GeologyEssential Knowledge for Naturalists
How Rocks relate to lifeEarth, the rock we all call home, is essentially a mass of rock, and soil, mostly covered with water, and a thin veil of air known as the atmosphere. Our rock is not just a consolidated ball of one material. It has layers, somewhat like an onion, but not as easy to peel.
The layers of Earth
The first layer of the Earth, the center of the onion, is the Core. The core is approximately 1600 miles thick, and is composed of two layers of its own. The inner core is composed of a ball of solid iron. Around this block of iron is the outer core, a thick layer of molten iron and nickel. It is this liquid outer core that we believe is responsible for Earth's magnetic fields that allow a compass to point us in the right direction.
The next layer of the Earth is known as the Mantle. The mantle is approximately 1300 miles thick. It is believed to be a thick soup of molten iron, magnesium and silicates. The top layer of the Earth essentially floats on this liquid middle layer.
The top layer of the Earth is known as the Crust. This layer is anywhere from 2 to 32 miles thick. The thinner parts of the crust are under the oceans, while the thickest are the portions under the continents. This portion of the Earth is where we humans spend our time, and where the results of turmoil below, such as convection currents in the mantle, are apparent to the human eye. Functionally, the crust, and an area of the mantle just below the crust, is known as the lithosphere. The lithosphere, like the Earth itself, is not just a homogeneous rock. It has its own dynamics, composition and structure. The predominant factor in the lithosphere is the existence and movement of the seven continental plates and a number of minor plates as well. The study of these plates is called plate tectonics.
The movement of these plates is responsible for earthquakes, volcanic activity, and the uplift of mountain chains. In areas under the oceans, the floor of the ocean spreads from new materials being pushed up from the mantle. Because the Earth's crust is a rigid body made mostly of granite and other fairly incompressible materials, in order to make room for this new material, something has to break, shift, lift or drop. Thus, the shifting and dipping of fault lines and uplift of mountain chains.
Upon this shifting crust, humans, plants and animals live. The land in the lithosphere upon which we lead our lives is layered as well as segmented by plates. The bottom layer is bedrock. The bedrock is what is left if you were to remove all of the soil from a region. It is the endoskeleton of the landscape. To a large extent, bedrock structure and form determine the shape of the landscape we see. Bedrock is not consistent in its composition. In some areas it may be granite, in others sandstone, limestone, or shale, for example.
There are three main types of rocks. Igneous rocks are those formed directly from molten lava. These are rocks such as granite, quartz, obsidian, feldspar and basalt.
Sedimentary rocks are those formed by the deposition and cementation of eroded rock particles, sand, and organic materials into a new solid amalgamation. These are stones such as limestone, sandstone, and shale. Their various properties depend on the size and origin of the parent materials, and the method of deposition.
Metamorphic rocks are those formed from sedimentary and igneous rocks, as well as other metamorphic rocks, under extreme pressure and heat. Stone such as slate, gneiss, and marble are examples of metamorphic rock.
Beyond being the underlying structure and support for the landscape, bedrock is a main source of material for soils. Wind and water break up stones into smaller and smaller fragments, which collect and become intermixed with organic materials to form soils.
Soil Formation and structureSoil is formed from a combination of parent material which is derived from underlying bedrock, and organic material from the plants and animals of an area. Climate plays a large role in soil formation, because weathering of rock is accelerated or slowed by the temperature, rainfall and wind of a particular area. These factors also dictate whether a forest grows in an area, or whether it will be a desert. Soil holds moisture, provides nutrients, and protects the roots of plants. The role of soil in plant, and therefore, animal, community development is far-reaching and important. Erosion is a key factor in soil formation, and in many other natural processes.
Soil's influence on lifeEdaphology is the study of soil and its influence on life. The moisture and nutrient contents of soil are the main determinants of the kinds of plants that will thrive in the area. The pH of the soil, whether it is acid, basic, or neutral, also plays a role in this determination. Soils are made up of four major components. Based on particle size the components can be classified as sand, silt or clay. The remaining major component of soil is organic matter.
