Topic 7: Terrestrial Surface Processes

 

EARTH MATERIALS

 

MINERALS: A mineral is a naturally occurring, inorganic substance with a definite crystal structure and chemical composition, and a diagnostic set of physical properties.

No two minerals have the same composition and crystal structure. Examples: Diamond (C; cubic crystal structure, hardness 10); Graphite (C; hexagonal crystal structure, hardness 2).

Minerals with the same chemical composition but different crystal structures (like diamond and graphite) are termed Polymorphs.

Crystal Structure - The crystal structure of each mineral is due to the regular repeating pattern of the atoms which make up the chemical composition of the mineral.

Example: Halite - Nacl (Sodium Chloride)

The chemistry and crystal structure of a mineral are the only characteristics that exactly determine the mineral type. But they can only be properly measured with precision laboratory instruments.

Physical properties - Physical properties are a more practical and easy (but non-unique) way to identify many minerals. Some of the more common physical properties include color, luster (the way a mineral reflects light), cleavage (the natural way a mineral breaks), taste, density (or specific gravity), magnetism, double refraction, and hardness.

The problem with specific physical properties, such as white color or platey cleavage, is that they can apply to more than one mineral. Therefore, one must look at several physical properties in order to try to identify a mineral. (Even then one might be wrong!)

One type of measurable physical property is mineral hardness. Mohs' Hardness scale defines hardness and is based on the relative hardness of 10 distinct minerals from softest=1 (talc) to hardest=10 (diamond). For example, a quartz crystal (hardness=7) will scratch a graphite crystal (hardness=2), but the quartz crystal will in turn be scratched by a diamond crystal (hardness=10). This is one of the most useful of the observable mineral properties.

Mineral Groups - Minerals can be broken up into two basic groups, silicates and non-silicates.

Silicate minerals have one atom of silicon and two atoms of oxygen (SiO2) in their (molecular) chemical structure. Silicates are the most abundant minerals on Earth.

Example: Quartz - SiO2 (silicon dioxide) is the best known mineral.

It has variable color, vitreous luster, h = 7, conchoidal fracture with no cleavage, and its crystal structure is a network of tetrahedrons of silicon and oxygen in a hexagonal array (six-sided crystals).

Feldspars are the most abundant group of silicates. They form a significant portion of most granites and other igneous rocks. Feldspars usually include K (potassium), Al (aluminum), or Ca (Calcium) in the mineral composition (along with silica).

Ferromagnesian silicates contain iron (Fe) or Magnesium (Mg). These elements produce dark mineral colors. The ferromagnesian minerals tend to look metallic in their luster, have relatively high density, and are often magnetic.

Clays are another important group of silicates. They are called sheet silicates because the chemical bond between layers of atoms is very weak and easily broken. Example: Mica (solid lines indicate a strong bond between neighboring atoms; dashed lines indicate relatively weak bonds)

Non-silicate minerals include a wide variety of minerals. They tend to be grouped on the basis of some shared chemical component. For examples: Carbonates contain CO3. The most common mineral is Calcite (CaCO3). Calcite is the primary building block of limestone, a rock. Sulfates contain SO4. One typical sulfate mineral is gypsum (CaSO4). Sulfides contain S without oxygen. One typical sulfide mineral is pyrite (FeS2). Oxides contain 02 as the sole cation. Metals often occur as oxides. For examples: magnetite (Fe3O4) or hematite (Fe2O3). Native elements are elements that occur by themselves in nature. Native elements also are commonly metals such as Cu - copper, Au - gold, or Ag - silver.

 

ROCKS: A rock is an aggregate or mixture of crystals from one or more minerals. For example, granite is a rock composed of interlocking crystals of quartz, feldspar, mica, and a mineral called hornblende. Sandstone is a rock composed of grains of the same minerals that make up granite. Sandstone, by the way, is just sand that has turned into rock. In other words, it has lithified.

Rocks are typically classified into three distinct types: Igneous, Sedimentary, Metamorphic.

Igneous Rocks - form from molten (melted) rock that crystallizes as it cools. This molten rock originates in the Earth's interior and is termed Magma.

