NEASON
Ch. 19 Notes
Section 1: Weathering

Weathering is the breakdown of rock into smaller and smaller pieces.
o
2 Types of weathering
1.
Mechanical weathering- the breakdown of rock into smaller pieces by physical means (ice,
wind, water, gravity, plants, and animals)

Ice-water expands when it freezes; water gets into cracks and freezes during cold
weather, which pushes against the sides of cracks and makes the cracks wider—
called ice wedging

Wind-when wind blows sand against rock, the sand slowly wears away the rock’s
surface; this is called abrasion
o
Abrasion-the grinding and wearing down of rock surfaces by other rock or
sand particles

Water-in rivers, rocks and pebbles bump into and scrape against each other;
abrasion

Gravity- when rocks fall, they grind against each other as they slide down a
mountain; abrasion

Plants- send their roots into cracks of rocks, when the root grows the cracks get
larger and can eventually split entire rocks

Animals- when animals dig or move soil, fresh surfaces are exposed to weathering;
examples-earthworms moving soil, humans hiking, fishing, and mining
o
Mixing and digging by animals often contributes to chemical weathering, as
well.
2. Chemical Weathering-the chemical breakdown of rocks and minerals into new substances
(water, weak acids, and air)

Water-dissolves rock; rocks dissolve at different rates

Acids ( in precipitation, ground water, and living things)
o
Precipitation-rain and snow is naturally acidic with carbonic acid; other
natural sources, like volcanoes, can increase the acid by contributing sulfuric
and nitric acids; example-statue damaged
o
Ground water-has weak acids like carbonic and sulfuric; as ground water runs
through rock it dissolves the rock; example-Carlsbad Caverns
o
Living things-plants and animals produce organic acids through normal bodily
processes; example-lichens break down rock by producing organic acids

Air-oxidation is a chemical reaction in which an element, such as iron, combines with
oxygen to form an oxide; example-rust
Section 3: From Bedrock to Soil
Soil is a loose mixture of small mineral fragments and organic material.

Sources of Soil and Soil Layers
NEASON
o
Soil depends on the source, so not all soil is the same. The type of soil that is formed depends on
the type of rock that weathers or the parent rock.

The layer of rock beneath the soil is bedrock.

Humus is the organic part of soil that is made of decayed plant and animal material. It
contains essential nutrients for plant growth.

Different layers of soil are known as horizons

Top layer of soil is called topsoil.

This layer has the most humus and is rich in nutrients that plants need to be
healthy.


Good topsoil is essential for farming.

It is in limited supply because it can take hundreds or thousands of years to form.
Climate affects soil.
o
It can have a big effect on what type of soil develops.
o
Tropical climate topsoil is very thin because of the large amount of rain. Water moves through the
soil and leaches the material from the topsoil. This leaves the tropical topsoil very thin.

Plants in the tropical topsoil keep rain from eroding it away and the plants depend on the
thin topsoil because the subsoil will not support lush plant growth.
o
o

Farming and mining disrupt this balance and expose topsoil to erosion.

Once the topsoil is gone, the plants will not return and no more topsoil will be made.
Temperate climate areas get just the right amount of rain to have very rich soils.

Chemical weathering also occurs at a high level.

Midwestern part of US is commonly known as the “breadbasket” for this reason
Desert and Arctic climates lack the proper amount of rain to have good soil.

Often salty.

Low rates of chemical weathering. (soil forms slowly)
Section 4: Soil conservation
Soil conservation is the way that people take care of the soil.

Soil holds water.

Provides homes for animals.

Soil provides minerals and nutrients that are essential for growing plants. However, poor agricultural
practices often cause rich soil to lose its nutrients
o
Ways to prevent soil erosion:

Cover crop-crop planted between harvests to reduce soil erosion and to replace nutrients.
Ex-soybean

Crop rotation-changing crops from year to year to keep soils nutrient-rich

Contour plowing-makes rows act like a series of dams instead of a series of rivers

Terracing-changes one steep field into a series of smaller flat fields