Chapter 13: Introduction to Landform Study

The Structure of Earth

The deepest existing mine shaft goes down 2.4 miles

Deepest drills for samples goes down 8 miles

Earth’s 4 Interior shells/regions:o The Crust-

Broad mixture of rock types Beneath oceans the crust is 4 miles (average) Beneath the continents the crust is 40 miles (average) Makes up less than 1% of Earth’s volume and 0.4% of Earth’s mass Has change in mineral composition at base [called Mohorovičić Discontinuity

(Moho)]o The Mantle-

Extends downward to approximately 1800 miles The largest of the 4 shells Makes up 84% of total volume and 2/3 of Earth’s total mass Three sub layers-

Lithosphere -o Thin yet hard and rigido 40-60 miles deepo Temperature believed to reach 1,832°F

Asthenosphere -o Rocks are hot and loose much of their strength and become

plastic (easily deformed, like tar) Centrosphere (lower mantle) -

o Rocks are believed to be largely rigid/firm againo Plays a major role in controlling the thermal evolution of the

planet

o The Outer Core- Molten Liquid Extend to depth of 3100 miles Thought to be made of iron/nickel or iron/silicate

o The Inner Core- Thought to be made of iron/nickel or iron/silicate Solid and very dense mass Radius of 900 miles

Earth’s Magnetic Field-o Primarily in the outer coreo Changes over timeo The location of the North Magnetic Pole does NOT match the geographic location of the

North Pole Slowly shifts several tens of kilometers each year

o Every couple of thousands of years, the polarity of the Earth’s magnetic field reverses (North becoming South)

Plate Tectonics and the Structure of Earth-o Earth’s Plates are slowly moving over the warm, soft asthenosphere below.o The plates are literally pulling apart, colliding, and sliding past each othero Volcanic Activity, Faulting, and Folding are linked to the plate action

The Composition of Earth

100 natural chemical elements are found in Earth’s crust, mantle, and core

Minerals - Naturally formed compounds and elements of Earth

Minerals Must Be-o A solido Naturally found in natureo Inorganico Have a specific chemical composition wherever found that varies only within certain

limitso Contain atoms arranged in a regular pattern to form solid crystals

The majority of known minerals are found only in the crust

The Families of Rock-Forming Minerals-o Silicates-

The largest and most important mineral family Combine Oxygen (O) and Silicon (Si) Hard and durable Major subcategories-

Ferromagnesian Silicateso Olivine

Nonferromagnesian Silicateso Quartz

They are distinguished by the presences or absence of iron and magnesium in their composition

Feldspar and quartz are the most abundant of the silicate minerals Quarts is composed of pure silica (SiO2)

o Oxides- An element combined with oxygen Megnetite

o Sulfides- Composed of sulfur in some combination with one or more other elements

Galenao Sulfates-

Contain sulfur and oxygen with some other element Gypsum

Calcium is the principle combining element Usually light-colored Mostly found in sedimentary rocks

o Carbonates- Light colored or colorless Composed of one or more elements in combination with carbon and oxygen Calcite

o Halides- Notably salty Flourite

o Native Elements- Discrete Elements - Not combined chemically with another element

Gold/Silver

Rocks

Rock - Solid material composed or aggregated mineral material

Outcrop - Solid rock found right at the surface

Rocks-o Solid rock exist as a buried layer of bedrock and covered by a layer of broken rock

(regolith) Soli comprises the upper portion of regolith

Three Major Rock Classes-o Igneous-

Formed by the cooling and solidification of molten rock Magma - Molten rock beneath the surface

The amount of silica present is an important variable to magma composition

Magmas with large amounts of silica cool to form felsic igneous rockso Contain large portions of light-colored silicate minerals

(quartz/feldspar)o Have lower melting temperatures than mafic minerals

Magmas with low amounts of silica cool to form mafic igneous rockso Contain large portions of dark-colored, magnesium- and iron-

rich silicate minerals (olivine/pyroxene) Lava - Molten rock when it flows out on, or is squeezed up onto, the surface Form directly from the cooling of magma or lava

Although some develop from the welding of tiny pieces of solid volcanic rock, called pyroclastics

Texture of igneous rock is determined by where/how the molten material cools The slow cooling beneath the surface leads to a coarse-grained texture The rapid cooling of lava on the surface leads to a fine-grained texture

