57
votes
Why is the pond in my backyard not frozen when it is -15 °C (5 °F) outside?
Water is a rather strange substance. With most substances, the solid phase is denser than is the liquid phase. This is not the case with water. Ice is less dense than liquid water. A side effect of ...
- 22.4k
55
votes
Accepted
Does gravity increase the closer to the core you get?
The below figure, taken from Wikipedia shows a model of the free fall acceleration, i.e., 'gravity'. The left-most point corresponds to the center of the Earth; then further right at $6.3\cdot1000$ km ...
- 1,979
53
votes
Accepted
Why is the pond in my backyard not frozen when it is -15 °C (5 °F) outside?
Water melts at 0 °C (32 °F) but freezing is a more complicated affair. It is safe to say water gains the ability to freeze at 0 °C, but it can get much cooler before it actually does so resulting in ...
- 14k
42
votes
Accepted
Where on Earth is the magnetic field intensity stronger?
If we consider only the magnetic field generated by natural sources, and not the ones generated by human activities. The general trend is that higher intensities of the magnetic field happen close to ...
- 17.3k
36
votes
What is the pressure at the center of the Earth?
It is the pressure gradient that is proportional to the local gravitational force. When that force is integrated over a distance, the pressure gradient is integrated to accumulate a total pressure.
...
- 3,271
31
votes
What are these undersea lines all around Hawaii?
Short answer: The lines radiating from the island are artifacts from a mapping process that superimposes high-resolution sonar data from standard ship tracks on top of r low- or average sonar data ...
- 3,189
28
votes
Accepted
Where is the calmest place on Earth?
The main resistance that winds have to their movements comes from the topography and surface obstacles. Therefore, as a general rule the closer to the surface the less wind you will find. But I guess ...
- 17.3k
26
votes
Accepted
Is it true that earthquakes are not felt in a cave?
Ground motion results due to passage of elastic waves. Now there are different kinds of waves, e.g., P waves, S waves, surface waves, etc. Most of the shaking (and therefore damage) is caused by ...
- 2,381
23
votes
What would be the first thing which will render the Earth uninhabitable?
I'll try to put time scales on each of these events.
The Sun as a red giant - 5 to 6 billion years$^1$
The Sun is currently on the main sequence, which means that it's a "full grown" star - think of ...
- 898
23
votes
Accepted
Why does sea level correspond to boundary between oceanic and continental crust?
Sea-level only sort-of corresponds to the oceanic–continental crust boundary.
In depth, they don't correspond at all:
It's the same story at an active margin: the plate boundary at a subduction zone ...
- 10.9k
22
votes
Why is the pond in my backyard not frozen when it is -15 °C (5 °F) outside?
Due to convection (the cold water sinks while the warm water rises), the entire pond needs to be brought to near-freezing temperatures before the surface can freeze. With only the top of the pond in ...
22
votes
Accepted
Does gravity differ from place to place on Earth
There is an entire field of Geophysics called gravimetry dedicated to measuring the magnitude of the gravitational field.
First, we should distinguish between weight (a force) and gravity (an ...
- 15.3k
22
votes
Accepted
Taking into account the decay products of transuranic elements, are the world's uranium reserves growing or shrinking?
Let's start with the easy question:
given our current known reserves of transuranic elements and again taking into account their decay products, where will the greatest uranium reserves be in, say, ...
- 22.8k
20
votes
How to quickly or easily prove the world is spherical?
I'm sorry that this answer will be unsatisfactory and, strictly spoken, off topic, but I want to give it anyway.
The answer is: You can't. Don't waste your time. Your relative has developed a paranoia....
19
votes
Accepted
How to quickly or easily prove the world is spherical?
To answer my own question based on the useful comments here, my favorite methods were:
The earth's shadow cast on the moon during an eclipse. It is easy to observe and the flat earth counter argument ...
- 401
19
votes
Accepted
Can ultrasounds be used to map the density of large underground regions?
Sound-like waves are routinely used to image the subsurface, but mostly well below the ultra-sound band.
Several methods involve sound-like vibrations:
Reflection seismic — the most important ...
- 10.9k
18
votes
Accepted
Change in earth mass since the time of the dinosaurs
I gather, but may be wrong, that the mass of earth at present increases by around 108kg/day. All else being equal, one would expect the earth to have gained a mass, since 75 million years ago, of:
$$ ...
- 22.4k
17
votes
Is it true that earthquakes are not felt in a cave?
There are three main types of waves produced during an earthquake: P, S, and L waves, which stands for Primary, Secondary, and Love. (There was a mnemonic I read many years ago that went P=pressure, S=...
- 2,157
17
votes
Accepted
How bad is geo-engineering?
The bad part about geo-engineering are the unknown unknowns, to paraphrase a certain US politician.
Our climate models are wrong. All models are wrong, but some are useful.. Our models are useful, ...
- 11.4k
17
votes
Why is the temperature between the earth core and surface not distributed linearly?
The temperature does indeed increase with depth, something that is a problem in deep mines or deep drilling, but a benefit for geotermal heating. The heat originates mostly from radioactive decay, but ...
- 5,926
17
votes
Accepted
How did the Ural mountains form?
The Ural mountains are one of the oldest mountain ranges on Earth. They started forming about 300 Ma ago by the subduction of the oceanic crust once attached to the Kazakhstania plate under the ...
- 17.3k
15
votes
Why does New Zealand and cold countries have volcanoes?
Surface temperatures on Earth are a consequence of solar heating and radiative cooling - in other words, the heat energy which drives climate comes from sunlight falling on the Earth "now".
On the ...
- 631
15
votes
Is it true that earthquakes are not felt in a cave?
Googling leads to a very old document: EFFECTS OF AN EARTHQUAKE IN A MINE AT TOMBSTONE, ARIZONA stating that in many cases, mine workers did not notice earthquakes which were felt above ground.
A ...
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15
votes
Why is the pond in my backyard not frozen when it is -15 °C (5 °F) outside?
The water in a pond is in contact with the ground and the ground is not even close to freezing even if the air temperature is 27° below freezing.
- 2,157
14
votes
Why Vp/Vs and not Vs/Vp?
After quite a bit of conversation on Twitter, I think it's arbitrary. Tradition is probably the prevailing reason.
It would be interesting to go back through the literature to see who first used it. ...
- 10.9k
14
votes
Are there water molecules in the oceans which will almost never fall down as rain?
Never is a very, very long time. Almost never is a vague term.
The age of deep ocean water has been studied with both models and observations, and both indicate a lower limit age in the order of ...
- 11.4k
13
votes
Are the oceans rising or the continents going down? How can we know?
To the best of our knowledge, sea-level is rising because the volume of water is increasing.
There is substantial local variation in sea-level change; it's falling in some parts of Canada. But of the ...
- 10.9k
13
votes
Is it possible that the recent droughts are signs of epic crust failure?
Is it possible that the recent droughts are signs of epic crust failure?
No. Even though your 5 points do not make much sense, I'll try to answer it anyway.
There are no continental plates. There ...
- 22.8k
13
votes
Accepted
Why is the South Pole colder than the North Pole?
The most obvious reason why the South pole is colder than the North pole is altitude. The Arctic is mostly covered by ocean and low lying land whereas the Antarctic continent is really quite high, ...
- 2,211
Only top scored, non community-wiki answers of a minimum length are eligible