Weathering Notes. When the mineral composition of the rock is changed

Size: px

Start display at page:

Download "Weathering Notes. When the mineral composition of the rock is changed"

Jocelin Lester
6 years ago
Views:

1 Weathering Notes Weathering Weathering is the breaking down on rocks into smaller pieces Two types of weathering o Physical (mechanical) When the mineral composition of the rock isn t changed Ex: Smashing a rock into two pieces o Chemical When the mineral composition of the rock is changed Ex: When a rock rusts Physical Weathering Frost Wedging o When water seeps into the cracks of rocks and freezes the water expands, splitting the rock o Occurs fastest in climates that constantly fluctuate between freezing and thawing Ex: mountains in the summer or early spring/late fall in mid latitudes (NYS) o The debris found at the base of mountains which is the result of frost wedging is called talus Abrasion o Grinding rocks down through friction created by rocks rubbing/smashing against each other o Causes rocks to become smaller, smoother, and rounder o Occurs in: Streams Oceans pounding along the coast creates beaches Wind picks up sand. This sand slowly wears away the base of rocks creating rocks that look like mushrooms mushroom rocks This occurs more in arid (dry) environments because the is little moisture and vegetation to hold the sand down Biological Activity o As trees grow, the roots will split rocks apart Unloading or Exfoliation o Intrusive igneous rocks, like granite, form deep inside the earth. As a result they are under tremendous pressure o Uplift and the erosion of the overlying layers causes these rocks to become exposed to the surface. o As a result there is little to no pressure pushing down on the rock and in response the rock expands. This causes the outer layers to peel off in layers, like onions.

2 Chemical Weathering Occurs on when rocks are exposed to the elements (hydrosphere and atmosphere) o So only of the surface of rocks Water is the primary agent of chemical weathering for a couple of reasons o Many minerals dissolve in water which removes materials and changes the chemistry of the rock in the process o Gases also dissolve in the water and those dissolved gases can chemically react with certain minerals in the rock Dissolved oxygen causes the oxidation of certain minerals Ex: Rust Dissolved CO 2 (CO 2 + H 2 O) creates a weak acid called carbonic acid This acid will dissolve rocks that are composed of calcite (like marble and limestone) o The dissolve effect of limestone leads to the formation of caves Cave Formation o Water absorbs CO 2 from the atmosphere and/or soil making it mildly acidic o Groundwater will slowly dissolve the limestone o If the land is lifted up, or the groundwater lowers, the water filled caverns empty creating caves. o Water containing dissolved limestone drips from the ceiling. As it drips it leaves behind (precipitates) a little bit of the calcite. Overtime the calcite builds up creating stalactites (on the ceiling) or if the water drips fast and hits the ground a stalagmite Biological Activity When plants die and decompose acids are created. o These acids can chemically weather rocks Some plants grow on the rock, lichen, and actually secrete acid which weathers the rock

3 Weathering Rates 3 major factors o Climate o Surface area o Rock composition Climate This is the most influential factor of how fast a rock will weather Temperate or cool wet climates favor physical weathering (mainly frost wedging) Tropical climates favor chemical weathering mainly because of abundant rainfall and higher temperatures o More vegetation = more decomposition = more acids o Chemical reactions speed up in warmer temperatures Surface Area Because chemical weathering only occurs on the surface, if more surface is exposed the rock will weather faster This is a good example of how physical and chemical weathering interact. o In the winter a rock my split into smaller pieces through frost wedging. This will open up new areas for chemical weathering to occur in the summer. Rock Composition The composition (mineral make up) and structure of a rock will determine how fast a rock will weather o Composition Rocks that contain a lot of iron will oxidize faster than those that don t This is why mafic igneous rocks weather quickly compared to felsic igneous rocks Rocks that contain calcite will weather faster than those that don t Rocks that contain minerals that dissolve are soluble in water (dissolve) will weather faster than those that don t o Structure Igneous and metamorphic rocks tend to be stronger than sedimentary rocks Igneous rocks are made of interlocking crystals, making it strong Metamorphic rocks are formed under extreme heat and pressure making the rock more dense and the minerals under such pressure grow into each other locking them together Many sedimentary rocks are made of sediments that are cemented together by minerals that are weak and dissolve in water. As a result the cement isn t that strong

