% over pre-industrial levels. (300 x more effective at trapping heat than CO2 over 100 year time spans)

Question 18

How do humans influence the global greenhouse effect?

Global Average Temperature Anomaly 1850-2023

Answer 18

The average global temperature on Earth has increased by about 1.4°C (2.3°F) since the Industrial Revolution of the mid-19th century and 1°C (1.8°F) since the 1970s
The plot shows that temperatures vary from year to year, with relatively stable or even cooling phases lasting a few years. But the long-term trend is toward a warming planet. Over the last few years, the rate of warming appears to be accelerating. It may be just a fluctuation, but it also may be a warning sign.

Question 19

How do humans influence the global greenhouse effect?

Atmospheric CO2 and Global Temperature since Industrial Revolution

Answer 19

Atmospheric CO2 is the primary driver of global temperature since the beginning of the Industrial Revolution (~1850)
- carbon dioxide (and other greenhouse gases) have been accumulating in the atmosphere, increasing carbon dioxide levels by 30% (from about 290 ppm in 1880 to 418 ppm in Oct 2023) (ppm = parts per million)
- the combustion of fossil fuels since the Industrial Revolution as well as intentional deforestation for agriculture and ranching are the dominant sources of the rapid increase in atmospheric CO2 content.
- the excess CO2 is enhancing the natural greenhouse effect and making our planet warmer.

Question 20

Atmospheric CO2 content is rising exponentially at an unprecedented rate, much faster than natural, pre-human increases in CO2 in the geologic past

Answer 20

CO2 concentrations have been much higher in the geologic past and were caused by natural mechanisms (e.g., volcanism, weathering), but the rate of increase was very slow relative to today’s extraordinarily rapid pace

Question 21

Global warming

Answer 21

Is an enhanced greenhouse effect triggered by a combination of fossil fuel combustion and deforestation that release greenhouse gases. It is best expressed by the upward trend in global temperatures during the 20th century, most notably since the 1970s.

Question 22

What level of CO2 might we expect in the near future?

Answer 22

The rate of modern CO2 increase is >10 times faster than during past interglacial warmings. It may be that humans, through the rapid burning of fossil fuels along with deforestation, are delaying
the expected natural swing back into a glacial phase. Perhaps we’re creating some sort of human-induced “super-interglacial” period?

projecting the current temperature trend into the future (red dotted line) suggests that global temperatures may rise by 2 to 4°C (3.6 to 7.2°F) by the year 2100, a rate of increase that would be very difficult for human civilization and the biosphere to adapt to
- it should be evident that the rate of temperature change in the modern world is unprecedented in the past 22,000 years (and is unprecedented in all of Earth history)

Question 23

Consequences of a warming planet

Answer 23

• Loss of continental ice mass
• increase in ocean heat content
• rising sea level

-extreme weather events

Question 24

Loss of continental ice mass

Answer 24

Remember that glaciers and ice sheets are physical indicators of climate change, growing during cold phases and shrinking during warm phases.
Both the Greenland and Antarctic ice sheets are losing mass, as measured by satellites and on-the-ground measurements. The mass is mostly being lost along the margins as the ice comes in contact
with warmer ocean water along the coastlines. (Gt = billion tons)
Most mountain glaciers are receding worldwide due to the warming planet, as well.

Question 25

Increase in ocean heat content

Answer 25

The world ocean absorbs more than 90% of the Earth’s net heat gain due to greenhouse gases. (water holds heat much better than the atmosphere)
- measurements are made from >2000 robotic buoys that constantly record ocean temperatures and other variables between sea level and 2000 m depths
- since the late 1980s, Earth’s oceans warmed at a rate eight times faster than in the preceding decades
- warmer water provides more energy for tropical storms, increasing the intensity and longevity of hurricanes and typhoons.
- warmer oceans also affect sea level because as the water warms it expands, causing the oceans to lap up onto the edges of continents. (called “thermal expansion”)

