11. Structure of the earth and Rocks

So far, we have emphasized atmospheric processes, how climate varies geographically, and how that helps shape distribution of vegetation and soils.

Now we turn to the configuration of the earth's surface and the underlying processes that shape it. We will begin by examining the earth's interior and the energy generated there.

11.1.structure of the Earth

A nearly perfect sphere (flattened at the poles, due to rotation). Radius of about 4000 mi (6400 km).

As we move from the center to the surface of the earth, we will observe a progressive decrease in density and temperature.

Highest density is concentrated in the center, because of the confining force of gravity.

The earth's interior is very hot. This geothermal energy is residual heat generated by the radioactive decay of earth minerals.

1. Core - starting from the center of the earth and working outward, we first encounter the core, which accounts for slightly over ½ of the earth's radius (but only about 15% of its volume).

(1).Inner core is solid iron (some nickel). The densest layer, with a specific gravity of about 13.5 g/cc. (For comparison, water = 1 g/cc, granite = 2.65 g/cc.)

a.It is the hottest region, with T = 3100°C.

b.Remains solid iron only because of intense gravitational pressure of overlying material.

(2).Outer core is liquid, or molten iron (some nickel). Somewhat cooler (T = 2800°C) and less dense (10.7 g/cc).

a.The fluid outer core generates the earth's magnetic field. (Why compasses work!)

2.Mantle - most of the remaining radius from the core to the crust is embodied in the mantle, which has several distinct layers. By volume, it is over 80% of solid earth.

a.The mantle is technically solid, but undergoes lots of local melting and plastic deformation near surface.

b.Mean T = 1800-2800°C for mantle material.

c.Mantle is comprised of silicate minerals (Si, O) that are rich in iron and magnesium. Mean density of mantle is 4.5 g/cc.

d.Mantle is divided into two layers: lower and upper mantle.

Upper mantle is further subdivided near the surface into two zones:

-Asthenosphere - mean density about 3.3 g/cc. Denser and hotter than lithosphere above. under tremendous pressure and heat so that it is "soft", near melting point, and flows plastically.

-Lithosphere - less dense and cooler than asthenosphere. It is solid, hard, brittle material. The lithosphere actually includes both the uppermost mantle and the crust.

The lithosphere it is vertically thin, only about 40-95 miles thick, depending on nature of overlying material.

The lithosphere is broken into pieces, or plates. These plates "float" atop the

asthenosphere, move around on it, and move independently of one another.

Pressure from plate collisions in the uppermost mantle lead to plate breakage (faulting-earthquakes) and warping (folding).

3.Crust - the very thin, brittle, outer shell of the earth. Only 5-25 miles (8-40 km) thick.

It is affixed to the lithospheric plates that comprise the uppermost mantle, so it is also broken into pieces, or plates.

Two types of crustal material:

(1).Oceanic crust - denser (3 g/cc), not very thick (5 mi). Made of iron and magnesium-rich silicate minerals, like basalt. Oceanic crust underlies all the major ocean basins.

(2).Continental crust - less dense (2.7 g/cc), often thick below mountain ranges (up to 25 mi.thick).

Made of less dense silicate minerals with lots of aluminum, calcium, potassium, like granite.

Continental crust is the geologic stuff of landmasses. It "floats" above iron and magnesium-rich layers of oceanic crust and uppermost mantle.

Summarizing overheads of upper layers and lithospheric plates:

-Outer layer of earth's upper mantle and crust broken into lithospheric plates, like sutures in a skull.

-These plates are rigid, and move independently

driven by molten convective plumes of energy and material from deep in the mantle

Some plates are moving apart, some are colliding.

10.2 Minerals and Rocks

Minerals are solid, inorganic substances with specific molecular composition and crystalline structure.

Different minerals exhibit distinctive properties:

such as degree of hardness (diamonds vs. soapstone)

crystal structure (like flat sheets of mica, the flakes of which sparkle in our local granitic bedrock).