Intrusive or plutonic igneous rocks cool slowly far beneath Earth's surface. Slow cooling allows individual mineral crystals to grow large enough that we can see them without a magnifying lens, so intrusive rocks are coarse-grained.

Extrusive or volcanic rocks cool quickly at or very near the Earth's surface. Fast cooling is analogous to quenching steel - cooling occurs so rapidly that crystals have very little time to grow, so they are very tiny and we cannot see them without powerful magnification. Extrusive rocks are mostly fine-grained.

Magma which comes to the Earth's surface before it cools is termed Lava.

Igneous rocks are easily classified on the basis of visual texture (individual crystals are visible - coarse grained; individual crystals are not visible - fine grained) and color (light, intermediate, or dark). Light igneous rocks are rich in quartz (SiO2) and most often found in continental crust; dark igneous rocks are rich in ferromagnesian minerals and are most often found in oceanic crust. Geologists currently use a more rigorous method for classifying igneous rocks that depends on exact chemical analysis of each rock. But the overall classification scheme shown below is both accurate and easy for anyone to use in the field.

Igneous Rock Classification

Texture Color(Light) (Intermediate) (Dark)

Coarse-grained: Granite Diorite Gabbro

(intrusive) (Most common)

 

Fine-grained: Rhyolite Andesite Basalt

(extrusive) Pumice (Most common)

Obsidian

 

Examples in intrusive and extrusive igneous rocks.

Example of igneous rocks in California - Sierra Nevada region. Mountains are composed mostly of granite; Owens Valley has several active volcanoes with visible lava flows.

 

Sedimentary Rocks - Sedimentary rocks are formed by lithification of sediment at the Earth's surface. Sedimentary rocks are composed of accumulations of mineral and/or rock grains (sediment). The sediment is derived by physical and chemical weathering of existing rocks at the Earth's surface.

Identification of sedimentary rocks in the field is based on their similarity to unconsolidated sediment you can see at the beach, along river banks, or in lakes.

Key features of sedimentary rocks are that they are usually layered and that they often contain fossils (remnants of prior life). One of the tenets of geology is the law of superposition: younger sedimentary rock layers lie on top of older sedimentary rock layers.

Sedimentary rocks are classified according to physical texture and chemical composition.

A rock with clastic (= detrital) texture is composed of deposited sediment grains. Clastic rocks are classified according to grain size. Small grains are 'clay' sized, larger grains are 'silt' sized, yet larger grains are 'sand' sized, and the largest grains are 'conglomerate' sized.

Non-clastic rocks are classified according to chemical composition. Non-clastic sediments are derived from the chemical precipitation of mineral crystals usually in water.

Sedimentary Rock Classification

Texture (grain size) Clastic Rocks | Non-clastic Rocks

Mud-sized grains Shale (layered) | Limestone (calcite)

Mudstone (no layers)

Sand-size grains Sandstone | Rock Gypsum

| Rock Salt (halite)

| Dolostone (dolomite)

Gravel-size grains Conglomerate

 

Metamorphic Rocks - Metamorphic rocks are rocks that have been changed by temperature, pressure, and/or chemical activity. This may happen at or close to the Earth's surface or deep in the Earth's interior.

The metamorphic changes may result in new textures, new structures, and/or new minerals. A new texture may occur when minerals in the rock recrystallize into larger crystals of the same mineral. A new structure, called foliation, may occur when minerals turn and flatten out in response to increased pressure, producing a distinct layering in the rock.

metamorphism can occur because of :chemical activity - ground water percolation. heat - push rocks to greater depths in the earth; intrusion of magma will heat rocks surrounding intrusion. pressure - burial of sediments; tectonic stresses (mountain building).

 

ROCK CYCLE: Processes at or near the Earth's surface constantly work to change existing rocks and form new ones. There is a continuous cycling of rock material, which we term the rock cycle. No single rock has remained unchanged since the Earth was first formed

- Examples of the rock cycle: Volcanism produces extrusive igneous rocks. Those rocks may be altered by wind, rain, and other weathering processes at Earth's surface. The products of those alteration processes are then deposited as sediment and eventually undergo lithification to become sedimentary rocks. Sedimentary rocks can be partially heated by a nearby igneous intrusion or squeezed during the tectonics of mountain building to form metamorphic rocks.

