Main Topics, Unit 5

Overview of the main topics you will encounter in Unit 5.

There are certain other forms of waste which could be entirely stopped—the waste of soil by washing, for instance, which is among the most dangerous of all wastes now in progress in the United States, is easily preventable, so that this present enormous loss of fertility is entirely unnecessary. The preservation or replacement of the forests is one of the most important means of preventing this loss."
—President Theodore Roosevelt, Seventh Annual Message to Congress, Dec. 3, 1907

The foggy drizzle of Redwood National Park may cause you to want a jacket for warmth as well as dryness, yet, above the clouds, Redwood gets about the same amount of sunshine as toasty Death Valley. Much of Death Valley's warmth can be traced to Redwood rain releasing heat that was stored during evaporation of water from the hot tropical ocean. Rain and other weather phenomena break down rocks, making soil, while washing some materials to the ocean to be used for shells or other things. Landslides, rivers, and other processes eventually remove soil from the land about as rapidly as the soil is produced, and deliver the soil to the ocean, where subduction and volcanoes recombine the soil, shells and other washed-away chemicals to make new rocks for weather to attack.

Weather, Weathering, and Landslides

• The Sun hits the equator just about straight-on, but gives the poles a glancing blow, so the equator gets more of the Sun's energy.
• The Sun heats the Earth, which drives convection in the atmosphere.
• This convection in the air, when combined with Earth's rotation, makes interesting winds, including onshore breezes on U.S. West Coast.
• These winds rise up the Coast Ranges and the Sierra Nevada, watering the redwoods and sequoias.
• These winds then sink down into Death Valley, drying it.

Why Redwood Wet, Death Valley Dry?

• Warmer air can hold more moisture.
• Rising air expands, which cools the air; sinking air is compressed, which warms the air.
• Evaporation requires heat (so you cool as your sweat evaporates, taking heat from you), and condensation releases heat.
• Air cools about 5^oF for 1000 foot rise, if water vapor is not condensing to form clouds.
• Air cools about 3^oF for 1000 foot rise with condensation (formation of clouds and then snow or rain).
• Air from the Pacific is "wet", carrying about as much vapor as it can.
• Cooling reduces how much vapor can be carried in air, so this air rains as it cools while rising above the redwoods.
• Dry air comes down the other side of the mountains into Death Valley.
• The air cools 3^oF for 1000 foot rise going up, warms 5^oF for 1000 foot fall coming down, and must go up about 15,000 feet to get over mountains that reach above 14,000 feet, so the air comes down about 30^oF warmer than it went up.
• This is a main reason Death Valley is hotter than Redwood; the lack of clouds over Death Valley also allows more warming from the Sun.
• The energy that warms the air 30^oF going from Redwood to Death Valley was stored in the air when water vapor evaporated from the ocean, and released to warm the air when condensation made the rain for Redwood.

Rocks Are Not Forever

• "Weathering" includes the physical changes that make small rock pieces from big ones, and the chemical changes that make new minerals.
• Physical weathering is caused by crystal growth in cracks (especially ice), and other processes.
• Granite is a common rock composed of quartz (silicon+oxygen, sometimes called silica), feldspar (which is silica+aluminum+(calcium or sodium or potassium), and a dark mineral (which is silica+iron+magnesium).
• Chemical weathering leaves the quartz as quartz sand with little change, changes the feldspar to clay (silica+aluminum+potassium) while the calcium or sodium wash away, and rusts the iron of the dark mineral while the magnesium and silica wash away.
• The "chunks"—rust+sand+clay—plus worm poop make soil.
• The calcium and silica go to make shells in the ocean.
• The magnesium reacts with hot sea-floor rocks to make new minerals there.
• The sodium makes the ocean salty.
• The soil eventually is washed to ocean.
• Subduction takes seawater, sediment, shells, soils, and the sea floor with its new minerals down to melt; the melt rises and solidifies as granite (or andesite, if it erupts), in a nearly balanced cycle.

Mass Movement

• Mass movement is the downhill transport of soil and rock without rivers or glaciers or wind.
• Mass movement ranges from huge, destructive landslides to barely measurable soil creep.
• Rivers usually pick up and carry away the material delivered by mass movement.
• In most places and times, there is a natural balance between soil production by weathering and soil removal by mass movement or other processes.
• Humans are upsetting this balance in many places, typically making soil removal much faster than soil formation, so that soil is getting thinner.
• We often can figure out where mass movement is potentially destructive, and stabilize slopes or stay out of the way.