Effusive volcanic eruptions are outpourings of low-viscosity lava from a volcano. The low viscosity of the erupted material allows for the slow dispersal of the lava in large sheets across Earth’s surface. An example of effusive eruptions is the large lava flows that creep across the surface of Hawaii. This eruptive style is comparatively calm, easy to avoid, and described as low energy. Rocks formed from effusive eruptions have very small crystal sizes due to rapid cooling. As such, the cooled lava has what is called a massive texture that is relatively featureless with no or little structure. However, gasses dissolved in this type of low-viscosity lava are able to escape the liquid, so small holes or vesicles produced by the escaping gas are often present in the rock.
Explosive volcanic eruptions occur when high-viscosity lava is suddenly released from a volcano. This eruptive style imposes significant violence upon Earth’s surface in a high-energy event. An example of an explosive eruption is the catastrophic eruption of Mount St. Helens. Rocks produced by explosive eruptions also have small crystal sizes due to rapid cooling, but they are chaotic and preserve features of the high energy of their deposition.
We can now use those basic features we identified in the outcrop to classify the two volcanic units. Our goal is to use our observations to recognize the processes that formed the volcanic units, from which we can then classify the two units.

Drag the appropriate items into their respective bins. Each item may be used only once.

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clearcutting reproduces a new even-aged stand by completely removing the mature stand.

clearcutting as a silvicultural treatment has several characteristics:

it causes a sudden environmental change

it removes the seed source from the regeneration area

the configuration of the clearcut may affect seed dispersal

it begins a new rotation

it will eliminate some pests that require forest cover

overall, it temporarily removes the forest cover, transforming the forest community and environment.

clearcutting followed by artificial (seeded or planted) regeneration has several advantages:

we can avoid delay in restocking the site

we can introduce a selected species, seed source, and genotype

we can control arrangement and spacing

we can achieve uniformity in the new stand

we can overcome the problems associated with securing sufficient natural regeneration.

the shelterwood method involves the removal of most of the mature stand at the end of the rotation, but a portion of the mature stand is left standing.   the shelterwood method serves three basic purposes:

to prepare the stand for production of abundant seed

to modify the environment in a way that promotes germination and survival of the selected species

to build up the amount and size of advance regeneration to ensure the prompt restocking of the new stand following overstory removal.

the shelterwood method involves a sequence of 3 cuttings:

preparatory cuttings: make the seed trees more vigorous and set the stage for regeneration. see our thinning page for more information.

establishment/seed cuttings: open up enough vacant growing space to allow establishment of the new regeneration.

removal cuttings: uncover the new crop to allow it to fill the growing space.

the residual trees in the shelterwood must:

be sturdy and windfirm

be able to survive exposure

flower and reproduce seed

be the best trees of the mature stand

keep in mind that the shelter trees must not occupy the entire site.   a void must be created in order to make room for the new regeneration. the amount of shelter to leave will depend on the following factors:

species characteristics (seeding and shelter requirements)

the number of cuttings in the cutting sequence (1, 2, or 3)

landowner objectives

the size of the area to be regenerated

the final removal harvest level required to be profitable

the shelter must be removed before it impedes the growth of the new stand or threatens its survival.

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