Les magmas siliciques étant plus froids que les magmas mafiques, les gaz dissous ont plus de difficulté à s'en dégager. Cela rend les magmas siliciques plus dangereux : plus le gaz s'accumule dans le magma, plus il devient explosif.
Le Piton de la Fournaise contient des magmas de basalte particulièrement pauvres en silice, ce qui signifie qu'ils sont beaucoup moins explosifs. Au lieu de cela, ils suintent et éclaboussent les proches alentours, créant des volcans boucliers - des formations fortement inclinées devenues des éléments distinctifs de nombre d'espaces insulaires.
Les différents types de coulées de lave
Les coulées qui descendent les pentes du volcan peuvent prendre deux formes différentes, typiques du volcanisme basaltique : elles peuvent être lisses, dites de type pāhoehoe, terme hawaïen signifiant « rivière de satin » ou en grattons, dites de type ʻaʻā, également un terme hawaïen signifiant « lave de pierre ». Au départ, il s'agit de laves de même composition : ce sont généralement des basaltes aphyriques ou à olivine dont certains, très riches en olivine, peuvent être qualifiés d'océanite. Si l'écoulement se produit sans heurts, la lave reste lisse en surface et en se refroidissant forme des figures de draperies (on parle de laves cordées). Si l'écoulement est plus brutal, la libération des gaz forme en surface une sorte de « mousse » de blocs irréguliers : les gratons. À tout moment une lave de type pāhoehoe peut se transformer en lave de type ʻaʻā, alors que l'inverse est impossible.
La vitesse d'avancement d'une coulée dépend du débit et de la pente. Cependant le front de coulée se solidifiant au contact du sol froid ne peut progresser qu'assez lentement (au plus quelques km/h). En revanche, dès lors qu'une coulée a tracé son emprise, la lave peut y circuler à grande vitesse (plusieurs dizaines de km/h).
Certaines coulées qui sont alimentées abondamment ou qui proviennent d'éruptions à basse altitude atteignent le rivage et se jettent dans l'océan. Le contact avec l'eau salée provoque un panache de vapeur d'eau condensée et la formation de gaz irritants, notamment d'acide chlorhydrique. Les laves qui continuent à s'écouler et à s'ébouler sous l'eau se refroidissent en formant des pillow lavas, tandis qu'une plate-forme rocheuse se construit peu à peu agrandissant le contour terrestre de l'île.
EARTHCACHE RÉUNION EARTHCACHE RÉUNION EARTHCACHE RÉUNION
Questions pour VALIDER CETTE EARTHCACHE
1. Devant quel type de coulée vous trouvez-vous ? Expliquez votre réponse.
2. En vous appuyant sur votre observation précédente et en vous aidant du descriptif de la page expliquez la formation de ce type de coulée.
3. Pouvez-vous observer un autre type de coulée dans l’enclos ? Si oui dans quelle direction. A) de la falaise B) de la mer C) du cratère Léo Formica D) du cratère Bory. Expliquez votre réponse
4. Touchez le sol au pied de cette coulée ; la taille des particules (appelée des scories) correspond à: A) du sable B) du gravillon C) des morceaux de la taille d’un poing D) des morceaux de la taille d’une noix. De quelle couleur sont-ils en majorité ? A votre avis pourquoi?
5. Une photo de vous, de votre GPS ou d’un objet personnel avec le cratère Léo Formica en arrière-plan. Voir les nouvelles règles des earthcaches : Guidelines Earthcaches
Loguez cette cache et envoyez-moi vos propositions de réponses soit via mon profil, soit via la messagerie geocaching.com. Je vous contacterai en cas de problème.
The Piton de la Fournaise reaches an altitude of 2632 metres. It is the only active volcano on Reunion Island. It was formed on the southeast side of the Piton des Neiges. For more than 500,000 years, these two volcanoes have been active simultaneously. But it has been 12,000 years since the Piton des Neiges was extinguished.
It is a conical-shaped volcano, with a vast volcanic crater in the middle. To date, it covers 26% of the island's surface area. It is a shield volcano because it is extensive and low.
