Download Chapter 7 Earth and the Terrestrial Worlds

Chapter 7
Earth and the Terrestrial Worlds
Mercury
craters
smooth plains,
cliffs
Venus
volcanoes
few craters
Radar view of a twinpeaked volcano
Mars
some craters
volcanoes
riverbeds?
Moon
craters
smooth plains
Earth
volcanoes
craters
mountains
riverbeds
Why have the planets turned out so differently, when they formed at the same time from the same materials?
7.1 Earth as a Planet
Our Goals for Learning
• Why is Earth geologically active?
• What processes shape Earth’s surface?
• How does Earth’s atmosphere affect the planet?
Why is Earth geologically active?
What do we know about the interior of the Earth, and how do we know it? And 4.45 billion years after its formation, why is the Earth still hot?
What cools off faster?
A. A large cup of coffee.
B. A teaspoon of capuccino in the same cup.
What cools off faster?
A. A large cup of coffee.
B. A teaspoon of capuccino in the same cup.
What cools off faster?
A. A big terrestrial planet.
B. A tiny terrestrial planet.
What cools off faster?
A. A big terrestrial planet.
B. A tiny terrestrial planet.
Why? (Hint: the answer is a ratio.)
Sources of Internal Heat in Planets
1) Gravitational potential energy of accreting planetesimals
2) Differentiation
3) Radioactivity
Differentiation:
Why do water and oil separate?
A. Water molecules repel oil molecules electrically.
B. Water is denser than oil, so oil floats on water.
C. Oil is more slippery than water, so it slides to the surface of the water.
D. Oil molecules are bigger than the spaces between water molecules.
Differentiation:
Why do water and oil separate?
A. Water molecules repel oil molecules electrically.
B. Water is denser than oil, so oil floats on water.
C. Oil is more slippery than water, so it slides to the surface of the water.
D. Oil molecules are bigger than the spaces between water molecules.
Differentiation
• Gravity sorts materials by density.
• Higher density materials sink below lighter density materials.
• Differentiation converts gravitational potential energy to heat.
How do we know what is inside the Earth?
A. We can drill deep inside the Earth.
B. We can use optical fibers to see deep inside the Earth.
C. X­ray machines allow us to view the inside of the Earth.
D. Seismic waves generated by earthquakes probe the Earth’s interior.
How do we know what is inside the Earth?
A. We can drill deep inside the Earth.
B. We can use optical fibers to see deep inside the Earth.
C. X­ray machines allow us to view the inside of the Earth.
D. Seismic waves generated by earthquakes probe the Earth’s interior.
How do we know what’s inside a planet?
• P waves push matter back and forth.
• S waves shake matter side to side.
How do we know what’s inside a planet?
• P waves go through Earth’s core, but S waves do not.
• We conclude that Earth’s core must have a liquid outer layer.
Internal Structure, by density
Internal structure, by rock strength
The lithosphere is the cool rigid rock that forms a planet’s outer layer: the crust and some of the mantle. The earth is relatively hot, so its lithosphere is relatively thin.
The lithosphere floats on the lower layers.
The thickness of the lithosphere affects many geological processes
Internal Heat Drives Geological Activity
Convection: hot rock rises, cool rock falls (because hot rock is less dense, just like hot air is less dense)
1 convection cycle on Earth takes about 100 million years
A large planet…
• Is warm inside from formation, differentiation, and radioactivity
• Has a convecting mantle
• Has molten rock nearer the surface
• Has a thinner, weaker lithosphere
...which makes it more geologically active
The four processes that shape the surfaces of planets, including Earth
1)
2)
3)
4)
Impact cratering
Volcanism Tectonics
Erosion
#1: Impact Cratering
#1: Impact Cratering
• Moon must be hit as often as Earth.
• Where are Earth’s craters?
• Erased by volcanic activity and erosion.
The more craters, the older the surface
#2: Volcanism
Molten rock rises when it is:
• Less dense than its surroundings.
• Squeezed by its surroundings.
• Pushed by expanding trapped gas (water vapor, nitrogen, carbon dioxide, sulfurous gasses)
#2: Volcanism
• Erases other geological features.
• Provided gas for our atmosphere (outgassing)...
• ...including steam (water in gas form), which cooled and became the water in our oceans.
Why doesn’t Mars have as much volcanic activity as Earth?
A. It’s too far from the Sun, so it cooled off faster.
B. It’s smaller than the Earth, so it cooled off faster.
C. It might, we just haven’t seen them erupt yet.
Why doesn’t Mars have as much volcanic activity as Earth?
A. It’s too far from the Sun, so it cooled off faster. B. It’s smaller than the Earth, so it cooled off faster.
C. It might, we just haven't seen them erupt yet.
#3: Tectonics
Do rocks s­t­r­e­t­c­h?
A. No ­ rock is rigid and cannot deform without breaking.
B. Yes ­ but only if it is molten rock.
C. Yes ­ rock under strain may slowly deform.
#3: Tectonics
Do rocks s­t­r­e­t­c­h?
A. No ­ rock is rigid and cannot deform without breaking.
B. Yes ­ but only if it is molten rock.
C. Yes ­ rock under strain may slowly deform.
#3: Tectonics and Plate Tectonics Tectonics: any surface reshaping from forces on the lithosphere
Internal Heat Required!
Plate tectonics: pieces of lithosphere moving around
Only Earth has plate tectonics now, but Mars did in the past.
#4: Erosion
• Wearing down or building up of geological features by wind, water and ice (weather)
• Important on Earth ­ why?
What have we learned?
• Why is Earth geologically active?
• Internal heat drives geological activity, and Earth retains plenty of internal heat because of its relatively large size for a terrestrial world. This heat causes mantle convection and keeps Earth’s lithosphere thin, ensuring active surface geology. It also keeps part of Earth’s core melted, and the circulation of this molten metal creates Earth’s magnetic field. What have we learned?
• What processes shape Earth’s surface?
• The four major geological processes are impact cratering, volcanism, tectonics, and erosion. Earth has experienced many impacts, but most craters have been erased by other processes. We owe the existence of our atmosphere and oceans to volcanic outgassing. A special brand of tectonics—plate tectonics—shapes much of Earth’s surface. Ice, water, and wind drive rampant erosion on our planet.