Rocks are combinations of minerals (usually more than one, but not always). Whereas minerals rarely occur in pure form, they are commonly the building blocks of rocks.

Granite is a combination of mica, quartz (pure silicon dioxide), and feldspars (orthoclase -- white,plagioclase -- pink). All three of these groups of minerals are silicates (Si and O).

Earth's crust is comprised primarily of oxygen (47%) and silicon (28%), in the form of silicate minerals (some combination of Si and O). Aluminum and iron also fairly prominent (5-8%) in crust.

Rocks are classified according to how they form

1.Igneous rocks are formed by cooling of molten mineral material, or magma. They comprise over 80% of rocks in earth's crust. Igneous rocks can be classified according to two features

(1).Chemical composition

a.Oceanic crust --> Fe and Mg-rich silicates, like basalt. These are richer in basic cations. Weather to more fertile soil.

b.Continental crust --> Al-rich silicates, like granite. These are richer in acidic cations. Weather to less fertile soil.

(2).Location of rock formation with respect to earth's surface.

a.intrusive igneous rocks, formed below the crust by gradual cooling in a magma chamber.

Slow cooling means large crystals grow. E.g. granite has conspicuous crystals of quartz, feldspar.

b.extrusive igneous rocks, or volcanics, formed by rapid cooling at or above the earth's surface.

rapid cooling means microscopic crystals. Basalts are volcanic, micro-crystalline rock.

occurs as lava, which flows over surface and cools.

c. occurs as tephra, which solidifies in air and falls to earth as volcanic ash deposits.

2.Sedimentary rocks are made of weathered and eroded rock particles.

(1).Formation:

Sediments are materials that are eroded from uplands and carried to lower elevations and deposited by running water, wind, ice.

Sediments may be deposited in lakes, streams, depressions, oceans basins.

As more sediments accumulate, buried sediments become increasing compacted and are often cemented into rocks.

Cementation may involve chemical changes (like the carbonates that make cement) or physical hardening.

Since sediments accumulate vertically, the oldest sediments are found on the bottom and the youngest on the top. Geologists use this simple rule (the law of superposition) to reconstruct ages of various rock layers, or strata. Dating is aided by fossils embedded in sediments and decay of radioactive minerals incorporated into sediments.

Common types of sedimentary rocks:

(1).Clastic - from mineral sediments

a.sandstone - from cemented sand grains

b.shale -- from consolidated and hardened mud

(2).Biogenic - from organic sediments

a.limestone (calcium carbonate) - from bones and shells of marine organisms (can also form inorganically by chemical precipitation from ocean water)

b.coal - from plants and animals accumulated in ancient tropical swamps.

3.Metamorphic rocks are made of other rocks that have been modified under high pressure and temperature into new forms.

Crustal movements associated with lithospheric plate boundaries generate lots of heat and pressure, as well as deform the crust.

This heat and pressure deforms and modifies the original crystalline structure of the rock. (In some cases, it actually subducts and melts the rocks, which makes new igneous rocks!)

Occurs along edges of plate boundaries, and in rocks surrounding magma chambers.

Both igneous and sedimentary rocks can be metamorphosed. Examples:

1.Sandstone --> quartzite

2.Shale --> slate

3.Limestone --> marble

4.Granite --> gneiss

5.Basalt --> schist

Metamorphic rocks are often very hard and resistant. Do not erode as quickly as sedimentary or igneous rocks. Form bedrock core of many major mountain ranges.

Thanks to lithospheric plate collisions, surface rocks are constantly being recycled as they are plunged into the mantle and melted. This allows new igneous rocks to form, and provides the material to renew the crust of the earth. (See overhead)

(1).Igneous or metamorphic rocks eroded to form sedimentary rocks.

(2).Igneous or sedimentary rocks modified by heat and pressure (without melting) into metamorphic rocks.

(3).Sedimentary and metamorphic rocks subducted and melted, minerals reassembled into new igneous rocks.