L'enclos Fouqué was named in honour of a famous geologist, Ferdinand André Fouqué, who was particularly interested in volcanic phenomena and earthquakes, minerals and rocks. This enclosure (also called caldera), in the "U" form, is completely surrounded by cliffs. The highest part has two craters: the Bory and the Dolomieu. It is 9 kilometres wide and 13 kilometres long.
The difference between magma and lava
The distinction between magma and lava is a simple matter of location. When geologists refer to magma, they refer to the molten rock that is still trapped underground. If this molten rock reaches the surface and flows along the walls of the volcano in liquid form, it is requalified as lava.
Magmas vary in their chemical composition, which gives them - them and the volcanoes that contain them - distinct properties.
- Mafic magmas such as those in Reunion tend to form when the heavier crust, which forms the ocean floor, melts. They contain between 47 and 63% silica, the mineral that makes up glass and quartz. As for melted rocks, mafic magmas are quite liquid, with viscosities ranging from molasses to peanut butter. They are also the hottest variety of magmas, reaching temperatures between 980°C and 1,200°C.
- Silica magmas tend to form when the lighter continental crust melts. These magmas contain more than 63% silica, which gives them a more viscous consistency: at higher concentrations, silicic magmas flow little or not at all. They are also less hot than mafic magmas. Rhyolite, a lava particularly rich in silica, reaches temperatures between 650°C and 815°C.
Since silicic magmas are colder than mafic magmas, dissolved gases have more difficulty in releasing themselves from them. This makes silicic magmas more dangerous: the more gas accumulates in the magma, the more explosive it becomes.
The Piton de la Fournaise contains basalt magmas that are particularly low in silica, which means that they are much less explosive. Instead, they ooze and splash nearby, creating shield volcanoes - steeply sloping formations that have become distinctive features of many island spaces.
The different types of lava flows
The flows that descend the slopes of the volcano can take two different forms, typical of basaltic volcanism: they can be smooth, called pāhoehoe, Hawaiian term meaning "satin river"; or scratching, called ʻaʻā, also a Hawaiian term meaning "stone lava";. Initially, they are lava of the same composition: they are generally aphyric or olivine basalts, some of which, very rich in olivine, can be qualified as oceanite. If the flow occurs smoothly, the lava remains smooth on the surface and as it cools forms drapery patterns (known as corded lava). If the flow is more abrupt, the release of gases forms on the surface a kind of "foam"; of irregular blocks: the scrapers. At any time a pāhoehoe type lava can be transformed into ʻaʻā type lava, while the opposite is impossible. Look at the picture above
The feed rate of a flow depends on the flow rate and slope. However, the pouring front solidifying in contact with the cold ground can only progress fairly slowly (at most a few km/h). On the other hand, once a flow has traced its path, lava can travel at high speed (several tens of km/h).
Some flows that are abundantly fed or result from low-level eruptions reach the shore and flow into the ocean. Contact with salt water causes a plume of condensed water vapour and the formation of irritant gases, particularly hydrochloric acid. The lava that continues to flow and collapse underwater cools to form pillow lavas, while a rocky platform is gradually being built to expand the island's land contour.
Questions to VALIDATE THIS EARTHCACHE
1. What type of flow are you facing? Explain your answer.
2. Based on your previous observation and using the description on the page, explain the formation of this type of flow.
3. Can you see another type of flow in the enclosure? If so, in which direction. A) the cliff B) the sea C) the Leo Formica crater D) the Bory crater. Explain your answer
4. Touch the ground at the bottom of this flow; the particle size (called scoria) corresponds to: A) sand B) gravel C) pieces the size of a fist. What color are they mostly? In your opinion why?
5. A picture of you or your GPS with the landscape of Léo Formica in the background. Look the news rules of earthcache Guidelines Earthcaches
Send me your proposals for answers either via my profile or via the geocaching.com messaging (Message Center) THEN log this cache "Found it", I will contact you in case of problem.