Chapter 11 The Dynamic Planet. The seemingly static landscape and mountains are constantly being shaped and reshaped by endogenic and exogenic forces, although slowly. Given enough time, any slow process can make a difference. . The Dynamic Planet. The Pace of Change
The seemingly static landscape and mountains are constantly being shaped and reshaped by endogenic and exogenic forces, although slowly. Given enough time, any slow process can make a difference.
Popular time: 1500 BC, 2009AD
Years Before Present (BP):
Earth age: 4.6 billion years
condensed and congealed from a nebula
of dust, gas and icy comets.
Scales of Geologic Time
Protero-: former, anterior
Phanero-: visible to the naked eye
nucleus: protons (+) & neutrons
For some matter, its protons and neutrons do not stay together indefinitely. The particles can break away and the nucleus can disintegrate and form new matter, in the mean time release radioactive energy. The decay rate is measured by half time, the time needed for the matter to decay half of its mass into new matter.
Measuring the ratio of original matter and its decayed matter, one can estimate the age of the object.
The oldest rock dated: 4.3 b.y.a, indicating continental crust was forming this many years ago.
Mantle includes lower and upper (upper mantle, asthenosphere and uppermost mantle) mantle, representing 80% of Earth volume. Temp increase with depth. Rich in oxides of iron, magnesium and silicates.
Note the density gradient
As Earth solidified, gravity sorted materials by density. Heavier substances (e.g. iron) gravitated toward the center; lighter elements (e.g. silica) welled upward to the surface.
Outer core generate 90% of Earth magnetic field to form magnetosphere, which protects Earth from the solar wind and cosmic radiation.
Inner core remains solid iron because of tremendous pressure though temp well above melting point,
Continental crust: primarily granite high in silica, aluminum, potassium, calcium and sodium
Oceanic crust: primarily basalt high in silica, magnesium and iron
Lithosphere: Crust + uppermost mantle.
Asthenosphere: plastic layer (least rigid in mantle), contains hot spots causing vertical convection, creates tectonic activity
Scientists cannot dig that deep. The deepest hole scientists ever dug is 12.23km (20 years of effort!)
Scientists infer the deep Earth structure indirectly through seismic tomography. The rate of transmittance of seismic waves depends on the density of the structural material. Rigid matter transmits the seismic waves faster. Plastic zones simply do not transmit certain seismic waves. Some seismic waves are reflected when density changes, whereas others are refracted (or bent) as they travel through Earth.
Continental crust is lightest, “floating” on denser layers. When there is heavier loads, such as mountains, glaciers, , the crust “sink” deeper (like a boat loaded with cargo). Unloading these cargos will results in isostatic rebound as shown here.
GPS can be used to study the rate of isostatic rebound. A group of scientists from UAF found that southern Alaska is rebounding much faster than they thought because of melting of glaciers
The cycle of matter within the Earth system caused by exogenic and endogenic forces.
Eight natural elements make up 99% of Earth’s crust! Oxygen and Silicon make up 74.3%. There are more Oxygen in the crust (47%) than in the atmosphere (21%) !
First person to notice the apparent fit of some continental coastlines was a Geographer (Abraham Ortelius) in 1500s.
The “continental drift” concept was formally introduced by a German geophysicist, Alfred Wegener in 1915. The Earth had a single giant continent, Pangean 225 MBP, then drifted apart as we have today.
Wegener’s continental drift concept was confirmed in the 1960s.
A remarkable feature of the sea floor:
An interconnected worldwide mountain chain, forming a ridge ~64,000 km in length and ~1,000 km in width.
Evidence of sea floor spreading:
1. The magnetic particles orient themselves in line with the magnetic fields when the
lava appeared and its orientation is frozen in the rocks.
2. Radioactive dating: the farther away from the ridge, the older the age of the rocks.
The oldest sea floor rock is 208 MBP (quite young compared to 4.6 Billion years of Earth).
Three kinds of plate boundaries:
7 major plates:
Arrows represent 20 million years of movement.
: upwelling material arrive at the surface
Plate boundaries are the primary location of earthquake and volcanic activity
Another hot spot: Yellow Stone National Park: a mega-magma chamber beneath which explodes 620,000 years. It can explode in the next 20,000 years. It is so big its ashes can cover half the country.
An Introduction to Physical Geography