1. A100 - Oct. 8 Shaping the Earth’s Surface
READ Chapter 5 – The Earth
Homework 4 Due Friday
Quiz on Friday
Today’s APOD
The Sun Today

2. Writing the History of Earth
Writing the History of Earth
Surface formations visible today have emerged only very recently compared to the age of Earth.

3. Earth’s Interior
Earth’s interior is hot in the center (6500K, as hot as the Sun’s surface)
Heat flows from the core to the surface as Earth slowly cools.

4. Motion in the Earth’s Interior
Heat generated by radioactive decay in the Earth creates movement of rock
This movement of material is called convection
Convection occurs because hotter material will be less dense than its cooler surroundings and consequently will rise while cooler material sinks

5. Convection Convection in the Earth’s interior
The crust and mantle are solid rock, although when heated, rock may develop convective motions
These convective motions are slow, but are the cause of: earthquakes, volcanoes, the Earth’s magnetic field, and perhaps the atmosphere itself

6. Plate Tectonics These plates “float” on top of the Earth’s mantle
Earth’s crust is fractured into more than a dozen plates
These plates “float” on top of the Earth’s mantle
Convection in the mantle cause the plates (and continents) to move

7. The shifting of large blocks of the Earth’s surface is called plate tectonics
Early researchers noted that South America and Africa appeared to fit together and that the two continents shared similar fossils
It was later proposed (1912) that all of the continents were once a single supercontinent called Pangaea
The Earth’s surface is continually building up and breaking down over time scales of millions of years

8. Earth’s Appearance: Past, Present, & Future
Plates (and continents) move at 2 cm/year
that’s 2,000 km in 100 million years
We can project the motion of continents into the past or future.
200 million years ago all continents were connected into one supercontinent
Total continental area has increased with time as new continental crust has formed

9. Earth’s Tectonic History
Earth’s Tectonic History

10. Plate Tectonics Tectonic plates move with respect to each other
Plate Tectonics
Tectonic plates move with respect to each other
Where plates move toward each other, plates can be pushed upward and downward → formation of mountain ranges, some with volcanic activity, earthquakes
Where plates move away from each other, molten lava can rise up from below → volcanic activity

11. Geological Features of Plate Tectonics
Continents are shaped by:
volcanism
stresses from plate tectonics
erosion
Subduction zones cause:
volcanic eruptions which form mountain ranges
islands to be scraped off seafloor plates onto continents
When two continental plates collide
one can not subduct under the other
crust is pushed up to form mountain ranges
e.g. the Himalayas

12. Geological Features of Plate Tectonics
When two continental plates pull apart
a rift valley is created
mantle convection causes eruption of basalt from valley floor
a new zone of seafloor spreading is created
e.g Arabian peninsula detached from Africa to form the Red Sea

13. Geological Features of Plate Tectonics
When two plates slip sideways against each other
rough grinding of plates builds up pressure along the crack between them
this crack is called a fault
pressure eventually breaks, causing a sudden shift, or earthquake

14. Rifting
Hot, molten material rises from deep in the Earth’s interior in great, slow plumes that work their way to the surface
Near the surface, these plumes spread and drag the surface layers from below
The crust stretches, spreads, and breaks the surface in a phenomenon called rifting

15. Subduction Subduction
Cool material sinks, and drags crustal pieces together buckling them upward into mountains
If one piece of crust slips under the other, the process is called subduction

16. Geological activity mostly occurs on plate boundaries
Plate tectonics is the dominant force that sculpt the Earth’s surface – it also triggers earthquakes and volcanoes

17. Volcanism on Earth – Subduction Zones
Volcanism on Earth – Subduction Zones
Volcanism on Earth is commonly found along subduction zones (e.g., Rocky Mountains).
This type of volcanism is not found on Venus or Mars.

18. Volcanism on Earth – Shield Volcanoes
Volcanism on Earth – Shield Volcanoes
Found above hot spots
Fluid magma chamber, from which lava erupts repeatedly through surface layers above.
All volcanoes on Venus and Mars are shield volcanoes

19. Example: The Hawaiian Islands
Shield Volcanoes
Tectonic plates moving over hot spots produce shield volcanoes → Chains of volcanoes
Example: The Hawaiian Islands

20. Only about 1/800 of the Earth’s radius!
Earth’s Atmosphere
Only about 1/800 of the Earth’s radius!

21. The Earth’s Atmosphere
The layer of gases around Earth constitutes its atmosphere
Relative to other planetary atmospheres, the Earth’s atmosphere is unique
However, studying the Earth’s atmosphere can tell us about atmospheres in general

