Pict. 1.10. Halitte                                     Pict. 1.11. Brown coal                          Pict. 1.12. Mineral oil

Petroleum is formed when large quantities of dead organisms, usually zooplankton and algae, are buried underneath sedimentary rock and undergo intense heat and pressure. Petroleum is recovered mostly through oil drilling. This comes after the studies of structural geology (at the reservoir scale), sedimentary basin analysis, reservoir characterization (mainly in terms of porosity and permeable structures) (Báldi 1991, Szakmány 2008a) (1.18. ábra).

3. Volcanoclastites or pyroclastites are sedimentary rocks gearing to explosive volcanism. It contains more than 75% primer volcanic clasts. Components of volcanoclastic rocks are juvenile components, crystals and lithic components.

Pyroclastites can be classified on the base of their grain size when they content more than 75% volcanic clasts (Table 1.4.).

grain size	incoherent sediment	diagenized rock
> 64 mm	block (angular)	pyroclastic breccia
	bomb (rounded)	pyroclastic agglomerate
2 - 64 mm	lapilli	lapillite
0,0625 – 2 mm	coarse grained ash	coarse grained tuff
< 0,0625 mm	fine grained ash	fine grained tuf

Table 1.4. Classification of pyroclastites (Szakmány 2008)

We can determine pyroclastic rocks on the base of their chemism also (for example rhyolite tuff, andesite tuff, basalt tuff).

When magmas reach the surface of the Earth they erupt from a vent. They may erupt explosive or phreatomagmatic. 1. -Explosive eruptions are favored by high gas content and high viscosity. Explosive bursting of bubbles will fragment the magma into clots of liquid that will cool as they fall through the air. 2. - Phreatomagmatic eruptions are produced when magma comes in contact with shallow groundwater causing the groundwater to flash to steam and be ejected along with pre-existing fragments of the rock and tephra from the magma. Because the water expands so rapidly, these eruptions are violently explosive although the distribution of pyroclasts around the vent is much less than in a Plinian eruption.Phreaticeruptions is a type of this. The magma encounters shallow groundwater, flashing the groundwater to steam, which is explosively ejected along with pre-exiting fragments of rock. No new magma reaches the surface.

Pyroclastites origin from the following types of the eruption’s process:

1. Ash falls: When a volcano erupts, it will eject a wide variety of material into the air above it (called pyroclastic fall). The fine material (millimetre-sized ash), which is derived from volcanic glass, rock and crystal particles, can be carried by currents in the eruption column to high above the volcano and pass into the downwind plume to rain out forming ash fall deposits.

2. Pyroclastic flows: If a large volume of volcanic debris is erupted quickly from a volcano, the eruption column can collapse, like pointing a garden hose directly up in the sky. As the eruption column collapses it can transform into an outwardly expanding flood of hot solid ejecta in a fluidizing gas cloud. This is known as a pyroclastic flow. The flow direction may be topographically controlled. Flows often travel at speeds up to 200 km/h, and cause total destruction of the areas they cover. Flows maybe very hot (several hundred ^oC) and can start fires. Some pyroclastic surges are cooler (usually less than 300^oC) and often deposit sticky wet mud.

3. Pyroclastic surges: Pyroclastic surges are low density flows of pyroclastic material. The reason they are low density is because they lack a high concentration of particles and contain a lot of gases. These flows are very turbulent and fast. They overtop high topographic features and are not confined to valleys. However, this type of flow usually does not travel as far as a pyroclastic flow. Pyroclastic surges can travel up to at least 10 kilometers from the source. There are three types of pyroclastic surges: 1) base surge, 2) ash cloud surge, and 3) ground surge. A base surge is usually formed when the volcano initially starts to erupt from the base of the eruption column as it collapses. It usually does not travel greater than 10 kilometers from its source. A ground surge usually forms at the base of a pyroclastic flow. http://www.geo.mtu.edu/volcanoes/hazards/primer/images/volc-images/basesurge.jpgAn ash cloud surge forms when the eruption column is neither buoying material upward by convection or collapsing (Báldi 1991, Szakmány 2008a).

1.4.3. 1.4.3. Metamorphic rocks