Question 26

Rising sea level

Answer 26

Water from melting ice sheets and mountain glaciers is entering the oceans, causing sea level to rise. When thermal expansion is added in, global sea level has risen >90 mm (3.5”) since 1993 when dedicated satellite measurements began.
- since 1900, sea level has risen 13-20 cm (~5-8 inches)
Sea level has been relatively stable over the past 6000 years, as determined from sediment cores and corals. Reliable tide gauge measurements began around 1880 and show an acceleration of sea level rise.
- projections of sea level rise to the year 2100 range from half a foot (at the optimistic low end) to 6.6 feet (at the pessimistic high end)
- ~400 million people live within 1 meter of sea level
- rising sea levels will cause a mass migration inland, mostly by the world’s poorest people who don’t have the means to cope with rising seas

Question 27

Extreme weather events

Answer 27

-The planet is getting hotter, with multiple, record-shattering high-temperature records set throughout 2021, 2022 & 2023.

-Parts of the planet are getting drier. Drought has recently affected large parts of South America, the southern United States, and China. Drought dried up major rivers, destroyed crops, sparked wildfires, and led to water restrictions in several places around the world in 2022-23

-Parts of the planet are getting wetter.

Question 28

Extreme weather events caused by water vapor

Answer 28

It may seem counterintuitive that the planet will see increases in ‘ both dry and wet events, but the physics of it are the same. What all these destructive events have in common is water vapor, which
plays an outsized role in fueling storms and accelerating climate change (because it’s a greenhouse gas). As the oceans and atmosphere warm, additional water evaporates into the air. Warmer air, in turn, can hold more of that vapor before it condenses into cloud droplets that can create flooding rains. So some regions will see more intense precipitation events.
In other regions, warmer air drives more evaporation from the surface then atmospheric circulation patterns transfer that moisture away from the area, enhancing drought.
The amount of vapor in the atmosphere has increased about 4 percent globally just since the mid-1990s.

Question 29

By the end of the 21st century:

Answer 29

• global temperature is expected to increase by 2 - 4°C (3.2 to 7°F), causing a latitudinal shift in climate/ecological zones as well as an expansion of deserts
• melting glaciers and ice sheets (along with the thermal expansion of warming seawater) will cause sea level to rise by 1-2 meters by the year 2100 that will flood large areas of low-elevation coastal plain, displacing millions of people living along the coasts (mostly in the developing world) In turn, the displacement of millions of people living along flooded coastlines will create “climate refugees” migrating inland to cities.
• extreme weather conditions (drought, floods, heatwaves) may become the norm
• wildfires will become more frequent and more intense
• higher sea-surface temperatures may feed hurricanes, making them more powerful
• society would be severely disrupted, disproportionately affecting the poor – exodus from flooded coastal regions and from expanding deserts . . .

Question 30

What’s the main thing we need to do?

Answer 30

-Energy Transitions

-Societal solutions

-Technological solutions

-Socioeconomic solutions

-Political Solutions

-Climate Activism

Question 31

Energy Transitions

Answer 31

What’s needed as rapidly as possible is a decarbonization of the global economy, a global revolution that alleviates our dependence on fossil fuels.
Decarbonization refers to the process of reducing carbon intensity, lowering the amount of greenhouse gas emissions produced by the burning of fossil fuels.
- we’re already at 418 ppm, so we don’t have that much time to work with
- requires rapid phase-out of fossil fuel sources, deployment of large-capacity renewable sources, enormous increases in energy efficiency, development of long-duration energy storage (batteries),
and other energy technologies not yet known.
- also requires land use emissions from agriculture and deforestation to fall steadily to zero

Question 32

Renewable energy sources

Answer 32

Renewable energy sources will need to supplant fossil fuels as our main energy supply. Renewable energy sources like wind and solar are today a cheaper form of electricity than coal and natural gas. (~$60 per megawatt-hour for solar & $50 for wind vs $70 for natural gas and $117 for coal)

Question 33

In 2022 wind and solar generated 12% of global electricity for the first time.

