Class Session 19>

I. Land

The earth is old. Scientists estimate that the earth is between 4.6 and 4.8 billion years old. It's hard for us to understand or imagine how old this is. If you were to begin counting at the rate of one number per second and continued 24 hours a day, 7 days a week and never stopped, it would take 2 lifetimes, about 150 years to reach 4.6 billion.

Look at it another way. Take 4.5 billion geologic years and compress them into 1 year. On that scale:

· the oldest rocks we know date from mid-March;

· living things first appeared in the sea in May;

· land plants and animals emerged in late November;

· the swamps that formed Pennsylvanian coal flourished for 4 days in early December;

· dinosaurs became dominant in mid-December, but disappeared 1 day after Christmas;

· about the same time, the Rocky Mountains were first uplifted;

· humanlike creatures appeared sometime during the evening of December 31;

· ice receded from the Great Lakes & Northern Europe 1 minute to midnight on the 31st;

· Rome ruled the Western world for 5 seconds from 11:59:45 to 11:59:50, and;

· Columbus discovered America just 3 seconds before midnight.

In this week’s notes we’ll look at land as a resource. To begin this week’s notes, let’s start down under, way down under in the center of the earth and gradually work our way back to the surface. The distance from the earth's center to the outer surface is 3,963 miles. Digging a hole to China may be tough; the interior of the earth consists of:

1. Inner core - solid center - 763 miles in depth;

2. Outer core - liquid - 1400 miles in depth;

3. Mantle - solid rocky layer - 1800 miles in depth, and;

4. Crust - 3 to 40 miles in depth.

The earth's crust and the upper section of the mantle make up a zone called the lithosphere. The lithosphere is a solid rocky layer. Just below the lithosphere is a layer with rocks that are slightly gelatinous in nature. These rocks have partially melted from great pressure and temperature. This layer is called the asthenosphere. This gelatinous material which makes up the asthenosphere is called magma. The lithosphere, being a solid, rocky layer tends to slide or slip over the asthenosphere below. Occasionally, cracks form in the lithosphere and molten material, or magma, from the asthenosphere rises to the earth's surface. Once on the surface, this molten material, which we call lava, cools and solidifies into rock.

II. Scratching the Surface
The surface of the earth is made up of rigid individual segments called tectonic plates. These plates slide or slip over the asthenosphere. There are about 20 major plates, including the Pacific Plate, Eurasian Plate, African Plate, American Plate, Antarctic Plate, and Australian Plate. There are also several minor plates, including the Nazca Plate, Cocos Plate, Caribbean Plate, Philippine Plate, Javan Defica Plate, and Scotia Plate.

Each plate moves as a different unit in relation to other plates at very slow speeds, about 1 to 10 centimeters per year. Some plates are moving apart from one another and the zone between them is known as a divergent boundary. Some plates are moving towards one another. The area between them is called a convergent boundary. Lastly, some plates are sliding laterally past one another. The boundary of these plates is known as a transform fault boundary. Since they move as distinct units, all the major interactions between plates occur along plate boundaries. Not surprisingly, most of the earth's seismic activity, volcanism, and mountain building occurs along these plate boundaries. Some plates are entirely underwater, such as the Nazca Plate. Some plates are composed of both areas under water and areas above water.

III. The Continents

The parts of the plates that are above water are called continents. The earth's major continents, Africa, Asia, Europe, North America, South America, Australia, and Antarctica are imbedded on these plates. North America is imbedded into the North American plate and Europe and Asia are joined together on the Eurasian plate.

At the center of the earth's continents is a solid, coherent aggregate of one or more types of mineral particles. Minerals are naturally formed inorganic solid compounds or elements with a specific chemical composition and characteristic crystal structure. Though there are some 3,500 known minerals with new ones still being discovered, about 95% of the earth's crust is composed of fewer than twenty minerals. The word aggregate implies a mixture in which the properties of the individual minerals are retained. These solid aggregates are known as rocks and the solid rock material at the base of plates is called bedrock.

On top of bedrock are rock fragments, some small and some large, which have broken away from bedrock by wind, water, rain, waves, ice, frost or any type of tectonic activity, or the action of roots. These fragments are called regolith.

The final layer closest to the surface, which sits on top of regolith and consists of tiny fragments of rock, is soil. It is important to note that this is not always a three layer process. Sometimes the bedrock is exposed and this exposed bedrock is known as an outcrop.

IV. Rock On

Rocks are solid inorganic aggregate mixtures of minerals, "mutts" in dog terms. The aggregate composition of rocks is usually visually apparent as many rocks appear striped or speckled with differing solid material. These different solid materials are actually different minerals.

There are 3 types of rock:

1. Igneous - formed when lava from asthenosphere cools and solidifies. Mica, quartz, obsidian and granite are all igneous rocks.