4 o Also these rocks form in layers which creates weak bedding planes where water can get between the layers of rocks and weather them faster Soil Soil is the final product of weathering It is a combination of weathered material and organic material (top soil or humus which is black in color) Parent material original rock that is the source of the soil Residual Soil soil that comes from the bedrock below it Transported Soil soil that formed in a different location and was later transported and deposited in a new location o NYS has a lot of transported soil that was deposited by glaciers at the end of the last ice age Different types of soil are found in layers (called horizons) o Top is rich in organic material o Middle contains more clays and silt o Bottom contains larger sediment that hasn t completely weathered

5 Mass Wasting Mass Wasting Notes mass wasting is erosion when gravity is the direct cause Causes water is the primary cause of mass wasting events o when the ground becomes saturated the water reduces the friction between the sediment allowing it to flow loss of vegetation o leaves act like umbrellas, roots soak up water and anchor soil in place over steepened slopes o rivers or road creation can undercut slopes leading to future collapses Examples rocks falls slumps mud/earth flows soil creep Wind Erosion and Deposition Notes Wind Erosion & Deposition occurs more in arid regions lack of vegetation to hold sediment down and dry soil allows smaller sediment to be removed by the wind This can lead to the total removal of sand sized sediment and smaller leaving behind the larger sediment. This process results in desert pavement. o This air born sediment can abrade the surface Eventually the sediment is deposited creating dunes.

Greenland and 2 in Antarctica western ice sheet in Antarctica is ~2.8 miles thick!

7 How Glaciers Modify the Landscape Notes Types of Glaciers Valley Glaciers o aka alpine glaciers o smaller, confined to mountain valleys at high elevations o flow down due to gravity like rivers of ice Continental Glaciers o aka ice sheets o huge, currently found in Greenland and 2 in Antarctica western ice sheet in Antarctica is ~2.8 miles thick! o Like a pancake, thickest at center and flow outwards due to weight at center pushing outwards Movement of Continental Glaciers Glacial Erosion glaciers erode in two ways o plucking o abrasion

U Shaped Valleys o Rivers cut downward creating V shaped valleys

straighten, deepen, and widen the valleys creating a U shape

deep into the land forming deep U shaped valleys or large basins

o When the ice melts those u shaped valleys/basins will fill

8 U Shaped Valleys o Rivers cut downward creating V shaped valleys o When glaciers flow down existing river valleys they straighten, deepen, and widen the valleys creating a U shape Glacial Lakes o When glaciers move over soft bedrock they cut deep into the land forming deep U shaped valleys or large basins (depressions). o When the ice melts those u shaped valleys/basins will fill with water creating lakes The Finger Lakes and Lake Champlain are occupy U Shaped Valleys The Great Lakes are the large depressions

o Kettle Lakes As glaciers retreat (melt) sometimes large blocks of ice break off Later this ice is buried by sediment from outwash plains Outwash plains from in front

can fill up with water creating a kettle lake Hanging Valleys o Hanging valleys are typically formed when the main valley has been widened and deepened by glacial

9 o Kettle Lakes As glaciers retreat (melt) sometimes large blocks of ice break off Later this ice is buried by sediment from outwash plains Outwash plains from in front of a retreating glacier when glacial meltwater carrying sediment is deposited When the ice that is now buried melts, the ground caves in creating a kettle hole which can fill up with water creating a kettle lake Hanging Valleys o Hanging valleys are typically formed when the main valley has been widened and deepened by glacial erosion, leaving the side valley cut off abruptly from the main valley below. The steep drop from the hanging valley to the main valley floor usually creates cascading waterfalls.