Answer 33

50 countries get more than a tenth of their power from wind and solar. (Unfortunately, due to the economic surge and the demand for electricity, coal power also rose at the fastest rate globally since 1985.)
- the countries leading the way were China (which leads the world in the amount of electricity produced by wind and solar), the Netherlands, Australia, and Vietnam.
- in the U.S., solar now employs more people than coal, oil and natural gas combined. Renewables are projected to produce 23% of electricity in 2023.
- the International Energy Agency projects that renewables will overtake coal (which now supplies 36% of global electricity) as the world’s largest source of electricity.
- more than $1.7 trillion is expected to be invested in technologies such as wind, solar, electric vehicles and batteries globally in 2023.

Question 34

Hydrogen energy

Answer 34

Hydrogen is a crucial storage medium for the clean energy future. It is capable of being stored as a liquid or a gas and then burned or converted into electricity through a fuel cell — all without producing
greenhouse gas emissions.
Isolating hydrogen takes a lot of energy. And right now, most hydrogen is produced by burning natural gas (CH4), which has an enormous carbon footprint. But electrolyzers, which split water into
hydrogen and oxygen, can be powered by renewable energy like wind turbines and solar panels.
- Hydrogen produced from natural gas is called “gray” hydrogen. Hydrogen produced from natural gas but where the carbon dioxide is captured rather than emitted is called “blue” hydrogen. Hydrogen
that uses energy from renewable sources to split water into H and O is called “green” hydrogen.
- Hydrogen fuel cells essentially work like batteries. They can be refilled with pure hydrogen, which is transformed through a chemical reaction into electricity, water, and heat.
- the price for hydrogen fuels varies by location but is typically far higher than traditional sources.
- the Biden administration recently announced plans to award up to $7 billion to create seven regional hydrogen hubs around the country. The money will be used to kick-start development of the
infrastructure and production facilities needed to make and transport hydrogen at scale.

Question 35

Electric vehicles

Answer 35

We need a complete transformation to electric vehicles with associated infrastructure of recharging stations (powered by renewable sources of electricity) to reduce our dependence on oil.
Electric vehicles are by far the fastest-growing segment of the auto industry, with record sales of 300,000 in the second quarter of 2023, a 48 percent increase from a year earlier. Federal tax credits of up to $7,500 have made the least expensive electric vehicles competitive with gas-powered cars. And about two dozen states offer additional tax credits, rebates, or reduced fees, further pushing
down their cost.
- mileage per charge and the availability of charging stations is slowing growth somewhat and has driven down prices, but those are problems that will be overcome in time
- the “upstream” issue of mining the array of minerals necessary for batteries (e.g., lithium, cobalt, manganese, nickel, and graphite) is also a significant problem, but can be done economically without
human rights abuses and environmental degradation is possible with monitoring and regulation.
- electric vehicles is not a red-state, blue-state thing, but rather is happening nationally. As the cost of batteries comes down, and the number of charging stations nationwide goes up, we should see
exponential growth.

Question 36

Societal solutions

Answer 36

There are many many ways to make this happen and they’re occurring now in many places.
• Conservation measures, including higher energy efficiency standards in new construction
• Redesigning cities and suburbs to make them less car-intensive and higher density, with people living closer to where they work (the New Urbanism movement with its focus on walking and biking and more livable communities)
• Better mass transit (including high-speed rail that can displace regional flights and the jet fuel they require)
• A corporate tax on carbon emissions by companies that pollute our atmosphere without repercussions
• Economies based on less consumption
• Voting in all elections, if feasible for the candidate who bests addresses the climate issue

Question 37

Technological solutions

Answer 37

Facilities that actively capture carbon from the atmosphere and then pump it underground (called sequestration)
CCS – (carbon capture and sequestration or carbon capture and storage) technologies are beginning to be deployed. The first large-scale plant opened in Iceland this year, mostly as a “test of concept” to assess its feasibility. It’s pulling a relatively tiny amount of carbon dioxide from the atmosphere.
- at the moment, the costs are high: about $600 to $800 per metric ton of carbon dioxide, far from the levels around $100 to $150 per ton that are necessary to turn a profit without the help of any
government subsidies.
Other technological fixes aim to actively change the amount of solar radiation reaching Earth’s surface, generally called geoengineering. For instance, one commonly proposed idea is to actively pump small
particles into the atmosphere to increase cloud cover and thus albedo. The increased clouds would reflect incoming sunlight back to space, cooling the planet. Geoengineering ideas have a wide variety of problems such as unintended consequences and cost.