Two Main Igneous Rocks Categories- Volcanic/Extrusive Igneous Rocks-

o Form from the cooling of lava or the bonding of pyroclastic materials on the surface

o Form on surface of Eartho Complete within hourso Mineral crystals in volcanic rocks as small (almost invisible)o Volcanic rocks that form from the accumulation of pyroclastics

may clearly show tiny fragments of shattered rock (from explosive volcano)

o Basalt - Black/dark gray Forms from cooling of lava Fine-grained rock Comprised of only dark minerals (plagioclase feldspar…) Makes up the bulk of the ocean floor crust

o Obsidian - Type of volcanic glass (no organized crystal material) Black in color Forms from extremely rapid cooling of lava

o Pumice - Forms from the rapid cooling of frothy, gas-rich, molten

material Light enough to float on water

o Tuff - Volcanic rock Consists of welded pyroclastic fragments

Plutonic/Intrusive Igneous Rocks-o Form from the cooling of magma below the surfaceo Cool and solidify beneath the Earth’s surfaceo Surrounding rocks serve as insulation around the intrusion on

magma that greatly slows the rate of coolingo Thousands of years to complete coolingo Individual crystals in plutonic rocks can grow to a large sizeo Granite -

Light colored Coarse-grained igneous rock Made of a combination of light & dark colored minerals

(quartz, plagioclase feldspar, potassium feldspar, …) Make up the core of many mountain ranges

o Sedimentary- The disintegration of rocks produces fragmented mineral material (Figure 1)

Removed by water/wind/ice/gravity/etc… much of the material is transported by water moving as sediment

Sediment is eventually deposited in a quiet body of water (floor of the ocean)

Over a long period of time sedimentary deposits build in thickness It exerts enormous pressure Causes individual particles in the sediment to adhere to each other and

to interlock Chemical cementing takes place

o Silica, calcium, carbonate, and iron oxide precipitate from the water into the pore spaces in the sediment

This transforms the sediments to sedimentary rock Most sedimentary deposits are built up in more or less distinct horizontal layers

(strata) Vary in thickness and composition Parallel structure

The strata may later be uplifted, tilted, and deformed by pressures from within the Earth

Two subcategories of sedimentary rocks- Clastic/Detrial Sedimentary Rocks -

o Sedimentary rocks composed of fragments of preexisting rocks in the form of cobbles, gravel, sand, silt, or clay

o Shale/Mudstones- Composed of very fine silt and clay particles

o Sandstones- Made up of compacted, sand-size grainse

o When the rock is composed of rounded, pebble-size fragments it’s called conglomerate (Figure 2)

Chemical & Organic Sedimentary Rocks -o Formed by the precipitation of soluble materials or more

complicated chemical reactiono Limestone is the most widespread result

Can be formed from the accumulated skeletal remains of coral and other lime-secreting sea animals

o Organically accumulates sedimentary rocks (like coal) are formed from the compacted remains of dead plant material

o Metamorphic- Igneous or sedimentary rocks that have been drastically changes by heat and/or

pressure

Figure 2

Figure 1

The effects of heat and pressure on rocks are strongly influences by such things as the presence or absence of fluids in the rocks and the length of time the rocks are heated and/or subjected to high pressure

Metamorphism can cause its mineral components to be recrystallized and rearranged

As a result the rocks are changes in structure, texture, composition, and appearance

Metamorphism can occur beneath the surface of the Earth where magma comes in contact with surrounding rocks (altering the surrounding rocks through heat and pressure)

Regional Metamorphism takes place where large volumes of rock deep within the crust are subjected to heat and/or pressure over long periods of time

Foliated - If the minerals in a metamorphic rock show a prominent alignment or orientation

May have platy, wavy, or banded texture Some rock change in predictable fashion when metamorphosed

Sandstone → Quartzite Shale → Slate

Metamorphic rocks are identified according to their physical characteristics and appearance

The Rock Cycle

The Study of Landforms Landform - An individual topographic feature of any size (cliff –mountain range)

Geomorphology - The study of the characteristics, origin, and development of landformers

70% of our Earth is covered with oceanic waters

58 million square miles of land are scattered over the continents and innumerable islands

Basic Elements for an analytical approach:o Structure-

The nature, arrangement, and orientation of the materials making up the landform being studied

o Process- Considers the actions that have combined to produce the landform

o Slope- The Fundamental aspect of shape for any landform

o Drainage- The movement of water either over Earth’s surface or down into the soil and

bedrock

Some Critical Concepts

Relief - The difference in elevation between the highest and lowest points in an area

Internal Processes-o Originate from within eartho Energized by internal heat that generates forces that apparently operate outside of

any surface or atmospheric influenceso Resulting in crustal movements (folding, faulting, and volcanic activity)o Increase the relief of the land surface

External Processes-o Draw their energy mostly from sources above the lithosphere (in the atmosphere or in

the oceans)o Behavior often predictableo Behavior influenced by the existing topographyo Thought of as wearing-down or destructive processes (called denudation)

Uniformitarianism-o Put forth by James Hutton in 1795o Means that the processes that are shaping the contemporary landscape are the same

processes that formed the topography of the past

Geologic Time-o Refers to the vast periods of time over which geologic processes operate

Scale and Pattern

A prime goal of any geographic study is to detect patterns in the areal distribution of phenomena