Physical and Chemical Weathering of Rock Weathering is defined as the chemical and physical processes that change the characteristics of rocks on the Earth’s surface.

Physical WEathering Physical weathering occurs when rocks are broken in to smaller pieces without changing the chemical composition of the rock. Think of a physical change (e.g., ripping a piece of paper) where the sample will change in size but all its other characteristics will remain the same

Physical Weathering Examples of physical weathering are: Frost Wedging Exfoliation Abrasion Biological Activity

Examples of Physical Weathering Frost action/ice wedging is the breakup of rock caused by the freezing and thawing (contracting and expansion) of water. Water can seep into the cracks of a rock and as the climate cools the water freezes and expands breaking the rock apart. A very similar process occurs on roads, which causes potholes.

Exfoliation is an example of Physical (Mechanical) Weathering Exfoliation is the peeling away of large sheets of loosened materials at the surface of a rock. Common in shale, slate, and mica. Rocks formed deep in the Earth are made under high pressure. When the pressure is released the rocks expand & crack. May also be caused by alternate heating and cooling of rocks by weather conditions.

In the rock below, layers have peeled from the outside caused by exfoliation

Abrasion as a form of physical weathering Abrasion is the physical wearing down of rocks as they rub or bounce against each other. This process is most common in windy areas, under glaciers, or in stream channels

Biological Activity: Physical Weathering by Plants Tiny root hairs seek out small cracks and pits in rock. Once the root hairs find a place they grow and expand. The expansion causes great pressure and cracks the rock.

Biological Activity: Moss and lichen (the light green patches) Moss helps to decompose rock into the sand that makes up part of soil through the action of the water it absorbs. Lichens, through the action of acid that it uses to create tiny holes for gripping, also act as decomposers of rocks.

Biological Activity: Burrowing Animals Prairie Dogs Burrow through rock, creating passage ways for water to enter. Other organisms that burrow are earthworms, ants, woodchucks and rabbits.

Chemical Weathering Chemical weathering is where the rock material is changed into another substance by reacting with a chemical. Some examples of Chemical weathering are: Oxidation Hydration Reaction to acid (Carbonation)

Oxidation Oxygen in the atmosphere chemically reacts with minerals. ex.: rusting of a nail Oxygen reacts with iron and causes iron oxide (rust) to form

Hydration When minerals absorb water

Carbonation When water is mixed with carbon dioxide it forms carbonic acid. This acid when it mixes with limestone will cause a chemical change that leads to the formation of caves. This process takes thousands of years to occur.

A cave formed by carbonation

Carbonation

Rates of weathering will be influenced by... Surface area exposed Chemical composition of the rock Most rocks are made of minerals (inorganic substances) One of the physical characteristics of minerals is hardness. The harder the mineral the more resistant it is to any form of weathering. Climate Conditions Particle Size

Exposed Surface Area Surface area exposed - weathering occurs on the surface. More surface exposed, the faster the weathering will occur.

Mineral Composition of Rock Mineral composition- some minerals are more resistant than others. ex.:Quartz is resistant to chemical and physical weathering In the diagram below the different layers of the rock are weathered at different rates depending on the minerals within the rock. The layers sticking up most are the most resistant

Resistance to Weathering Not only is quartz the most stable of the common rock forming minerals in chemical weathering, its high hardness and lack of cleavage make it quite resistant to mechanical weathering. Quartz is itself an agent of mechanical weathering in the form of blowing dessert sand.

Climate Cold and/or dry climates favor physical weathering. Warm and wet climates favor chemical weathering. Frost action works best in areas where the temperature fluctuates wildly.

Particle Size When rock particles are smaller, the total surface area per unit volume exposed to weathering is greater. For example, granulated sugar dissolves faster than a sugar cube.

Soil - The product of weathering Soil is made from rocks, minerals (mainly sand and clay), and organic material (regolith and organic matter) Soil forms layers of different characteristics called horizons.

What is Soil? Soil is a mixture of weathered rock and the remains of living organisms where plants can grow. Soil formed at any location depends on: the composition of local bedrock climate time for development

How does climate affect weathering? Warm and moist climates favor chemical weathering Cold and dry climates weather physical weathering

How are residual and transported soil different? Residual soil formed from rock that originated in that particular area and was weathered. Transported soil formed from rock that weathered some place other than where it is found. True fact: Most of NYS is made from sediment transported by glaciers from Canada.