Question 38

Socioeconomic solutions

Answer 38

Other policies that need to be implemented in the developing world to move toward a more sustainable planet involve reducing the disparity between the rich and poor.
- research shows that the costs of climate change will be disproportionately absorbed by the poorest people on the planet who are also the least responsible for contributing to the problem. Boosting their lives gives them more options to cope with rising sea level, flooded coastal farmlands, severe weather events, and prolonged drought.
- this issue relates to “climate justice” - the global north historically has been responsible for the vast majority of carbon pollution. And the global south has been responsible for almost none of it.
Currently, the emissions attributable to the richest 1 percent of the global population account for more than double those of the poorest 50 percent.
- the world’s 85 richest people have as much wealth as the poorest 3.5 billion
- greater inequality within countries makes them less able to tackle climate change. The more wealth is concentrated at the top, the more powerful people tend to insulate themselves from the effects of warming and resist meaningful climate action.

Question 39

Political Solutions

Answer 39

Earlier in 2022, the Biden administration and Congress passed the Inflation Reduction Act (a politically motivated name that hides its potential for attacking the climate crisis. It really should be called the
Climate Action Act.), which is intended to reduce US emissions by 40% from 2005 levels by 2030. This is by far the most ambitious attempt to tackle climate change in American history. The Inflation Reduction Act in the US will spend 370 billion dollars to push the country closer to decarbonizing, spurring trillions in private investment in electric vehicles & charging stations, upgrades to the national electrical grid, sustainable batteries, solar energy, and direct air carbon removal. Through tax incentives, the US government is subsidizing the clean energy transition. International collaboration on climate, fair trade, technology transfer, population growth
- working collectively to reduce our ecological footprint on the planet
- the consensus at this point suggests that governments weren’t doing enough to meet commitments made previously (e.g., the Paris Climate Accords of 2015)
- the problem is that there is no enforcement mechanism to get countries to honor their pledges
- the long arc of history points to our benevolence and cooperation as a species

Question 40

Climate Activism

Answer 40

A global climate movement is happening today, focused on pressuring governments and large corporations to take action on the climate crisis.Several environmental non-profits are guiding the direction and strategy of the movement.
- some groups focus on advocacy and lobbying, others on ending fossil fuels, yet others on environmental and climate justice

Question 41

natural divisions of Earth

Answer 41

geologists look at the worlds natural divisions of Earth is not defined by the arbitrary position of the shoreline, but rather the location of natural geologic boundaries that define plates, huge slabs of rock that are in constant slow motion relative to one another.

Question 42

California plate

Answer 42

a place where plates grind laterally past one another, occasionally getting stuck until the pent-up energy is released as an earthquake.

Question 43

plate boundaries occur

Answer 43

Most plate boundaries occur along the seafloor, deep beneath the water. But some cut across the continents, like the Himalayas or the San Andreas fault in California

Question 44

Alfred Wegener, a German meteorologist/geologist.

