What is the Earth?

Earth is our home planet. Scientists believe Earth and its moon formed around the same time as the rest of the solar system. They think that was about 4.5 billion years ago. Earth is the fifth-largest planet in the solar system. Its diameter is about 8,000 miles. And Earth is the third-closest planet to the sun. Its average distance from the sun is about 93 million miles. Only Mercury and Venus are closer. Earth is the only planet known to have large amounts of liquid water. Liquid water is essential for life. Earth is the only planet where life is known to exist.

Origin of the Universe:Big Bang Theory

•       Evidence:

–      The galaxies are rapidly moving apart (Hubble's Law); indicates that galaxies were closer together in the past
(This was discovered in 1929 by Edwin P. Hubble.)

–      observed temperature of the universe today (background microwave radiation) 3 degrees above absolute zero

–      present abundances of hydrogen and helium
interpretation: the universe is expanding; everything began together at a point, and a big explosion occurred, causing things to move apart rapidly

•       How old is the Universe?

–      Calculations of the age of the universe depend on the calculation of the Hubble Constant, a number which refers to the rate of expansion of the universe.

–      Controversy arose in Fall 1994
Age of the universe has been calculated to be about 10-15 billion years; calculations show that the age must be less than 20 billion years.
BUT 1994 data from Hubble Space Telescope was interpreted to indicate a high rate of expansion, resulting in an age of only about 8 billion years.
This stirred up a lot of excitement in Astronomy. For details, see the March 6, 1995 issue of TIME Magazine, p. 76-84.

•       Update: April 7, 1997, front page, New York Times. National Academy of Sciences held a colloquium on the age of the universe in March 1997. Many cosmologists now think that the age of the universe is likely to be between 12 and 14 billion years. Later observations from the Hubble Space Telescope observations of Cephids (pulsating stars) have given somewhat lower expansion rates. Lower expansion rates have been suggested by other recent studies as well. This seems to mean that the age of the universe is most likely 15-20 billion years.

•       Origin of the Solar System

•        Planets begin

•       Our Solar System

•       The planets

•       Terrestrial Planets (Inner planets, small, dense, and rocky. No rings. Few or no moons.)

•       Mercury

•       Venus

•       Earth

•       Mars

•       Asteroid belt lies mainly between orbits of Mars and Jupiter.

•       Jovian Planets (Outer planets, large, low density, and gaseous(H, He, CH4). All have rings. All have many moons)

•       Jupiter - largest planet

•       Saturn

•       Uranus

•       Neptune

•       Other planet that does not fit into the two previous categories: Pluto (the outermost planet - small, icy, low density. one moon)

Origin of the Solar System

The Milky Way and our solar system are thought to have originated instantaneously in what’s called the Big Bang. The Universe and all matter as we know is originated from conversion of energy to mass during the tremendous explosion called the "Big Bang."

Formation of the Solar System – Solar Nebula Hypothesis:

1) 10-20 billion years after Big Bang

2) Very cold clouds of H, He, and some heavier elements

3) Wave of gravity (nearby exploding star?) set cloud in motion

4) Cloud began to rotate and contract / condense

5) Rotating cloud assumed disk-like form

6) Contraction / condensation continued due to gravity - generated tremendous heat

7) Eventually, heat and pressure were high enough to produce nuclear fusion in proto-sun.

8) Planetesimals (early form of planets) formed from rotating disk of matter around proto-sun.

Dimensions of the Earth

Major Spheres of the Earth

•       Atmosphere

•       Hydrosphere

•       Lithosphere

•       Biosphere

How do we know what the Earth's Interior is like?

•       Drilling

–      Wells drilled into Earth are mostly in the upper 7 km of the crust

–      Deepest well = Soviet (Russian) well in northern Kola Peninsula 4 year effort to drill a 12 km (12,000 m) hole, starting in 1970.

–      Deepest U.S. well was a gas well drilled in Oklahoma. Drilling starting in 1974 and stopped at 31,441 ft (9,583 m or 9.58 km) when it struck molten sulfur.

