Test Review 2
5.6 What processes influence the composition of a magma?
- Felsic Crystals float to TOP
- Mafic Crystals sink to BOTTOM
2. Mixing: When two different magmas come in contact with one another
- Creates INTERMEDIATE ROCK
3. Assimilation: When a magma chamber is hotter than the melting temp of the felsic
rock separating two chambers and melts through the wall and melts the rock into the
next magma chamber
- Host rock is dislodged, melts and is incorporated into magma body
5.5 What causes a rock to melt?
1. Decompression: Pressure is released; causing the (at least slightly hot) rock/magma
to uplift so quickly that it does cool.
- Ex: Ocean Ridge/ Divergent Boundary
2. By Water: Adding water lowers melting temperature of rocks
- Will see partial melting in subduction zone with water (Ocean-ocean convergent
3. Heating: Raise the temperature and some rocks will melt
- Raise the temperature and raising the pressure will cause the rock to stay solid
because: pressure acts to keep rocks solid
- Heating by burial is in SUBDUCTION ZONES when magma rises
5.9 What types of igneous rocks form along mid-ocean ridges? 1. Pillow Basalts: Upper part of oceanic crust consists of basaltic flows
- When magma is introduced to water
2. Sheeted Dikes: Thin, vertical intrusions of finely crystalline basalt through which
3. Gabbro (magma chamber): Form deep in oceanic crust (mid ocean ridge)
A. How is magma generated (melting) along subduction zones?
- When an oceanic plate or lithosphere mantel converges with another one of
those plates; pressure and temperature increase and the subducted plate
- Because of the increase in temp and press. The minerals convert into new
ones. Magma is then created and may crystallize at a depth of erupt at surface.
B. What magmatism accompanies continental collisions?
- Have high water content, do not reach the surface (crust is too thick): Do not
have many volcanoes at convergent boundaries
5.11 What type of magmatic activity occurs at hot spots?
1. Hot spots and mantle plumes
2. Hot spots in oceans
3. Hot spots in continents
5.7 What controls how easily magma moves?
a. Low temperature: Flows with difficulty- VERY VISCOUS
b. High temperature: Flows very easily- LESS VISCOUS
- Mafic magma is hotter than felsic magma and less viscous than felsic
2. Precentage of Crystals:
a. Abundant Crystals: Flow slow. More viscous
b. Few Crystals: Flows easily. Less viscous
3. Composition: a. Abundant Silicate Chains: Very viscous
b. Few Silicate Chains: Less viscous
5.8 Under what conditions does magma solidify?
a. Slow Cooling: Coarse-grained granite
b. Medium Cooling: Medium-grained granitic rock
c. Very Fast Cooling: Very fine-grained rhyolite
d. Slow then Fast Cooling: Porphyritic Intrusive Rock
5.12 How do large magma chambers form and how are they expressed in landscapes?
a. Irregular Plutons: LARGE SCALE
- Intrusive, high resistance to erosion, usually steep
b. Sheetlike Plutons: LARGE SCALE
- Magma chambers that have spread out
c. Batholiths: LARGE SCALE
- Multiple plutons; cover huge regions over long periods of time
- Ex: Sierra Nevada’s
5.13 How are small intrusions formed and expressed in landscapes?
- Dike: Magma intrudes perpendicular to sediment beds (Cuts across any layers
present in the host rock)
- Sill: Come up through cracks, along or parallel to sediment plains (Parallel to
layers in host rock)
6.1 What are the characteristics of a volcano?
- Craters, volcanic rocks, classic cone shape, vent (where magma comes out)
- Evidence that indicates an eruption: Layer of ash on hillslopes or lava and ash flows
6.1 What are some different types of volcanoes?
1. Scoria/ Cinder Cone: - Small, flat top, “cinders make up the size of particles and scoria is the type or
2. Shield: MAFIC/BASALTIC ROCK (DARK)
- Largest, highest, widest
- Lava flow
- Starts all the way at the bottom of the ocean
3. Composite: INTERMEDIATE AND FELSIC AND MAFIC ROCKS
- Classic shape (stereotypical volcano)
- Steep sides, crater at top and lava flows at base
- Pyroclastic flow
4. Volcanic Dome: FELSIC/INTERMEDIATE
- Ash and lava solidify, viscous magma (very slow)