Answer 44

First serious ideas about shifting continents and the existence of an ancient supercontinent.
1915 book suggested that the continents were once combined into one huge continent that he called Pangea. Continents subsequently drifted apart to their current configuration. His ideas were
based in evidence such as the fossil record, the alignment of mountain belts on separated continents, and the jigsaw fit of the continents.
- at the time, we knew nothing about the 70% of the planet beneath the oceans, so the emphasis was on the continents.
- his ideas of continental drift were met with resistance by the establishment of the day because no one could conceive of a way for continents to plow through the oceanic crust, which were thought
to be old, immobile, and fixed through time.
- Wegener’s ideas were based on data from continents, but a mechanism for moving continents across ocean basins couldn’t be determined
- It wouldn’t be until WWII and the Cold War when the ocean basins were more completely explored that a mechanism could be found to explain ancient continental configurations.
- in the 1960s and 70s, after more became known about the seafloor, the original ideas of continental drift were modified and evolved into the comprehensive theory of plate tectonics

Question 45

Continental Drift

Answer 45

the movement of continents resulting from the motion of tectonic plates

Question 46

Exploration of the Oceans

Answer 46

WWII and the Cold War triggered the exploration of the ocean basins and the transition from the original ideas about continental drift to the modern theory of plate tectonics. Remember, the seafloor covers 70% of the planet and until the 1960s and 70s we knew very little about it.

Question 47

Echo sounding

Answer 47

(recording the echo of a sound wave bounced off the ocean floor) provided information about the bathymetry of the ocean bottom. Bathymetry is simply the depth and topography of the sea floor.
- information about the composition of the seafloor itself was determined by a coordinated program of seafloor drilling from specially outfitted research vessels.
- ships sailed back and forth along traverses, recording echoes and converting the time of the sounding and the time of the echo to a vertical distance, in the process creating bathymetric profiles of the sea floor. These profiles were combined to make maps of the sea floor.
- average depth of the world ocean revealed to be ~ 4 km below sea level (For comparison, the average height of the continents is less than 1 km above sea level. And relative to the 6371 km radius of Earth, the 4 km deep oceans are a mere puddle.)

Question 48

first maps

Answer 48

The first maps revealed the presence of elongate mountain chains that extended beneath all the world’s
oceans - called mid-ocean ridges (about 2-2.5 km beneath the surface)
- mid-ocean ridges occur in the mid-Atlantic, the East Pacific, the Indian Ocean and between Australia and Antarctica in the Southern Ocean
- e.g., Mid-Atlantic Ridge, East Pacific Rise (a “rise” is slightly less steep and high than a ridge)
- mid-ocean ridges are roughly symmetrical - bathymetry on one side is a mirror image of bathymetry on the other
- between the mid-ocean ridges and the continents are broad, relatively flat regions of the ocean called abyssal plains (about 4-5 km deep)
Mid-ocean ridges are bisected at right angles by steep-walled fracture zones that parallel one another, effectively segmenting and offsetting ridges into smaller pieces

Question 49

deep ocean trenches

Answer 49

Other parts of the oceans, especially regions adjacent to chains of active volcanic islands, exhibit narrow, deep ocean trenches that may reach depths of 8-11 km (~36,000’ deep!)
- the Marianas Trench in the western Pacific contains the deepest point on Earth, the Challenger Deep (11,035 m or 35,838’)
- relative to the 6371 km radius of the Earth, the 11 km deep Challenger Deep is a mere scratch on the surface

Question 50

Continental margins

Answer 50

Continental margins slope gently seaward along a relatively smooth continental shelf before abruptly steepening into a continental slope (Nance Fig. 17-33b). The slope eventually gives way to a broad
continental rise that ends in relatively flat abyssal plains of the ocean floor.
- the continental shelf is just the gently steepening underwater continuation of the coastal plain along the seaward edge of the continents. The coastal plain meets the continental shelf at the shoreline.
- the continental shelve extends from the shoreline out to about 130 m (~400 ft) water depth where it abruptly drops off onto the continental slope
- the continental shelf and slope are covered by a wedge of sediment a few km thick, derived from erosion of the adjacent continent and transported seaward by rivers
- the continental slope marks the true edge of continents because it reflects underlying changes from thick continental crust to thin oceanic crust.
- the shoreline that marks the geographic edge of continents on maps is an ephemeral feature that shifts position through time as sea level rises and falls - it does not mark the true geologic edge of
continents.
- the continental rise is a gentle slope covered with a wedge of sediment derived from the continental shelf that acts as a transition zone between the steep continental slope and the flat abyssal plains.