The top layer of the soil is known as topsoil. This is the darker, richer soil in which plants grow, and what is most apparent to the naturalist if he or she were to pick up a handful of soil and examine it. The qualities of topsoil are much more important to plant community development than the subsoil. However, subsoil conditions can greatly influence topsoil conditions. For example, clay subsoil may cause an otherwise well-drained topsoil to develop hydric properties, thereby facilitating development of a wetland plant community.
The clay, silt, and sand content determines the texture of the soil, and thus the capacity of the soil to hold or drain water. This, of course, directly relates to the available moisture for plants to grow. It also determines whether there is oxygen available in the soil, or whether anaerobic conditions will prevail.
Coarse soils, containing mostly sand, do not hold water, unless there is a clay layer below the sand. Because sandy soils are coarse, there are large spaces between each grain of sand, and water flows through quickly. This may also allow nutrients to be leached from the soil quickly, resulting in dry, low-nutrient conditions. If your soil feels gritty when you pick it up, and it falls apart easily, you likely have soil with a high sand content.
After sand, the next smallest particles in soil are known as silt. Silt particles, being smaller than sand, tend to mass closer together, and thus, water moves more slowly though silt soils. Therefore, they hold more water, drain more slowly, and tend to be more fertile than sandy soils. If you soil feels smooth, and will not hold together if you try to form it into a ribbon between your thumb and forefinger, you probably have more of a silty soil.
Clay is the smallest particle in soil. Clay, being so small, clings tightly together with very little space between particles for water or air. Pure clay is impermeable to water. Soils with a high clay content, therefore, are subject to poor drainage, which limits the types of plants growing in that soil to those which can stand high moisture content, and dense soil conditions. If your soil is sticky and tends to be clumpy, you have high clay content. Clay soils will hold together fairly well when trying to form them into a ribbon between your thumb and forefinger.
It is generally the case that you will find a mixture of sand, silt, and clay particles, rather than each occurring separately. A mixture of approximately even proportions of sand, silt and clay is known as loam. Loamy soils tend to be fairly loose, yet able to hold a fair amount of moisture, which is useful to plants trying to grow. Because loam soils contain coarse and fine materials, nutrients are not leached as readily from the soil, and so the nutrient content of loams tends to be higher.
Organic content, such as decaying leaves, or animal residues, provide nutrients, as well as structural diversity to soil. Soils with high amounts of organic content are more sponge-like, retaining water, but also draining excess water. Nutrient content of these soils is also high.
The components of soil comes from a variety of sources. It can be blown in on the wind, it can result from local erosion, it can be dropped by floodwaters, glaciers can deposit the soil components as they recede, and the plants that grow on the soil can contribute to its development over time. The physical content of the soil sets the stage for the plant communities that may thrive upon that soil, and determines the ability of the soil to drain or hold water. However, that is not the end of the story.
The landscape position of the soil determines nutrient content (because at a lower level on the hill, more of what is above the soil can contribute, and more from nearby streams can flood in and add nutrients as well) and moisture content. For example, soils on north facing slopes tend to have more moisture content than a similar soil on a south facing slope, because the north facing slope is shaded, whereas the south-facing slope gets more sunlight, which in turn dries the soil. North-facing slopes, hence, tend to be more mesic (neither dry nor wet), whereas south-facing slopes have a tendency to be more xeric (dry).
Hydric conditions, where there is a great abundance of moisture, tend to appear based on topography, rather than landscape position. For example, a slope with a flat area of clay soils, whether north or south-facing, may develop into a perched wetland (hydric soil conditions) if rainfall sufficient to keep the soil saturated.
This is all important, because certain plants grow in certain conditions. For example, pin oak trees (Quercus palustris) tend to grow best in hydric conditions, whereas, northern red oak (Quercus rubrum) tends to favor more mesic to xeric sites. You can predict the plants you will see by the soils, and you know much about the soils by observing the plants. Like everything else in Nature and in Life, one aspect is connected to all other aspects. Learn more about Plants in Nature by exploring the rest of our web pages.