An	aerial	view	of	Kilauea	volcano,	Hawaii,	showing	the	summit	caldera.	Halemaumau	Crater	within	the	caldera	is	about	1	km	in	diameter.		Although	volcanoes	are	topographically	high	areas,	their	summits	are	often	marked	by	areas	of	subsidence,	forming	depressions.		Smaller,	depressions	where	the	magma	commonly	erupts	from	are	called	craters,	but	larger	areas	of	subsidence	are	referred	to	as	calderas.		Calderas	are	described	on	Ch.	4,	p.68. Magma	vs.	lava:		Magma	is	a	term	for	any	high	temperature	liquid	originating	in	the	Earth’s	interior	(molten	rock).		Lava	is	when	magma	erupts	onto	the	surface.
Chapter	4	starts	with	an	“In	the	Field”	segment	that	includes	some	text	and	an	image	on	the	Kilauea	volcano	in	Hawaii.		The	top	left	image	comes	from	Fig.	4.1,	and	shows	volcanologists	making	observations	a	short	distance	from	the	Pu'u	O'o	cone	on	the	slope	of	the	Kilauea	volcano.	Red-hot	lava	fountains	high	above	the	cone	and	feeds	lava	flows	with	a	dark,	solidified	crust.		The	lavas	flow	very	easily;	they	are	very	fluid.		A	way	of	describing	the	ability	of	a	fluid	to	flow,	is	a	property	called	viscosity.
Two	main	type	of	lava	flow	develop	in	Hawaii:	pahoehoe	and	A’a.		These	are	also	knows	as	ropy	and	blocky	lavas.		These	textures	only	form	in	subaeriallavas,	as	opposed	to	subaqueouslavas,	which	erupt	underwater.