Question 51

ocean basins

Answer 51

The exploration of the ocean basins revealed that the seafloor wasn’t just a flat, featureless surface, but rather has a rugged topography marked by distinct features like mid-ocean ridges, fracture zones, continental margins, abyssal plains, and deep ocean trenches.
The recognition of all these underwater topographic features in the post-WWII years prompted geologists to explain them all, forcing a reconsideration of the long-forgotten ideas of continental drift theory. Eventually plate tectonic theory superseded the incomplete ideas of continental drift. Most plate boundaries occur on the seafloor and coincide with mid-ocean ridges, deep-ocean trenches,
and fracture zones. These bathymetric features of the seafloor are fundamental geologic boundaries that mark the edges of thick slabs of rock called plates.

Question 52

How are plate boundaries defined?

Answer 52

Continental drift evolved into today’s theory of plate tectonics - truly a scientific revolution when it was conceived and proven in the 60s and 70s, in large part due to the exploration of the oceans.

Question 53

Maps of earthquake distribution

Answer 53

Maps of earthquake distribution follow somewhat linear patterns. They tend to be located along the edges of continents or strung out along linear trends down the middle of oceans. The global pattern of volcano distribution follows a similar linear trend as the earthquake locations. The distribution of active volcanoes around the Pacific Ocean is called the “Ring of Fire” because they are among the most violent and deadly volcanoes in the world.

Question 54

tectonic plate boundaries

Answer 54

The locations of earthquakes and volcanoes, in concert with large ‘ bathymetric features on the seafloor, mark the locations of tectonic plate boundaries
- they occur irrespective of the geographic boundaries of continents and oceans based on the arbitrary position of sea level. - some tectonic boundaries occur in the middle of oceans (marked by mid-ocean ridges). Others occur close to the margins of continents (like the western Americas). Still others cut through narrow seaways
(the Caribbean, the Red Sea, Persian Gulf and Mediterranean). Yet others cut through continents (Himalayas, SAF).
- so the bottom line is that tectonic plate boundaries only rarely coincide with the geographic boundaries
between continents and oceans

Question 55

The surface of the Earth is in continual motion

Answer 55

The surface of the Earth is in continual motion. New crust is being formed as we speak. Old crust is being destroyed. And all the surface of the planet is in constant slow motion. This process is called plate tectonics.

Question 56

Plate tectonics

Answer 56

Plate tectonics = the continual motion, creation, and destruction of parts of the planet’s active surface.
“plate” refers to large, tabular sections of the Earth’s outer surface that are in constant motion..
~ 12 major plates plus several smaller plates; plates typically include both continents and parts of oceans, but some are all ocean floor and a few are mostly continental
- the locations of earthquakes and volcanoes essentially define plate boundaries since the boundaries are where plates interact . . .
“tectonics” refers to large scale movement and deformation of Earth’s outer surface (crust plus upper mantle)

Question 57

How thick are tectonic plates?

Answer 57

~ 100-150 km (~60-90 miles)
To answer this question, we need to differentiate between layers in the Earth based on composition and layers of the outer Earth based on strength
•Layers of the Earth based on composition = crust, mantle, and core
•Layers of the outer Earth based on strength = lithosphere, asthenosphere
- you can think of ‘strength’ as the potential to flow under certain conditions

Question 58

Asthenosphere

Answer 58

Asthenosphere = upper mantle down to ~ 400-600 km
- boundary between lithosphere and asthenosphere defined by a temperature of ~1280°C, the temperature at which rock (at high pressures) begins to slowly flow when acted upon by a force
- “hot, weak, semi-plastic” strength properties – the asthenosphere is solid, but mobile
- heat moves by convection (more on this later) in the asthenosphere

Question 59

lithospheric plate

Answer 59

So, in sum, rigid lithospheric plates move above weak, slightly molten asthenospheric rock.