Glacial Deposition When a glacier melts it drops all of the sediment embedded in the ice These

is sorted vertically by size, glacial till is unsorted Sediment Deposited by Water or Wind

sediment Moraines o at the front of a glacier till can pile up into ridges (hills) called

last ice age erratic a boulder deposited from a glacier drumlin small tear drop shaped hill

10 Glacial Deposition When a glacier melts it drops all of the sediment embedded in the ice These glacial deposits are called glacial till o Unlike sediment deposited by wind and water, which is sorted vertically by size, glacial till is unsorted Sediment Deposited by Water or Wind Sediment Deposited by Glaciers Cobbles and boulders randomly mixed together with smaller sediment Moraines o at the front of a glacier till can pile up into ridges (hills) called moraines o moraines can help scientists determine the furthest extent of the ice sheet from the last ice age erratic a boulder deposited from a glacier drumlin small tear drop shaped hill formed from glacial deposition o orientated in same direction so like striations they show direction of glacial movement

11 Stream Erosion and Deposition Stream Erosion Sediment Transportation 1. Solution 2. Suspension a. Smaller sediments float in current 3. Bed load or Saltation a. Larger sediments slide, roll, or bounce along the bottom of the channel Upstream vs. Downstream Upstream Known as the source or headwaters Higher gradient Smaller/narrower stream channels Tend to be more straight Less discharge o Discharge is the total volume of water moving through a stream at a given point in a given amount of time Usually measured in cubic feet per sec Down cutting leads to narrow v shaped valleys, gorges, or canyons Downstream Lower gradient Higher discharge o Because there is more water flowing into it due to tributaries upstream Tributaries are streams that flow into other streams Wider and deeper channels Meandering channel o Outside bend erosion is greater here because water movement is faster known as the cut bank channel is deepest here o Inside bend deposition is greater here because water movement is slower as a result sandbars form channel is not as deep here Floodplains o due to streams meandering overtime

Why Low Gradient Streams Meander Over Time Due to erosion occurring more on the outside of the meander while deposition occurs on the inside, some streams

Eventually the meander becomes so much that a new channel cuts through, cutting off a meander creating an oxbow lake.

12 Why Low Gradient Streams Meander Over Time Due to erosion occurring more on the outside of the meander while deposition occurs on the inside, some streams begin to meander (wander or curve) more. Eventually the meander becomes so much that a new channel cuts through, cutting off a meander creating an oxbow lake. Formation of a Flood Plain Low gradient streams continual meandering result in horizontal erosion of the landscape more than downward. Formation of Natural Levees This creates broad flat areas on the side of the river that could flood during periods of rapid snowmelt or excessive rain known as flood plains.

13 Stream Deposition when water velocity decreases the sediment the water was carrying is deposited water sorts sediments both vertically and horizontally at the mouth of the river deposition of sand and silt form deltas o the mouth is when a stream enters a large body of water Drainage Basin or Divide a river and all of its tributaries empty its water into a basin (lake or ocean) the total area of land that drains into a river system is known as a watershed or drainage basin the border, a ridge or hill, that divides two or more watersheds is known as a drainage divide the Continental Divide runs through the Rocky Mountains and divides the continental United States into two watersheds.

14 Groundwater Notes Groundwater Rain water seeps down into the ground in a process called infiltration. Water will then move through the joints/cracks found in some rocks or through the pore spaces that exists between sediments and becomes groundwater. Porosity Porosity is the percentage of pore space in sediment or rock Porosity remains the same even if sediment size changes o Smaller sediment has smaller pore spaces but there are more of them, while larger sediment has bigger spaces but there are fewer of them Poorly sorted sediment (big and small mixed together) has a lower porosity than well sorted sediment because the small sediment fills in the gaps of the larger sediment Permeability Permeability refers to how easy water can move through the ground When sediment gets smaller or less sorted permeability decreases o Ex: gravel is used in landscaping and construction for drainage o Clay, because it is small and flat, is impermeable (meaning water basically can t move through it), which is why clay is used to line the bottom of ponds or why you see wetlands. The water won t drain their Aquifer Rock or sediment that is saturated and water can easily be pumped from o High porosity and permeability The boundary between the ground that is unsaturated and saturated is called the water table Spring Formation When groundwater hits an impermeable layer it is forced sideways. Eventually the water will exist on the side of a hill and when water flows out of the ground it is called a spring Springs can also form when the water table intersects the ground o This is how many rivers get their water

What are the different ways rocks can be weathered?

Romano - 223 What are the different ways rocks can be weathered? Weathering - the breakdown of rocks and minerals at the Earth s surface 1. 2. PHYSICAL WEATHERING Rock is broken into smaller pieces with