 

RIVER SYSTEMS

A) Hydrologic Cycle:

Water Distribution at the Earth's Surface: The hydrosphere refers to all water at or near Earth's surface. It includes:

- water vapor in the atmosphere,

- ground water underground,

- the polar icecaps,

- rivers,

- lakes,

- oceans, etc.

Water Distribution in Hydrosphere

Oceans 97.54%

Icecaps 1.81%

Ground Water 0.63%

Everything Else 0.02%

Water Circulation Within the Hydrologic Cycle: The hydrologic cycle is the movement of water in the hydrosphere. The amount of water involved in the hydrologic cycle is estimated to be 1017 gallons per year.

B) Origin of Rivers:

Drainage Basins: A drainage basin is the area drained by a particular river and its tributaries. You can define a drainage basin on any scale: A big river has a drainage basin that includes the drainage basins of all its tributaries, and each tributary has a drainage basin that includes the drainage basins of all of its small tributaries, and so on.

The drainage basin collects surface runoff from rainfall (or springs etc.) and concentrates the water in rivers and streams.

 

Drainage basins typically develop in a dendritic pattern as surface water accumulates in small rivulets which feed into larger streams which, in turn, feed into larger rivers.

Seasonality of Stream Flow: There are seasonal variations in river water level that are fairly predictable.

The stream discharge, Q, is the amount (volume) of water flowing past a point along the stream in a specified time interval. Discharge is usually reported in cubic feet per second.

Stream records of seasonal discharge are called Hydrographs.

 

If the water level is extra high it is called a flood. Floods may result from heavy, prolonged rain and/or rapid snow melt.

The probability of a certain flood discharge occurring can be calculated from records kept over many years.

To calculate the probability of a certain flood discharge, record the peak (maximum) discharge for each year and rank them in order from highest Q (rank = 1) to second highest Q (rank = 2) and so on. N = the number of years for which there is a record, and M = rank; the recurrence interval, the number of years between floods of a certain rank and discharge, is given by the equation

Recurrence interval = (N+1)/M

By making a graph of Q vs. recurrence interval using measured values, the recurrence intervals of Q values that were not actually recorded can be found. The problem is the need for good long-term historical records for Q.

 

C) Flood Control:

1. Construct flood control dams or debris basins.

To prevent the annual Nile flood, 20th century man built the Aswan Dam. The soil along the river is now depleted because it is not replenished by the flood deposits. The shrimp industry off the delta has waned because without a nutrient supply from the river the shrimp went elsewhere in search of food. Snails have taken over the irrigation canals along the Nile, bringing with them parasites that have infected the area with schistosomiasis.

The Mississippi River system has an extensive series of large dams on its tributaries in order to minimize flood damage. Fort Peck, Sakakawea, Oahu, all on the Missouri river, etc.

2. Construct concrete river channels. Or dredge river channels to deepen them.

3. Construct man-made levees. Unfortunately, a levee built at one point along the stream causes worse flooding further downstream.

D) Geologic Work Done by Rivers:

Erosion is the weathering and transportation of earth materials. Weathering can be mechanical, as when freezing/thawing of water in cracks, or when root growth wedges rock apart; or it can be chemical, as when water or solutions alter or dissolve rock.

Transportation of sediments by streams may be via suspension (suspended load) for fine particulate material, traction (dragging or rolling along the bed, bed load) for larger particulate material, or in solution (dissolved load) for dissolved minerals.

The amount of load depends on the stream discharge, Q.

Stream erosion features include valleys and waterfalls. Valleys evolve from youthful to mature to old age. A youthful stream valley is V-shaped in cross-section, has a steep gradient (the slope it runs down) of 10-15 feet of drop per horizontal mile, has rapids (usually just downstream of where a tributary joins it), and has waterfalls because the stream course has not had enough time to smooth its path. A mature stream valley is characterized by a meandering river and a flood plain not much wider than the meander belt. The most fertile land ("bottom land") is on the flood plain, where the river deposits sediment when it floods. An old stream valley has a very low gradient of only 1-2 feet per mile, a wide flood plain, tight meanders, oxbow lakes, swamps, and natural levees.