–      Germans drilled 3.5 km pilot hole and found bottom temperature was 118 º C (instead of the expected 80 º C)

•       Volcanic activity

–      Materials are brought up from below. Xenoliths = foreign rock (pieces of the mantle in lava); example: coarse-grained olivine (peridotite) xenoliths in basaltic lava. Only useful to depth of about 200 km

•       High pressure laboratory experiments

•       Samples of the solar system (meteorites)

•       Study of seismic waves generated by earthquakes and nuclear explosions

Probing the Earth's Interior with Seismic Waves

•       P and S wave travel times depend on properties of rock materials that they pass through. Search for differences in travel times which will correspond to differences in rock properties.

•       Major layers of the Earth were detected before 1950.
Fine details were delineated in 1960's during nuclear testing.

•       Wave velocity depends on density and elasticity of rock.

•       Seismic waves travel faster in denser rock.

•       Speed of seismic waves increases with depth (pressure and density increase downward).

Probing the Earth's Interior with Seismic Waves

Probing the Interior

Differentiation of early Earth

Planetary differentiation

•       Crustal differentiation

–      Gravity causes heavier elements to sink toward center (Fe, Ni core)

–      Lighter elements "float" upward (Si, O, Al, K, Na, Ca, etc.)

•       Evidence: Earth is differentiated or layered; highest density in center, lower densities progressively outward

•       Crust - rocky outer layer, brittle (5-40 km)

–      Differentiation of crust into:

•       Continental - thick (30-40 km), granitic (sialic)

•       Oceanic - thin (approx. 5 km), basaltic (mafic)

•       Lithosphere (crust and uppermost mantle, 0-100 km deep), cool, rigid, brittle

•       Asthenosphere (upper mantle, 100-700 km deep), hot, weak, solid that flows

•       Mantle - solid rocky layer, dense, high pressure, flows

•       Outer Core - molten Fe-rich

•       Inner Core - solid Fe, Ni

Interior of the Earth

Elements that build the Earth

The Elements of the Crust

Earth's Surface

•       Continents 40% (avg. elevation 840 m above SL; 2750 ft)

•       Ocean basins 60% (avg. depth 3800 m; 12,500 ft)
(Elevations are largely a reflection of their densities - granite vs. basalt)

•       Most prominent features of continents are linear mountain belts

–      Circum-Pacific Belt

–      Alpine-Himalaya Belt

•       Highest point on Earth?

–      Mt. Everest (Himalayas) 8848 m or 29,028 ft

•       Most prominent features of oceans are ocean ridge systems. Continuous belt 65,000 km or 40,000 mi

•       Lowest point on Earth?

–      Mariana Trench -11,033 m or -36,198 ft

Sea Floor Features

•       Contain abyssal plains, deep sea trenches, mid-oceanic ridges and seamounts

Cover about 30% of Earth's surface

•       Abyssal Plain
Flat, deep ocean floor.
Depth may be 2 - 3 miles or more
Thick accumulations of sediment bury topography of oceanic crust

•       Deep Sea Trenches
The deepest part of the oceans
May exceed 10,000 m deep (30,000 feet, or nearly 5 miles)
Deepest is Mariana trench in Pacific Ocean (more than 11,000 m or 33,000 ft)
Occur at subduction zones where oceanic crust is forced downward into the mantle
Associated with earthquakes (Benioff Zones) and volcanoes

•       Mid Oceanic Ridges

•       The global mid-ocean ridge system is the largest single volcanic feature on the Earth, encircling it like the seams of a baseball. Here the Earth’s crust is spreading, creating new ocean floor and literally renewing the surface of our planet. Older crust is recycled back into the mantle elsewhere on the globe, typically where plates collide. The mid-ocean ridge consists of thousands of individual volcanoes or volcanic ridge segments which periodically erupt.

•       Seamounts
Undersea volcanic peaks which formed along mid-ocean ridges or over hot spots
May be eroded flat on top and called guyots
Subsidence occurs after volcanic activity ceases; crust moves away from ridge or off hot spot.
May be ringed by coral reefs called atolls (circular reef surrounding lagoon over now-submerged volcanic peak)