2. Sedimentary - made up particles of gravel, sand, silt and clay that were eroded from existing rocks. As more sediment accumulates, compression occurs and the chemical action of water and certain minerals causes sedimentary rock to form. Sedimentary rocks tend be found near oceans, marshes, lakes, and tidal basins because water plays an important role in transporting the individual rock sediments which later become sedimentary rock. Limestone, sandstone, and shale are all sedimentary rocks.

3. Metamorphic - formed when igneous and sedimentary rocks are changed by heat, pressure or chemical reaction into a new mineral structure. This heat or pressure is caused by the movement and collision of lithospheric plates. Limestone, for example, under certain conditions becomes marble.

Rocks are continually being created and destroyed, although it takes a long time for these processes to occur. Generally, the rock cycle consists of the hardening of magma or lava from the asthenosphere into igneous rock. Igneous rock is weathered and eroded and, over time, turns into sedimentary rock. As sedimentary rock is thrust downward into the asthenosphere by tectonic activity, it turns into metamorphic rock. Eventually, metamorphic rock is pushed down further into the earth and turns back into magma. So, you see that nothing on the earth, even rocks, are permanent.

V. The Dirt on Soil

Sitting above regolith on the surface of the earth is soil. Soil is the bridge between the inanimate world and life and where all the spheres - lithosphere, atmosphere, hydrosphere and biosphere come together. When we think of soil, we tend to think of little fragments of rock, but soil is really much more than this. Soil is composed of:

1. inorganic materials - insoluble mineral material, little pieces of rock

2. water -the origin of which is precipitation

3. air -in some cases as much as 50% of voids between individual particles is air

4. organic matter - decayed remains of plant and animal material - humus

There are four principal characteristics of soil. These include:

1. Color - The color of soil ranges from almost black to almost white and includes yellowish, orange, and reddish. A dark brown or blackish soil indicates high level of humus or organic matter. A red or yellow soil is an indication of a high presence of minerals such as iron and a white soil indicates a high proportion of salt or sand.

2. Texture - The texture of soil varies according to the size of particles. Clay soils have the smallest particle size, with diameters less than .002 mm. Silt has the next larger soil size, with particle diameter between .002 and .05 mm. The next size up is sandy soils with a diameter between .05 to 2.0 mm. Once you get above 2.0 mm, you no longer have soil but gravel. One last important soil type is loam in which no particular particle size dominates.

3. Structure - Structure refers to how the individual particles of soil clump together. These clumps are called peds and there are two important characteristics of peds, porosity, which is an indication of the amount of airspace between soil particles and permeability, which refers to the ability of water and other fluids to pass through soil. Generally, the smaller the particle size, the more tightly compacted the soil is and the less porous and permeable the soil is. Clay soils, for example, have very little air space between the particles and water tends to runoff, rather than through, clay soils.

4. Acidity and alkalinity - The soils acidity is expressed as the degree to which a soil departs from chemical neutrality either towards acidity or alkalinity. Acidity is an important characteristic in determining soil fertility. A soil that is alkaline may not break down organic matter fast enough for nutrients to be absorbed by plants. A soil that is too acidic, on the other hand, may decompose nutrients too quickly. Most plants tend to thrive in soils that are slightly acidic - just enough, but not too much or too little. The pH scale is used to measure acidity.

There are five factors that affect the formation and development of soil. They include:

1. The type of inorganic, solid, rock & mineral fragments known as parent material. The older a soil is, the less it resembles its parent material because the greater the length of time that other factors have to affect its development.

2. Climatic factors such as temperature and precipitation. The warmer the temperature, the more biological decomposition that takes place. Generally, the more rain a place receives the greater the amount of nutrients and chemicals are found throughout the soil, though it's possible to have too much micro organic activity. Tropical areas often have an overabundance of micro organic activity, resulting in very little accumulation of humus. On the other extreme, soils near the poles have little micro organic activity, resulting in layers of undecomposed organic materials. In the mid latitudes, the climate slows microorganisms enough to allow accumulation of rich layers of organic matter and humus. Not surprisingly, the richest soils, and the best farmland, are found in the mid-latitudes.

3. The topography where the soil is found. Soils found on hillsides tend to erode and are often thinner and less fertile than soil found on flat land. Soils with poor drainage may be waterlogged. Slope orientation is also important. South facing slopes in the northern hemisphere are warmer and the reverse is true in the southern hemisphere. The warmer the soil, the more micro organic activity that takes place.

4. The amount of biological materials or biomass. Generally the more biological activity that takes place in or on soils, the more fertile they are as living organisms go through the eventual processes of death and decay. 5) The age of the soil. The older a soil is the more opportunity for factors other than parent material have to affect the development of the soil. If you have soil in your yard, don't hold your breath and wait for it to age. Generally, it takes hundreds of thousand years for a mature soil to develop.