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Tomatoes 2008
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How Rocks are Formed There are three major ways
that rocks are formed. The first of these is from heat. They are called igneous
rocks. They are the volcanic rocks, among which are obsidian, and granite. Both
obsidian and granite contain the same minerals but their different appearances
are caused by differences in the speed of their cooling. Obsidian cooled very
rapidly and has a glassy texture. In fact, it is a glass and amorphous or
non-crystalline. Granite cooled very slowly and the separate minerals
crystallized into larger crystal. The three major minerals of granite and
obsidian are quartz, feldspar, and mica. The third method of rock formation is called metamorphic. The name means changed form. Three factors can cause the changing form. They are Heat, Compression or Pressure, and Infiltration of solutions containing other minerals such as Quartz, Calcite, and other minerals. Examples of metamorphic rocks are marble (changed by heat and pressure from limestone) and petrified wood (changed by infiltration of quartz). All soils are formed from
rock at some time or another. The factors that cause soil formation are as shown
in the chart below.
Soil Components As rocks are decomposed from the action of wind, rain, freezing, cooling and the action of plants and animals as shown in the above chart the resulting debris forms soil. The mineral components of rocks become the mineral components of the soil. Some of the minerals further degrade into their constituent minerals. These minerals are then further broken down by soil organism into forms that can be used by plants. Since all of the rocks of the earth's crust were at one time igneous rocks we shall look first at the major minerals which form the crust. Minerals from Granite and other igneous rocks
In addition to the
minerals, soil is composed of the following: Moh’s Scale of Hardness Each mineral has its own characteristics such as color, crystal form, hardness, solubility etc. To understand the way minerals and rock break down into smaller particles to form clay and sand we need to know which minerals will break down more quickly. To help in understanding this, Moh devised the following scale of hardness. Diamond is the hardest of all natural minerals and is pure carbon. It will scratch all other minerals and is scratched by none. Thus it is a good gem material when it is relatively free of inclusions, fractures and foreign matter. Its hardness also makes it valuable industrial material. The other minerals on Moh’s ten point scale are as follows:
Virtually all continental sands are composed of quartz because at a hardness of 7 it is harder than most of the other minerals that are common in igneous rocks. The very hard minerals such as diamond, ruby and topaz are relatively rare and because of their weight are usually deposited at depths. Gold, another rare minerals, is also deposited at lower levels of sediments because it is very heavy. It can be found in some sands in the Rio Grande which are underlaid by bedrock. Panning for gold has been and still is a popular recreation. Evolution of Rio Grande Valley Evidence from the top of the Sandia mountains shows that at one time where Albuquerque now is, there was a shallow sea. It was inhabited by crustaceans whose shells were deposited in the bottom of that sea. Over time that layer of calcium rich material grew thick and formed limestone which you can now see as a ribbon of lighter rock across the top of the Sandias. In many places fossil shells can be found on the surfaces of this limestone formation. These rocks then were lifted up slowly by the upward movement of the continental plate and as a result, the water of the inland sea drained away to southeast. This period of mountain building lifted up the Sandia mountains and caused a fault or break in the earth’s crust to form from north to south. The land on the east was uplifted and the land on the west was depressed.. The amount of uplift on the east was equal to the amount of depression on the west. The Sandias are thus block fault mountains. Erosion filled the valley with alluvium which is the debris from the rock decomposition. As the mountains were eroded cirques formed. A cirgue is caused by the progressive erosion of the steeper sections of the mountains. It is usually semi-circular in form and as more erosion occurs eventually adjacent cirques form ridges extending at right angles to the mountain chain. At the bottom of the cirque alluvial fans developed extending out into the valley. These fans have rather deep soils that in the steeper portions will be rocky or gravely and as the water carries the soils down to lower levels and the speed of the water slows the sands are deposited. On the level areas then the clays are the last to be deposited in the river valley. Meanwhile, volcanism formed the west mesa. To the west other mountains were breaking down and the westerly winds caused progressive layers of sands to build up the land. Lava flows from the volcanoes spilled out to cover those sands. The prevailing winds brought in more sand which covered the lava in some places and more lava flowed out to again cover the sand. Since the lava flows are relatively recent, the layers of sand above the lava flows are usually rather thin. Water from the valley dissolved calcium carbonate from the limestone that was at one time on the surface but which was now deeply buried. Evaporation of that water on the surface brought more water from deep below toward the surface where evaporation caused the sands, and other soils to be cemented together with the calcium carbonate. This caused the soils to be very alkaline. The arid environment did not allow much vegetation to grow but that which did decomposed into humus. The very low humus levels then made most soils except in rare sites in the valley to be very short of nitrogen. Meanwhile the minerals that formed the soil continued to decompose and as they did so, abundant sources of potassium and phosphate were created. These minerals accumulated because the low rainfall did not allow them to be leached away. The low rainfall also allowed deposits of caliche or calcium carbonate and gypsum or calcium sulfate to accumulate. White mountain which is just south of San Ysidro on highway 14 is almost pure gypsum. It has been mined to make plaster for many years. Additional deposits of gypsum occur to the east of White mountain and also at White Sands National Monument. In Tijeras, the mountains are being mined to make cement from the limestone. Human intervention has subsequently altered the contours of the land and also altered the composition of the soil. In the short time that humans have occupied the valley, water courses have been altered. Soils have been extensively amended. Some areas of Albuquerque have been filled in an attempt to prevent water from creating streams and arroyos. This is usually unsuccessful in the long run. For example, should we have a 100 year rain in the water shed that runs through Winrock Center it is possible that the shopping center would be filled for a short period with six feet or more of moving water. This, understandably, would cause a great deal of damage to the shopping center. The texture of soil is determined by particle size and shape. Sandy soils have rounded or irregular particles that are usually mostly quartz. Gravels may be rounded or irregular depending on whether the particles have been moved by water or not. Clay particles are very small and usually flat, platy or flaky so that they can be very closely compacted. They are usually secondary silicate minerals. Organic soils are dark and light in weight. The accumulation of any soil is determined by its position on a slope. Soil Particle Size in Relation to Deposition on a Slope and/or Water Velocity
On the west mesa the Calabacillas arroyo drains the water from a very large acreage of land to the west. In heavy rainstorms that usually occur in the summer monsoon season, huge auto size boulders sometimes can be seen being rolled down the arroyo by the force of the water as if they were mere pebbles. In an attempt to control the water, dams and retention ponds have been built on both the east and west mesas. These then alter the water flow and the soils around them. Near the river where the water flows very slowly, in such places as Corrales, there are great quantities of silt and clay. Methods of gardening in Albuquerque thus vary a great deal depending on the soils, the water flow and similar factors. |
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Go to nmmastergardeners.org/ to find these and other articles in a pdf. format that you may use to print out a book with much of this material from that web site. Those articles, however, may not have been modified since they were originally printed in 2001.
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