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Volcanoes
These iconic pointy mountains
are characteristic of many states
such as Hawai'i and Alaska and parts of the Pacific Northwest
They look different depending on where we see them
Why is that?
Lets find out.
The type of volcano can help geologists interpret how violent the next eruption might be and to evaluate the potential risk for different types of volcanic hazards.
Our learning objectives for this lesson are that you will be able to tell the difference between some common types of larger and smaller volcanoes.
Let’s start with a definition:
A volcano is a cone shaped hill or mountain created by the eruption of magma in the form of lava, tephra and other debris.
The presence of a volcano is indicative of an underlying magma source in the crust or mantle, escaping gases that help drive the eruption,
a range of potential volcanic hazards, and one or more eruptions needed to build the cone.
We will begin by examining the distribution of active volcanoes on Earth’s surface as represented by the green triangles on this spinning globe.
These are dominated by two main types of volcanoes, the gently-sloping shield volcanoes, and the steeper slopes of composite volcanoes.
Most of those triangles near the middle of the oceans are probably shield volcanoes
which typically form at hot spots and along oceanic and continental rift systems.
That line of green along the rim of the Pacific Ocean and along the southern boundary of the Eurasian plate
represents chains of composite volcanoes that formed above subduction zones at convergent plate boundaries.
We will describe shield volcanoes first.
Shield volcanoes have a distinctive low angle profile
that forms broad triangular landforms that are thought to resemble an overturned warrior’s shield.
These volcanoes are built up by a series of lava flows
that can emerge from vents or from networks of fissures, or deep cracks, along the volcano’s flanks.
These lava flows are thin and fluid and can flow easily.
For example, let’s take a few moments to watch this time lapse video of lava emerging from fissures near the Hawaiian coast.
Notice how thin each flow is.
Lava from shield volcanoes can travel long distances along a series of lava tubes
or channels that can carry the hot lava tens of kilometers before it can cool off and solidify.
The largest shield volcanoes make up the main island of Hawai’i.
The biggest volcano, Mauna Loa, is more than 100 km across and it rises more than 9 kilometers from the sea floor making it taller than Mt. Everest.
That’s one impressive volcano.
But keep in mind that many shield volcanoes are much more modest in size (a little more on that later).
In contrast to shield volcanoes, composite volcanoes have a distinctive steep triangular profile.
Eruptions of composite volcanoes are much more violent than those of shield volcanoes,
and are characterized by the production of a mix of tephra and thick, viscous lava.
In addition, some of the magma will force its way into the interior of the volcano
and provide support for the structure’s growth
Volcanic rocks in composite volcanoes are more silica rich, resulting in more viscous lava
and more violent eruptions often separated by hundreds or thousands of years.
Many volcanoes look pretty big up close,
but it is important to keep a sense of scale in mind when we are thinking about classifying volcanoes.
Shield and composite volcanoes are an order of magnitude larger
than typical smaller types of volcanoes such as cinder cones and lava domes.
These figures are approximate but we measure the width of shield and composite volcanoes
in 10’s of kilometers and their heights in thousands of meters.
In contrast, their smaller cousins have heights measured in hundreds of meters.
One way to distinguish these volcanoes from each other is to compare the size of the volcano to nearby vegetation.
Images of larger volcanoes are often show them surrounded by forests.
In contrast, you can often make out individual trees in images of cinder cones and lava domes.
Let’s take a closer look at these volcanoes.
These small cones can be found on their own
or on the flanks of larger volcanoes and are typically formed by a single eruption.
They are composed of smaller pieces of tephra
produced when lava is blasted into the air and cools as it falls to ground.
The cooled lava fragments often contain air bubbles and are known as scoria.
Lava domes form when the most viscous lava oozes out at the surface
and slowly crawls along and cools to form steep-walled, bulbous domes.
The steep slopes along the dome flanks may collapse to create dangerous hazards such as pyroclastic flows.
While lava domes often form in the craters of composite volcanoes,
they can also be found in other volcanic environments where high-silica magmas are generated.
Take a look at these sped up time lapse images of the growth of a lava dome in the crater of Mount St. Helens
over a period of nearly three years. Notice the steepness of the slopes.
This looks fast because it all happens in less than 30 seconds but it actually represents very slow, gradual movements.
Now, let’s see how well you have assimilated all this information.
We are going to show you five pictures of different types of volcanoes
and you will have four seconds to classify the image as a shield or composite volcano or a cinder cone or lava dome.
Jennifer will be playing along with you, she hasn’t seen these images before either, let’s see how you all do.
Okay here comes the first image.
Composite Volcano
Cinder cone
Shield Volcano
Lava Dome
And...
Shield Volcano
Alright lets look and see how well you did
Oh, I got one wrong!
Not bad!
We had two learning objectives for this lesson.
How confident are you that you could accomplish each of these tasks?
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How to Classify Volcanoes

845 タグ追加 保存
羅紹桀 2015 年 11 月 3 日 に公開
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