April 23- May 4th
Products of Weathering - Sediment, Soils, and Dissolved Minerals
Now that you are familiar with HOW rocks and minerals on Earth surface are broken down, it is time to learn what is left over when that happens? Imagine a planet with NO weathering at all. The entire surface would be solid rock, mountains or fresh lava flows, but no loose material.
The products of weathering are:
- Dissolved Minerals in water (like NaCl, or salt, in the oceans. This is just the most famous and probably most notable dissolved substance in water, but there are others like Iron, Magnesium, and sulfur, some of which combine to form minerals when water evaporates like Gypsum and Talc.
Of these three, it is my opinion that Soil is the most important result. Without soil, trees, grasslands and most importantly cropland and agriculture could not take place on Earth's surface, which would make life on land much more difficult.
It will take hundreds of years to form a few inches of soil on this fresh lava flow in Hawaii. First acid rain (chemical weathering) will dissolve some of the minerals on the surface. As you see here, roots of vegetation will help break it up along with exfoliation of warming and cooling during day and night. Leaves and animals will fall on the surface, decay and add what we call humus (picture material from a compost pile) to the soil.
Some places on Earth like the Mississippi River Valley and areas of Texas, Oklahoma, and other plains states have hundreds to a few thousands of feet of soil beneath them before getting to solid bedrock. Around here in WNY, most areas if you dig down 10 or 15 feet, perhaps 20 feet you will hit shale bedrock. So we have about average thickness soil, but its only been developing for about 10 thousand years (short geologically) because when the glaciers came through in the last ice age, they bulldozed the soil we had before away and scraped down to bare rock. A lot of the material that we have now in fact, was left behind when those glaciers melted. Kind of like having a pile of stones or gravel left after a snowbank melts away.
The rest of this lesson will focus on sediment first, the next one will be on soils. The dissolved minerals part you will not do.
Watch the following video and observe and think about these questions as you watch. Read the questions both before and after you watch the video.
What are the largest sizes of sediment (rock pieces) that you see?
What size are the particles that make the water look muddy and brown?
When do you think a tiny particle, smaller than a sand grain will stop moving in the river?
What would happen to a large sediment piece, the size of a car, if you dropped it in the middle of the river?
All of these questions were to get you thinking about sediment. The following chart shows you sediment sizes (it should look familiar from the section on sedimentary rocks earlier in the year.
This photo includes most of the sediment sizes in the charts above. Without using a ruler, you should be able to see tiny little piles of clay or silt, sand, and then certainly a lot of pebbles and a few cobbles too.
So at this point, you should just understand, that most surface rock on Earth, is broken rock that we call sediment. There are a few places like Hawaii with fresh lava flows or Mountain Chains such as the Alps with the Matterhorn that is still solid rock, but most rock has been split and weathered into sediment. In our next section (after soils), we will learn about erosion and deposition, which is the movement of that sediment around the surface of Earth.
Soil is the most important result of weathering on Earth's surface. As solid rock and mineral is broken into pieces, it mixes with decaying plant and animal material like leaves, twigs, logs, animal carcasses, animal feces, insects, and other materials to turn into what we call soil. It is in this soil, that seeds can germinate, plants and trees can grow, and forests and grasslands can be established. Please watch the following video on Soils and there importance.
Layers of the Soil Profile.
These are the layers that are present in a very well developed, old soil.
Photo of what a real Soil Profile looks like
Humans often take soil for granted, but it is critical for agriculture (farmland) and forests to thrive. We have learned over the past few hundred years, how to conserve soil, as it can wash away by floods or blow away in the wind, like happened during the dust bowl in the plains states during the 1930's. This brings us to our next chapter: Erosion, Deposition and Landscapes.