22. Composition of the Earth’s Atmosphere
Primarily nitrogen (78.08% by number) and oxygen (20.95% by number)
The remaining gases (about 1%) include: carbon dioxide, ozone, water, and argon
This composition is unique relative to the carbon dioxide atmospheres of Mars and Venus and the hydrogen atmospheres of the outer large planets

23. Sources of Atmospheric Gas
Sources of Atmospheric Gas
Outgassing: Release of gasses bound in compounds in the Earth’s interior through volcanic activity
Later bombardment with icy meteoroids and comets

24. Loss of Atmospheric Gas
Loss of Atmospheric Gas
Chemical reactions in the oceans
Energetic radiation from space (in particular, UV)

25. The early atmosphere was different
Earth’s original atmosphere formed from gasses captured during formation of Earth
Contained much more methane (CH4) and ammonia (NH3)
Atmospheric composition changed through a combination of several processes:
Solar UV was intense enough to break out hydrogen atoms from CH4, NH3 , and H2O leaving carbon, nitrogen, and oxygen behind while the hydrogen escaped into space
Ancient plants further increased the levels of atmospheric oxygen through photosynthesis

26. Structure of the Earth’s Atmosphere
Atmosphere extends to hundreds of kilometers becoming very tenuous at high altitudes
The atmosphere becomes less dense with increasing altitude
Half the mass of the atmosphere is within the first 4 kilometers
The atmosphere eventually merges with the vacuum of interplanetary space

27. The Temperature Structure of Earth’s Atmosphere
The Temperature Structure of Earth’s Atmosphere
Exosphere: Heated by UV and X-rays from space
Thermosphere: Heated by X-rays from space
Stratosphere: Heated by UV radiation from space
Top of Ozone Layer
Ozone Layer
Troposphere: Heated by greenhouse effect
Atmosphere gets colder at larger distance from heat sources.

28. The Ozone Layer Oxygen shields against solar UV radiation
O2 provides some shielding, but O3, or ozone, provides most of it
Most ozone is located in the ozone layer at an altitude of 25 km
Shielding is provided by the absorption of UV photons by oxygen molecules (both O2 and O3) and their resultant dissociation
Single O atoms combine with O and O2 to replenish the lost O2 and O3
Life probably could not exist on the Earth’s surface without the ozone layer

29. The Greenhouse Effect
Visible light reaches the Earth’s surface and is converted to heat
As a result, the surface radiates infrared energy, which is trapped by the atmosphere at
infrared wavelengths
This reduces the rate of heat loss and makes the surface hotter than it would be otherwise

30. The CO2 cycle is a feedback mechanism which regulates Earth’s climate

31. Stability of Earth’s Climate
Plate tectonics causes the relative stability of Earth’s climate.
plate tectonics makes the CO2 cycle work
it takes about 40,000 years for the CO2 cycle to restore balance
There have been temporary episodes of extreme cooling and heating in Earth’s history.
these ice ages & hothouse period have their own feedback mechanisms

32. Earth’s Atmosphere is Unique
Outgassing from volcanoes on Venus, Earth, & Mars released the same gasses:
primarily water (H2O), Carbon dioxide (CO2), and Nitrogen (N2)
So why did Earth’s atmosphere end up so different?
why did Earth retain most of its H2O – enough to form oceans?
why does Earth have so little CO2 in its atmosphere, when it should have outgassed just as much CO2 as Venus?
why does Earth have so much more Oxygen (O2) than Venus & Mars?
why does Earth have an ultraviolet-absorbing stratosphere?

33. Why Earth’s Atmosphere is Unique
Earth’s H2O condensed because of the temperature
oceans formed, in which the CO2 gas dissolved
chemical reactions bound the C of CO2 into rocks like limestone
low level of atmospheric CO2 causes moderate greenhouse effect
temperatures on Earth remain where H2O can be a liquid
There was once liquid H2O on Mars and maybe Venus.
before CO2 could dissolve out, temperatures fell/rose so that
oceans boiled away on Venus and froze out on Mars
Earth’s O2 was not outgassed by volcanoes.
O2 is a highly reactive chemical
it would disappear in a few million years if not replenished
no geologic process creates O2

34. Life makes Earth’s atmosphere unique
Earth’s O2 was created through the evolution of life.
plants & microorganism release O2 via photosynthesis
they convert CO2 into O2
In the upper atmosphere, O2 in converted into ozone (O3).
via chemical processed involving Solar ultraviolet light
O3 absorbs Solar UV photons which heats the stratosphere
Venus & Mars lack plant & microbial life.
so they have no O2 in their atmospheres and no stratospheres

35. Dates to Remember ASSIGNMENTS this week READ Chapter 5 – The Earth
Homework 4 Due Friday
Quiz on Friday