As rivers meander, they erode sediment from their bankswhere current directions meet the bank, and deposit sediments where current directions diverge from the banks (like how spits are formed on coastlines). Eventually this can result in a new river path being cut that bypasses part of the older river, thus forming a 'meander cutoff' or 'oxbow lake'.

Deposition is the leaving behind of erosion products when transporting agents can no longer carry them. Alluvial deposits are stream deposits; all material deposited by streams is called alluvium.

Sediments are also deposited along the margins of rivers when they flood. Since more sediment is deposited close to the banks and less far away, eventually natural ridges called levees are formed along river channels.

Base level is the elevation to which a stream can erode. Base level may be sea level, or it may be the elevation of water in a lake or reservoir,.

River terraces form when a river erodes down through its own gravel deposits as the ground undergoes uplift:

Where a river empties into standing water a delta forms. Sediment deposition builds the delta out farther and farther from the rest of the shoreline.

 

The Mississippi River delta is an example of a birdsfoot delta, named for its shape when seen from overhead (or on a map).

The Nile River delta is an example of an arcuate delta. The World War II battle El Alamein took place in the coastal sand dunes around the Nile delta. The Nile made possible the existence of an Egyptian civilization; without it there would be no agriculture because there would be no water, and there would be only sand dunes instead of pyramids and temples.

Deltas face hazards from two directions: flooding from the river, and storm surge and other attacks from the ocean. Hurricane damage is greatest on deltas, where the land is already low and swampy.

As rivers flow out of mountains onto flatter plains, they lose velocity and deposit sediment as alluvial fans. The alluvial fans along the base of the mountains form the piedmont, a slope intermediate between the slope of the mountains and the slope of the plain. Many Los Angeles suburbs (La Canada, La Crescenta, Sierra Madre, Azusa, and others) are built on alluvial fans, as are many desert communities (parts of Palm Springs and Palm Desert, for example).

Piedmont is apron of fans (changed slope from mountains to piedmont to basin plain).

 

GROUNDWATER

A) Groundwater Zone

97% of the Earth's readily available fresh water (liquid H20) is underground. Only 3% resides in rivers and lakes.

The permeable zone that has water filling all void space is called the saturated zone; the zone with water filling only part of the void space is the aerated (vadose) zone.

Recharge of groundwater comes from surface water.

Aquifers are saturated zones of groundwater with sufficient supply and permeability that water can be removed as a resource.

B) Aquifers

Porosity/Permeability: Ground water comes from aquifers, rock layers with porosity and permeability saturated with retrievable water.

Porosity is a measure of the amount of void space in a rock that can be occupied by water:

Porosity is usually between 20% and 40%.

Permeability tells how well the void spaces are connected and, therefore, how easy it is for water to move through the rock. A rock with high porosity may have no permeability if it has lots of unconnected void space. Permeability is essential for an aquifer because if water cannot flow through the rock, wells will dry up. Wells will also dry up if they pump out water faster than it can flow through the rock.

Types of Aquifers: An aquiclude is a layer of rock that acts as a barrier to ground water flow because of low porosity and/or low permeability.

 

unconfined aquifers are overlain by permeable rocks or soil. water must be pumped from aquifer. recharge occurs over broad area above aquifer.

Confined aquifers are surrounded by aquacludes. Water in confined aquifers is usually under pressure. This causes wells drilled into confined aquifers to flow freely (artesian) due to excess pressure.

Confined aquifers have a severely limited recharge area because of the bounding aquacludes.

 

WIND

A) Global Wind Patterns

Here's a simplified diagram of wind patterns near the Earth's surface:

As air rises or falls, the Earth's rotation causes it to be deflected (remember the Coriolis force ?) as shown on the diagram, creating the northeast trade winds, westerly winds, and other winds.

 

B) Desert Types

Deserts are products of wind. The Sahara and Gobi Deserts are trade wind deserts. The Mojave and Colorado Deserts of North America are rain shadow deserts.

C) Wind as a Geologic Agent

Wind erodes by abrasion (blowing things against and across each other) and by deflation (blowing things away). Desert pavement forms as wind blows fine material away leaving behind a flattened rocky mosaic.

