Volcanology. The study of volcanoes

Transcription

1 Volcanology The study of volcanoes

2 Magma forms wherever temperature and pressure are high enough to melt rock. Some magma forms at the aesthenosphere Magma also forms at plate boundaries, where intense heat and pressure develops from friction between the plates.

3 Volcanoes Melted rock expands as it heats up, becoming less dense than surrounding rock. Magma moves upwards through fractures; If magma reaches the surface, it erupts through an opening called a volcano.

4 Types of Magmas Felsic magmas: high silica, thick, light-coloured, slowmoving Mafic magmas: low silica, thinner, dark-coloured, flow more easily

5 Volcanic Eruptions Magmas contain dissolved gases (i.e., water vapour, carbon dioxide) As magma reaches the surface, pressure is reduced and dissolved gas comes out of solution as bubbles of gas. Bubbles expand and explode therefore LOTS OF GAS = MORE EXPLOSIVE ERUPTIONS!

6 Mafic magmas are more fluid and let gases escape more easily than felsic magmas MAFIC MAGMAS = LESS EXPLOSIVE FELSIC MAGMAS = MORE EXPLOSIVE

7 Lava and Lava Fragments If magma reaches the surface of the earth, it is called lava Explosive eruptions produce lava fragments called TEPHRA TEPHRA CLASSIFICATION: Ash Lapilli < 2mm Bombs >64 mm 2 mm 64 mm

8 Pyroclastic Flow in explosive eruptions, tephra combines with gases to form a dense, superheated cloud traveling at very high speeds (> 100 km/h)

9 Volcano Hazards 1. Ash Gritty sand-sized particles blasted from erupting volcano Can reach very high altitudes. Large amounts can block sunlight, causing world temperatures to drop. Can destroy crops, buildings, clog rivers, damage machines.

10 Volcano Hazards 2. Lava Molten rock may flow over large areas, destroying everything in its path.

11 Volcano Hazards 3. Pyroclastic Flow Destructive mix of superheated gas, ash and debris which can move faster than 100 km/h

12 Volcano Hazards 4. Lahar Flow of mud, water, ash and debris that can result when snow-covered volcanoes erupt

13 Types of Volcanoes There are three main ways volcanoes are formed: Convergent boundaries Divergent boundaries Hot Spots

14 1. Convergent Boundaries (Composite Volcanoes) As ocean plate sinks beneath the continental plate, increasing heat melts the rock, forming magma. Magma is lighter than the surrounding solid rock, so it rises up through the edge of the continental plate to form a volcano.

15 1. Convergent Boundaries (Composite Volcanoes) Form classic, cone-shaped volcanoes that erupt ash and lava Cone shape results from layers of ash and lava building up over time Magma is usually felsic (very thick), and traps gas producing explosive eruptions i.e, Mt. Baker

16 2. Divergent Boundaries (rift eruptions) Magma flows out of volcanoes at mid-ocean ridges; occasionally, volcanoes grow high enough to rise above the surface of the ocean and produce islands i.e, Iceland

17 3. Hot Spots (Shield Volcanoes) Sometimes extremely high temperatures are found beneath the middle of oceanic plates, in areas called hot spots. At these locations, the melting of crustal rock produces magma that rises up to form a volcano.

18 3. Hot Spots (Shield Volcanoes) As the plate moves, it carries the hot spot with it, so that chains of volcanic islands are formed Magma is usually mafic, so eruptions are not explosive and volano s sides have a shallow slope. i.e., Hawaii

19 Plutons and Volcanism Magma squeezes through fractures and rock layers as it rises. Igneous rock masses that form inside other rocks are called PLUTONS. Structures are named according to size and orientation.

20 Dikes sheets of igneous rock that cut across the rock layers they intrude (usually mafic)

21 Sills sheets of igneous rock that are parallel to the rock layers they intrude (also usually mafic)

22 Laccoliths Dome-like masses of igneous rock formed by stiff magma which, instead of spreading, bulges upwards (usually felsic)

23 Volcanic Neck Plug of magma that hardens inside the vent of a volcano

24 Batholiths and Stocks Batholith: Largest of intrusions; forms the core of many mountain ranges (usually felsic) Stock: Small batholith (less than 100 square kilometres exposed at surface)