Question 60

Plate Tectonics - three types of plate boundaries

Answer 60

• Divergent boundaries marked by mid-ocean ridges
• Convergent boundaries marked by deep-ocean trenches
• Transform boundaries marked by fracture zones and elongate faults, like the SAF

Question 61

Divergent plate boundaries

Answer 61

Boundary between two plates that contributes to the growth of ocean basins or the break-up of continents.
- divergent boundaries commonly occur along mid-oceanic ridges and contribute to the continual growth of older ocean basins (e.g., Atlantic, Pacific, Indian Oceans) or they may occur within
continents where they act to open new ocean basins (continental rifting, see below).
- the primary force at divergent boundaries is extension – ‘stretching’ caused by the motion of the two plates away from each other
- two main types of divergent boundaries: mid-ocean ridges and continental rifts

Question 62

New lithosphere

Answer 62

New lithosphere is created by seafloor spreading along divergent margins such as the 60,000 km underwater mountain chain known as mid-oceanic ridges that circle the globe. Mid-ocean ridges are
commonly called “spreading ridges.”

Question 63

lithosphere

Answer 63

the rigid outer part of the earth, consisting of the crust and upper mantle

Question 64

Magma and lava= Seafloor spreading

Answer 64

Seafloor spreading is the process where magma wells up along fractures in the lithosphere near the mid-ocean ridge axis and pours out as lava onto the seafloor. (both magma and lava are molten rock. The only difference is that magma is below ground, whereas lava is above.)
- the lava erupts and solidifies along a narrow, central rift valley (aka ‘axial rift’) that occupies the ridge axis. The axial rift valley has typical dimensions of about 500 m deep and 10 km wide, bordered by
steep cliffs.

Question 65

Seafloor spreading part1

Answer 65

As plates are pulled apart along the spreading axis by extensional (divergent) forces, the rocks of the brittle crust break along faults, with blocks of rock sliding downward to create the axial rift valley and adjacent ridges
- the underlying rock of the asthenosphere passively rises beneath the thin lithosphere above and begins to melt as the overlying weight of rock is reduced, producing magma (this is called “decompression melting” and occurs by the release of pressure rather than any increase in temperature)
- magma accumulates in magma chambers beneath the axis of the mid-ocean ridge, sort of a ‘holding pen’ for the magma supply
- most of the magma solidifies in place beneath the surface, while some finds its way to the surface where it pours out as lava, cools and solidifies, forming brand-new seafloor
- the lava interacts with cold seawater to form bulbous pillow shapes with the lava solidifying to form
the main rock of the ocean floor – pillow basalt
- with continued tectonic extension of the plates bordering the mid-ocean ridge, this new oceanic lithospheric rock eventually gets split along faults and fractures, then is displaced to either side of the
mid-ocean ridge as newer molten material rises along the axial rift in the middle of the mid-oceanic ridge.

Question 66

Seafloor spreading part 2

Answer 66

Because all seafloor forms at mid-ocean ridges and spreads laterally with time, the youngest seafloor lies along the ridge axis and the oldest seafloor lies along the outer margins of the oceans adjacent to continents
- all ocean floor on the planet is less than 200 million years old (extremely young compared to the continents which have rocks ranging back to 4.0 billion years old)
- of all tectonic settings, underwater mid-oceanic spreading ridges produce the greatest sheer volume of volcanic material (essentially produces all of the seafloor rock)
- 80% of all volcanic activity on Earth takes place under water along the mid-ocean ridge axis
- eruptions on the seafloor are generally benign since they occur under the pressure of >2 km of water
- as ocean basins open up between continents, it creates the illusion that continents are drifting, but in reality new seafloor is forming in between, widening the ocean and increasing the distance between
the two continents (in reality, continents ride along on the growing oceanic plate like suitcases on an airport conveyor belt) Recognition of the process of seafloor spreading in the 1960s resolved Wegener’s problem of forcing continents to plow through oceanic crust - instead, continents rode along passively as ocean basins opened up between them. Due to seafloor spreading, the map of Earth’s surface has changed continuously through time as ocean basins open and close and continental configuration evolves progressively.