The best-known wind deposits are sand dunes. Wind blows sand up the more gradual slope of the dune; sand falls from the dune crest down the steeper slope, creating a deposition pattern called crossbedding. Crossbedding is preserved in sedimentary rock, so it is possible to determine if the sediments that make up a given rock were deposited by the wind. (Ripples on stream bottoms have crossbedding, too, but dunes are usually much larger than ripples and contain a greater percentage of sand grains close to the same size. It's pretty easy to tell sand dune rock from stream ripple rock.)

Dunes migrate in the direction to which the wind blows. The three types of sand dunes are barchan, sief (longitudinal), and transverse. LAX is built right over the front line of transverse dunes that back the South Bay beaches (Hermosa, Manhattan, Dockweiler, etc.)

Wind transports erosion products via saltation and suspension. Saltation is the movement of sand grains by impact-caused jumps: one grain hits another and causes it to jump; when the jumping grain lands it hits another grain, causing it to jump, and so on. The saltation layer is only about 18 inches high, but jumping sand grains have tremendous erosive power and and can do considerable destruction in those 18 inches. Suspension is carrying material into the atmosphere, mostly dust.

Now back to wind deposition and dunes. The steep side of a sand dune is at the angle of repose of sand, which is 32°. Dry sand can be piled up at any angle up to and including 32°, but no steeper.

Some famous dune areas are:

- White Sands, New Mexico. Here the dunes are gypsum sand and are very white. Animals living on the dunes are also white. If you go to this place, do not go too early in the morning when the ground is dewy, unless you like gypsum grains stuck all over your body.,

- El Segundo, California. This is where desert movies were filmed in the silent era, including the Rudolph Valentino "Sheik" pictures.

- Death Valley, California. When George Lucas needed some emergency desert scene re-shoots for Star Wars he had them filmed here, rather than going back to Tunisia.

- Sahara Desert, Africa. (of course.)

- Indiana Dunes, Warren Dunes, Sleeping Bear Dunes. These dunes areas are on the east shore of Lake Michigan and are characterized by transverse dunes migrating inland through forests and swampy lowlands. They are also endangered by sand mining companies - the nice sand makes good industrial molds.

 

GLACIERS

Glaciers are accumulations of snow and ice moving downhill due to gravity.

The weight (pressure) of overlying snow and ice essentially metamorphoses deeper snow and ice, creating foliation planes along which the material is able to slip and flow.

One-tenth of Earth's surface is ice-covered.

Glacier Types

1. Valley or Alpine glaciers at high altitude (in the mountains).

Examples are valley glaciers in the Sierra Nevada and Alps.

2. Ice caps, as found in Alaska, Iceland, and the southern Andes.

3. Ice sheets or continental glaciers, which cover everything. The

only two continental glaciers on Earth today are at Greenland

and Antarctica.

Glacial Anatomy

A glacier advances by flowing downhill. A glacier wastes by melting or by calving. Calving is the splitting off of ice blocks from the glacier toe, making icebergs, when the toe is over water. If the rate of advance equals the rate of wasting, the toe is stationary. If the rate of advance exceeds the rate of wasting, the toe advances. If the rate of wasting exceeds the rate of advance, the toe retreats.

Glacial Landforms - Valley Glaciers

Valley glaciers carve out the valleys they fill. When the glaciers recede, the valleys left behind a 'U' shaped in cross-section. (Remember mountain valleys formed by streams are 'V' shaped.)

Valley glaciers leave rugged topography and high relief in the mountainous regions where they occur. Even after glaciers have melted completely, there are features such as U-shaped valleys, horns, aretes, and cirques that are left behind.

Glacial Landforms - Continental Glaciers

Continental glaciers leave subdued, rounded topography, including such features as deranged drainage and numerous lakes. Notice all the lakes as you fly over Michigan or Minnesota

The numerous small lakes all over Michigan, Wisconsin, and Minnesota that were formed by glaciers are called kettle lakes.

 

Glacial Landforms - Moraines

All rock material deposited by glaciers is called till. Where the till is deposited and what happens to it after deposition determines further classification. Till deposited at the toe of a glacier is called end moraine - Long Island is an end moraine. Till deposited along the side of a glacier is called lateral moraine. Till deposited in the middle of a glacier formed by tributary glaciers is called medial moraine.

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