The Standard Model

For a better understanding of the topics covered on this website, a brief overview of the key concepts and processes of modern geology is necessary [Grotzinger/Jordan]:

The mean diameter of Earth is 12,742 km. It consists of core, mantle and crust. The core is divided into a solid metallic inner core and an outer liquid core. Its convective flow leads to a current that creates a magnetic field – a kind of geo-dynamo. This magnetic field is similar to that of a dipole, but it also has a small non-dipole component. The shell is similarity divided: At 2890 km depth, the transition from Earth’s core to the lower mantle happens. The upper mantle begins at about 650 km. The crust of the Earth (lithosphere) consists of the surface rocks, the granitic crust (7-70 km) and the uppermost mantle made of a rock named Peridotite, which slides on the upper mantle.

Geology has been fundamentally reformed in the sixties and seventies of the last century by the acceptance of the theory of plate tectonics. According to the theory of plate tectonics, the lithosphere of the Earth is divided into about a dozen moving plates. The movement of the plates is driven by convection in the mantle. Three types of plate boundaries are discussed: divergent boundaries, convergent boundaries and transform faults. Since the surface of the earth in the course of time is regarded as constant by the prevailing doctrine, an increase in an area (diverging boundaries) corresponds to the destruction of land by subduction (descent) at converging boundaries. Large-scale geological phenomena (transformations) are bound to plate boundaries, such as elongated mountain ranges, volcanoes and earthquake zones.

The lithosphere of the Earth is formed by rocks and minerals. They have a key role in reconstructing the geological history as their composition, their structure, their appearances and their stability allow conclusions about the processes and events that led to their creation. The most common minerals of the earth’s crust are by far the silicates (salts and esters of ortho-silicic acid and its condensates), followed by carbonates (salts and esters of carbonic acid). The carbonate minerals consist of carbonate ions which are bound to calcium or magnesium and also to other elements. Oxidic minerals on the other hand are compounds of oxygen and metals. Sulfides and sulfates consist of sulfur atoms in combination with metals. The chemical composition and the respective crystal lattice establish the physical properties of minerals: hardness, cleavage, fracture, luster, color, density and crystal form.

Rocks are further divided into primary and secondary:

• Igneous rock is primary, the source of all other rocks. It is formed when molten magma cools and crystallizes. This can be done either on the surface of the Earth during eruptions (extrusive) or inside the Earth (intrusive, when so called plutons form). The cooling process within the Earth is slow, and it produces large crystals. On the surface the cooling process is faster, and there are no or only microscopic crystals. Granite and Rhyolite for example are chemically identical, but granite is intrusive while Rhyolite is extrusive. Basalt is also extrusive, but contains significantly less silicates compared to Rhyolite and instead more iron and magnesium.
• Sedimentary rock is secondary. It is caused by weathering, chemical or mechanical erosion of primary rocks and is a fine and partly dissolved material that is first deposited as loose sediment. The conversion process of diagenesis – determined by moderate pressure and temperature, and chemical processes – turns the loose sediment over a longer period of time into hard rock.
• The so-called metamorphic rocks are a third form that also belongs to the secondary group. They develop as recrystallization of other rocks (primary or secondary), substantially without melting. The main causes are excessive heat and pressure inside the earth. Typical creation depths of 10-40 km are indicated. In extreme cases, there is also talk of conversion depths of 100-250 km [Dobrzhinetskaya, 8]. A metamorphosis of rock can be repeated several times.

Rocks are created and destroyed over millions of years. They melt and solidify, they erode and sediment, they are transformed by pressure, temperature and chemical processes. Plate tectonics provides the framework for these processes.

The world ages

The following summary is based on information from the textbook Understanding Geology [Grotzinger/Jordan] and from Wikipedia [wiki/Geologic time scale]:

Eon	Era	Period	Epoch	Age (until millions of years before present)
Hadean				4000
Archean				2500
Proterozoic				542
Phanerozoic				0
	Paleozoic				251
		Cambrian				488,3
		Ordovician				443,7
		Silurian				416
		Devonian				359,2
		Carboniferous				299
		Permian				251
	Mesozoic				65,5
		Triassic				199,6
		Jurassic				145,5
		Cretaceous				65,5
	Cenozoic				0
		Paleogene				23,03
			Paleocene				55,8
			Eocene				33,9
			Oligocene				23,03
		Neogene				2,588
			Miocene				5,332
			Pliocene				2,588
		Quaternary				0
			Pleistocene				0,0117
			Holocene				0

Next: Rock Metamorphism ^[2]

Literature

Carey, Jennifer H. (2009): What’s so great about Granite?; Missoula

Dobrzhinetskaya, Larissa (2011): Ultrahigh Pressure Metamorphism: 25 Years After the Discovery of Coesite and Diamond; Burlington

Grotzinger J. / Jordan T. H. (⁶2010): Understanding Earth, New York

Wikipedia (2012): Geologic time scale; https://en.wikipedia.org/wiki/Geologic_time ^[3]